tag:theconversation.com,2011:/au/topics/dwarf-planets-1293/articlesDwarf planets – The Conversation2022-06-05T12:22:26Ztag:theconversation.com,2011:article/1778892022-06-05T12:22:26Z2022-06-05T12:22:26ZCurious Kids: Why does it matter if Pluto is a planet or a dwarf planet?<figure><img src="https://images.theconversation.com/files/466839/original/file-20220602-20-45czb4.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C4500%2C2997&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Pluto was recategorized from a planet to a dwarf planet in 2006.</span> <span class="attribution"><span class="source">(Shutterstock)</span></span></figcaption></figure><iframe style="width: 100%; height: 100px; border: none; position: relative; z-index: 1;" allowtransparency="" allow="clipboard-read; clipboard-write" src="https://narrations.ad-auris.com/widget/the-conversation-canada/curious-kids--why-does-it-matter-if-pluto-is-a-planet-or-a-dwarf-planet" width="100%" height="400"></iframe>
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<img alt="" src="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=293&fit=crop&dpr=1 600w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=293&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=293&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=368&fit=crop&dpr=1 754w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=368&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=368&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<p><em>Curious Kids is a series for children of all ages. Have a question you’d like an expert to answer? Send it to <a href="mailto:curiouskidscanada@theconversation.com">CuriousKidsCanada@theconversation.com</a>.</em></p>
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<p><strong>Why does it matter if Pluto is a planet or a dwarf planet? Because for me it just makes it more confusing in our solar system. I know that some things in space are planets and some are stars and some are other names like moons or comets. Dwarf planet is a more different name and I think it just makes it more confusing. —
Timmy, 11, Kitchener, Ont.</strong></p>
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<p>“Comet,” “star” and “planet” are category names that immediately tell you something important about what they describe.</p>
<p>Our solar system consists of the sun, planets (which orbit around the sun) and small bodies (which either orbit around the sun or planets). The “small bodies” category is divided up into even <a href="https://nineplanets.org/small-solar-system-bodies/">smaller categories</a>, mostly depending on the shape and size of orbits.</p>
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Read more:
<a href="https://theconversation.com/what-is-a-dwarf-planet-178844">What is a dwarf planet?</a>
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<p>In 1801, astronomers discovered Ceres, which was <a href="https://earthsky.org/space/jan-1-1801-discovery-of-ceres/">initially categorized as a “planet.”</a> Astronomers measured that it was much smaller than the other known planets. Soon, a lot of smaller objects were discovered on orbits very close to Ceres. These small bodies were categorized as “asteroids” and we have since discovered <a href="https://solarsystem.nasa.gov/asteroids-comets-and-meteors/asteroids/in-depth/">hundreds of thousands of these in the asteroid belt</a>.</p>
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<a href="https://images.theconversation.com/files/466843/original/file-20220602-183-9xrt9e.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A black and white photograph of Ceres, which looks like an eclipsed moon" src="https://images.theconversation.com/files/466843/original/file-20220602-183-9xrt9e.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/466843/original/file-20220602-183-9xrt9e.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/466843/original/file-20220602-183-9xrt9e.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/466843/original/file-20220602-183-9xrt9e.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/466843/original/file-20220602-183-9xrt9e.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/466843/original/file-20220602-183-9xrt9e.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/466843/original/file-20220602-183-9xrt9e.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Ceres, seen from NASA’s Dawn spacecraft.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/content/ceres-seen-from-nasas-dawn-spacecraft">(NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)</a></span>
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<h2>New discoveries</h2>
<p>A similar process of discovery and re-categorization happened for small bodies further out in the solar system. </p>
<p><a href="http://pluto.jhuapl.edu/">Pluto</a> was <a href="https://lowell.edu/discover/telescopes-exhibits/pluto-discovery-telescope/">discovered in 1930</a> and was called the ninth planet in our solar system for many decades. But astronomers soon learned that Pluto was pretty different from the other eight planets: it’s on a tilted orbit and it’s way, way smaller than the other planets. </p>
<p>Over the years, astronomers discovered more and more small, planet-like objects that crossed Pluto’s orbit. These are now categorized as “<a href="https://nineplanets.org/kids/the-kuiper-belt/">Kuiper Belt objects</a>.” It was looking more and more like Pluto might fit in better with the category of Kuiper Belt objects than with planets.</p>
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<em>
<strong>
Read more:
<a href="https://theconversation.com/how-we-discovered-840-minor-planets-beyond-neptune-and-what-they-can-tell-us-96431">How we discovered 840 minor planets beyond Neptune – and what they can tell us</a>
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</p>
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<p>In 2005, a new object was discovered in the outer solar system, <a href="http://web.gps.caltech.edu/%7Embrown/planetlila/">Eris</a>, that is even heavier than Pluto. This led astronomers to consider if both Eris and Pluto are planets or not. Astronomers thought this was an important enough decision that the International Astronomical Union <a href="https://earthsky.org/human-world/pluto-dwarf-planet-august-24-2006/">voted on it in 2006</a>. Astronomers decided that rather than demoting Pluto to a plain old Kuiper Belt object, they would make a new category of small body called a “<a href="https://calgary.rasc.ca/dwarfplanets.htm">dwarf planet</a>.” Pluto and Eris would both be part of this new category.</p>
<h2>How planets form</h2>
<p>Solar systems like ours form from big clouds of dust and gas that collapse into disks around young stars, but astronomers are still learning exactly how that process works. We use telescopes to <a href="https://almascience.eso.org/alma-science/planet-forming-disks">look carefully</a> at forming solar systems far away, but they are so far that it’s really hard to see the planets forming directly. </p>
<p>A planetesimal — a baby planet — <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/planetesimal">first forms from clumps of dust in a disc orbiting a young star</a>. Planetesimals then grab nearby pebbles, dust and sometimes even smaller planetesimals with their gravity, which gets stronger as they get bigger. When they get to be a few hundred kilometres across, they have enough gravity to pull themselves into a round shape, which is the <a href="https://www.iau.org/public/themes/pluto/">definition of a dwarf planet</a>.</p>
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<a href="https://images.theconversation.com/files/466836/original/file-20220602-24-oxee8w.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="two photographs and corresponding illustrations of debris around a young star" src="https://images.theconversation.com/files/466836/original/file-20220602-24-oxee8w.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/466836/original/file-20220602-24-oxee8w.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=595&fit=crop&dpr=1 600w, https://images.theconversation.com/files/466836/original/file-20220602-24-oxee8w.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=595&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/466836/original/file-20220602-24-oxee8w.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=595&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/466836/original/file-20220602-24-oxee8w.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=747&fit=crop&dpr=1 754w, https://images.theconversation.com/files/466836/original/file-20220602-24-oxee8w.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=747&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/466836/original/file-20220602-24-oxee8w.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=747&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">The two images at top reveal debris disks around young stars uncovered in archival images taken by NASA’s Hubble Space Telescope. The illustration beneath each image depicts the orientation of the debris disks.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/press/2014/april/astronomical-forensics-uncover-planetary-disks-in-nasas-hubble-archive/">(NASA/ESA, R. Soummer, Ann Feild (STScI))</a></span>
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<p>Measuring small bodies in our solar system, including dwarf planets, and comparing them with computer simulations is another way to see how our solar system formed. Our current theory is that there must have been <a href="https://baas.aas.org/pub/2021n7i202p01/release/1">a lot of dwarf planets that formed in our solar system</a>.</p>
<p>Ceres, in the asteroid belt, and Pluto, Eris and <a href="https://calgary.rasc.ca/dwarfplanets.htm">about a dozen other Kuiper Belt objects</a> are big enough to be in the dwarf planet category. This means that while they are planetesimals that grew big enough to be round, they did not develop a gravity strong enough to grab all the other planetesimals near their orbit.</p>
<h2>Other solar systems</h2>
<p>Astronomers have now measured more than <a href="https://www.jpl.nasa.gov/news/cosmic-milestone-nasa-confirms-5000-exoplanets">5,000 exoplanets</a>, planets in other solar systems. We won’t be able to measure dwarf planets there for a very long time, but the ones we’ve found in our own solar system can teach us about how planets form everywhere.</p>
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<p><em>Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to <a href="mailto:curiouskidscanada@theconversation.com">CuriousKidsCanada@theconversation.com</a>. Please tell us your name, age and the city where you live.</em></p>
<p><em>And since curiosity has no age limit — adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.</em></p>
<hr><img src="https://counter.theconversation.com/content/177889/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Samantha Lawler receives funding from the Natural Sciences and Engineering Research Council of Canada.</span></em></p>A curious kid asks: Why does it matter if Pluto is a planet or a dwarf planet?Samantha Lawler, Assistant professor, Astronomy, University of ReginaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1788442022-04-04T12:28:37Z2022-04-04T12:28:37ZWhat is a dwarf planet?<figure><img src="https://images.theconversation.com/files/452922/original/file-20220317-13-1u6oyiu.jpg?ixlib=rb-1.1.0&rect=0%2C3%2C1039%2C713&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Pluto, the largest of the dwarf planets. This image was taken by NASA's New Horizons spacecraft.</span> <span class="attribution"><a class="source" href="https://solarsystem.nasa.gov/resources/699/pluto-dazzles-in-false-color/">NASA/JHUAPL/SwRI</a></span></figcaption></figure><figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=293&fit=crop&dpr=1 600w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=293&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=293&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=368&fit=crop&dpr=1 754w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=368&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=368&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<p><em><a href="https://theconversation.com/us/topics/curious-kids-us-74795">Curious Kids</a> is a series for children of all ages. If you have a question you’d like an expert to answer, send it to <a href="mailto:curiouskidsus@theconversation.com">curiouskidsus@theconversation.com</a>.</em></p>
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<p><strong>What is a dwarf planet? – Myranda, age 8, Knoxville, Tennessee</strong></p>
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<p>The word “planet” came from the ancient Greek words that mean “wandering star.” That makes sense, because for thousands of years, people have watched planets change position in the night sky – unlike stars, which appear fixed and unmoving to the naked eye. </p>
<p>That’s how the ancients discovered five of the planets: <a href="https://solarsystem.nasa.gov/planets/mercury/overview/">Mercury</a>, <a href="https://solarsystem.nasa.gov/planets/venus/overview/">Venus</a>, <a href="https://solarsystem.nasa.gov/news/1679/mars-resources/?">Mars</a>, <a href="https://solarsystem.nasa.gov/planets/jupiter/overview/">Jupiter</a> and <a href="https://solarsystem.nasa.gov/planets/saturn/overview/">Saturn</a>. Astronomers using telescopes found <a href="https://solarsystem.nasa.gov/planets/uranus/overview/">Uranus</a> in 1781, <a href="https://solarsystem.nasa.gov/planets/neptune/overview/">Neptune</a> in 1846, and <a href="https://solarsystem.nasa.gov/planets/dwarf-planets/pluto/overview/">Pluto</a> in 1930. </p>
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<img alt="Artist's impression of the dwarf planet Eris, a white and pale gray sphere." src="https://images.theconversation.com/files/454273/original/file-20220324-13-18yzsb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/454273/original/file-20220324-13-18yzsb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/454273/original/file-20220324-13-18yzsb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/454273/original/file-20220324-13-18yzsb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/454273/original/file-20220324-13-18yzsb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/454273/original/file-20220324-13-18yzsb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/454273/original/file-20220324-13-18yzsb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Artist’s impression of the dwarf planet Eris.</span>
<span class="attribution"><a class="source" href="https://www.eso.org/public/images/eso1142a/">ESO/L.Calçada and Nick Risinger</a></span>
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<h2>Solar system leftovers</h2>
<p><a href="https://dornsife.usc.edu/cf/faculty-and-staff/faculty.cfm?pid=1063926">I’m a space scientist</a> with a passion for astronomy and the exploration of the Solar System. I received my Ph.D. in physics in 1994, about the time astronomers began to find more and more objects beyond Neptune, in the <a href="https://spaceplace.nasa.gov/kuiper-belt/en/">Kuiper belt</a>. That’s a place in space that holds the “leftovers” of the solar system – particularly small icy bodies. </p>
<p>Three of those icy bodies – <a href="https://solarsystem.nasa.gov/planets/dwarf-planets/eris/in-depth/">Eris</a>, <a href="https://solarsystem.nasa.gov/planets/dwarf-planets/haumea/in-depth/">Haumea</a> and <a href="https://solarsystem.nasa.gov/planets/dwarf-planets/makemake/in-depth/">Makemake</a> – were discovered in the early to mid 2000s. They seemed large enough to be planets; all of them are roughly the same size as Pluto.</p>
<p>Astronomers then surmised that there were likely many more of these icy bodies in the Kuiper belt. They began to wonder: How many planets might we end up identifying in our solar system? Twenty? Thirty? A hundred? More?</p>
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<img alt="An artist's illustration of the dwarf planet Haumea, an oval shaped world surrounded by its ring." src="https://images.theconversation.com/files/454305/original/file-20220325-27-j10f5i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/454305/original/file-20220325-27-j10f5i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=480&fit=crop&dpr=1 600w, https://images.theconversation.com/files/454305/original/file-20220325-27-j10f5i.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=480&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/454305/original/file-20220325-27-j10f5i.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=480&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/454305/original/file-20220325-27-j10f5i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=603&fit=crop&dpr=1 754w, https://images.theconversation.com/files/454305/original/file-20220325-27-j10f5i.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=603&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/454305/original/file-20220325-27-j10f5i.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=603&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">An artist’s illustration of the dwarf planet Haumea, surrounded by its ring.</span>
<span class="attribution"><a class="source" href="https://science.nasa.gov/haumea-outer-solar-system">Instituto de Asrofísica de Andalucía</a></span>
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<h2>Dwarf planet defined</h2>
<p>In 2006, and after much debate, the <a href="https://www.iau.org/">International Astronomical Union</a> came up with a new definition for a planet. And for the first time, the term “dwarf planet” was used.</p>
<p>Here’s what the IAU said: A planet has to orbit the Sun directly. It also must be large enough to have a round, or spherical, physical shape. </p>
<p>And the planet must “<a href="https://solarsystem.nasa.gov/planets/in-depth/">clear its neighborhood</a>.” That means, aside from any moons it might have, the planet can’t share its orbit with other objects of comparable size.</p>
<p>An object that satisfies only the first two criteria – but not the last – is now called a dwarf planet. </p>
<figure class="align-center ">
<img alt="Artist's illustration of Makemake, a dwarf planet in the Kuiper Belt. Nearby is its moon, MK 2. Off in the distance: the Sun." src="https://images.theconversation.com/files/454461/original/file-20220325-25-1th3ksp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/454461/original/file-20220325-25-1th3ksp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/454461/original/file-20220325-25-1th3ksp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/454461/original/file-20220325-25-1th3ksp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/454461/original/file-20220325-25-1th3ksp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/454461/original/file-20220325-25-1th3ksp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/454461/original/file-20220325-25-1th3ksp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Artist’s illustration of Makemake, a dwarf planet in the Kuiper belt. Nearby is its moon, MK 2. Off in the distance: the Sun.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/sites/default/files/thumbnails/image/makemakemoon100mile.jpg">NASA/ESA/A. Parker/Southwest Research Institute</a></span>
</figcaption>
</figure>
<h2>Pluto is demoted</h2>
<p>That’s why Pluto <a href="https://www.kidsnews.com.au/space/pluto-lost-its-spot-in-our-solar-system-but-still-holds-a-place-in-many-hearts/news-story/4b4f0862a47b089ea5850d9522f211ca">lost its status as a planet</a> and is now classified as a dwarf planet. It failed the final item on the checklist – other icy Kuiper belt bodies are within its orbital path. The decision, a controversial one to be sure, is <a href="https://www.sciencenewsforstudents.org/article/pluto-dwarf-planet-definition-iau-astronomy">debated by scientists to this very day</a>. </p>
<p>At the same time Pluto got demoted, another solar system object was promoted. <a href="https://solarsystem.nasa.gov/planets/dwarf-planets/ceres/overview/">Ceres</a>, once considered an asteroid, is now classified as a dwarf planet. It’s nowhere near the Kuiper belt; instead, Ceres is in <a href="https://solarsystem.nasa.gov/asteroids-comets-and-meteors/asteroids/overview/?">the main asteroid belt</a>, orbiting between Mars and Jupiter.</p>
<p>Add them up – Pluto, Ceres, Eris, Haumea and Makemake – and that brings the number of dwarf planets in our solar system to five. But that list is sure to grow. Already, hundreds of candidates, nearly all in the Kuiper belt, potentially satisfy the criteria to be a dwarf planet. </p>
<figure class="align-center ">
<img alt="A photo of the dwarf planet Ceres. To the human eye, it appears a sandy brown color and is pockmarked with craters." src="https://images.theconversation.com/files/454567/original/file-20220328-13-1wwgmnd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/454567/original/file-20220328-13-1wwgmnd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/454567/original/file-20220328-13-1wwgmnd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/454567/original/file-20220328-13-1wwgmnd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/454567/original/file-20220328-13-1wwgmnd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/454567/original/file-20220328-13-1wwgmnd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/454567/original/file-20220328-13-1wwgmnd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">This photograph of Ceres, a dwarf planet in the main asteroid belt, was taken by NASA’s Dawn spacecraft.</span>
<span class="attribution"><a class="source" href="https://solarsystem.nasa.gov/resources/622/ceres-in-color/?category=planets/dwarf-planets_ceres">NASA/JPL-Caltech/UCLA/MPS/DLR/IDA</a></span>
</figcaption>
</figure>
<h2>About the dwarf planets</h2>
<p>Dwarf planets are nothing like Earth. </p>
<p>As their name implies, they are much smaller. Pluto and Eris, the largest of the dwarfs, have less than one-fifth the diameter of the Earth. </p>
<p><a href="https://spaceplace.nasa.gov/planets-weight/en/#:%7E:text=Mass%20stays%20the%20same%20regardless,mass%20is%20the%20same%20everywhere!">They have less mass, too</a>. For example, Earth has <a href="https://theplanets.org/ceres/">about 6,400 times more mass</a> than Ceres. That’s like comparing two <a href="https://en.wikipedia.org/wiki/Orca">killer whales</a> to a <a href="https://en.wikipedia.org/wiki/Guinea_pig">guinea pig</a>.</p>
<p>And dwarf planets are cold. <a href="https://solarsystem.nasa.gov/planets/dwarf-planets/pluto/in-depth/#:%7E:text=On%20average%2C%20Pluto's%20temperature%20is,orbits%20in%20our%20solar%20system.">Pluto’s average temperature</a> is around minus 400 degrees Fahrenheit (minus 240 Celsius).</p>
<figure class="align-center ">
<img alt="A photograph of Pluto and one of its five moons, Charon." src="https://images.theconversation.com/files/454578/original/file-20220328-25-rfa5po.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/454578/original/file-20220328-25-rfa5po.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/454578/original/file-20220328-25-rfa5po.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/454578/original/file-20220328-25-rfa5po.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/454578/original/file-20220328-25-rfa5po.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/454578/original/file-20220328-25-rfa5po.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/454578/original/file-20220328-25-rfa5po.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A photograph of Pluto and one of its five moons, Charon. Except for Ceres, all the dwarf planets have at least one moon. Charon is nearly half Pluto’s size.</span>
<span class="attribution"><a class="source" href="https://solarsystem.nasa.gov/resources/853/pluto-and-charon/?category=planets/dwarf-planets_pluto">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</a></span>
</figcaption>
</figure>
<h2>Could life exist on a dwarf planet?</h2>
<p>Three things are needed for life: liquid water, an energy source and organic molecules – that is, molecules containing carbon. </p>
<p>More than 100 miles (161 kilometers) below Pluto’s surface, an enormous ocean of liquid water may exist; this might also be true for other Kuiper belt worlds. <a href="https://astronomy.com/news/2020/08/ceres-an-ocean-world-in-the-asteroid-belt">Ceres also has subsurface water</a>, remnants of what might have been an ancient global ocean. </p>
<p>Organic molecules, in abundance <a href="https://www.extremetech.com/extreme/271914-organic-molecules-on-ceres-are-more-abundant-than-previously-thought">everywhere in our solar system</a>, have been found on Ceres and Pluto. </p>
<p>But the one missing ingredient for all the dwarf planets is a source of energy. </p>
<p>Sunlight won’t work, particularly for the Kuiper belt dwarfs; they are simply too far away from the Sun. To reach the belt, the light must travel <a href="https://imagine.gsfc.nasa.gov/features/cosmic/solar_system_info.html">more than 2.7 billion miles</a> (4.4 billion km). By the time the sunshine arrives at these distant worlds, it’s <a href="https://solarsystem.nasa.gov/solar-system/kuiper-belt/in-depth/">too weak to significantly heat</a> any of them.</p>
<p>And all the dwarf planets are too small to hold the inner heat that remains from the <a href="https://kids.britannica.com/kids/article/solar-system/353789#:%7E:text=The%20solar%20system%20was%20formed%20about%204.7%20billion%20years%20ago,that%20it%20got%20very%20hot.">solar system’s formation</a>. </p>
<p>Yet scientists have discovered life on Earth in the most hostile places imaginable – near the bottom of the ocean, miles deep in the soil and even inside an active volcano. When it comes to life in our solar system, never say never. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/nJiw2NxqoBU?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Take a tour of the dwarf planet Ceres.</span></figcaption>
</figure>
<hr>
<p><em>Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to <a href="mailto:curiouskidsus@theconversation.com">CuriousKidsUS@theconversation.com</a>. Please tell us your name, age and the city where you live.</em></p>
<p><em>And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.</em></p><img src="https://counter.theconversation.com/content/178844/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Vahe Peroomian has received funding in the past from the National Science Foundation and from the National Aeronautics and Space Administration.</span></em></p>The dwarf planets in our Solar System are cold, dark, far away and full of surprises.Vahe Peroomian, Professor of Physics and Astronomy, USC Dornsife College of Letters, Arts and SciencesLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1409762020-06-22T15:08:41Z2020-06-22T15:08:41ZLife inside Pluto? Hot birth may have created internal ocean on dwarf planet<figure><img src="https://images.theconversation.com/files/343057/original/file-20200621-43196-1m9elpx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Pluto, with its basin Sputnik Planitia on the right.</span> <span class="attribution"><span class="source">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker</span></span></figcaption></figure><p>Pluto, along with many other dwarf planets in the outer solar system, is often thought of as dark, icy and barren – with a surface temperature of just −230°C. But now a new study, <a href="https://www.nature.com/articles/s41561-020-0595-0">published in Nature Geoscience</a>, suggests that the body has had a warm interior ever since it formed, and may still have a liquid, internal ocean under its icy crust.</p>
<p>It could mean that other sizeable icy dwarf planets may have had early internal oceans too, with some possibly persisting today. This is exciting, as where there’s warm water, there could be life.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/342509/original/file-20200617-94036-c8xr6e.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/342509/original/file-20200617-94036-c8xr6e.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=256&fit=crop&dpr=1 600w, https://images.theconversation.com/files/342509/original/file-20200617-94036-c8xr6e.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=256&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/342509/original/file-20200617-94036-c8xr6e.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=256&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/342509/original/file-20200617-94036-c8xr6e.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=322&fit=crop&dpr=1 754w, https://images.theconversation.com/files/342509/original/file-20200617-94036-c8xr6e.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=322&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/342509/original/file-20200617-94036-c8xr6e.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=322&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Near-sunset view of Pluto’s rugged, icy mountains and flat plains.</span>
<span class="attribution"><a class="source" href="http://pluto.jhuapl.edu/Galleries/Featured-Images/pics/Pluto-Wide-FINAL-9-17-15.jpg">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</a></span>
</figcaption>
</figure>
<p>As soon as NASA’s New Horizon’s probe began to send back its haul of pictures and other data from its <a href="https://theconversation.com/new-horizons-finally-gets-up-close-with-pluto-for-15-minutes-44603">2016 flyby of Pluto</a>, it became clear that this is <a href="https://theconversation.com/stunning-crystal-clear-images-of-pluto-but-what-do-they-mean-47517">one of the most interesting worlds</a> ever seen. Beneath its haze-layered atmosphere is a frigid, cratered surface of impure water-ice and one major impact basin (Sputnik Planitia) that has been flooded by frozen nitrogen.</p>
<p>The water-ice crust is cut by numerous fractures, all of which appear to be the result of stretching of the surface. Those cracks in the ice provided the first hints that there might be liquid water flowing underneath, in the form of an internal ocean between the icy shell and rocky core. More evidence <a href="https://www.nature.com/articles/nature20148">soon emerged</a> in favour of this, such as hints that the icy shell has been able to re-orient itself, gliding over an essentially frictionless (hence liquid) interior.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/342511/original/file-20200617-94036-z0e0a0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/342511/original/file-20200617-94036-z0e0a0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=360&fit=crop&dpr=1 600w, https://images.theconversation.com/files/342511/original/file-20200617-94036-z0e0a0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=360&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/342511/original/file-20200617-94036-z0e0a0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=360&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/342511/original/file-20200617-94036-z0e0a0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=452&fit=crop&dpr=1 754w, https://images.theconversation.com/files/342511/original/file-20200617-94036-z0e0a0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=452&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/342511/original/file-20200617-94036-z0e0a0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=452&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Artist’s impression showing Pluto’s interior. An ocean of liquid water between the icy crust and rocky core.</span>
<span class="attribution"><span class="source">Pam Engebretson/Physics Org</span></span>
</figcaption>
</figure>
<p>If it does have an internal ocean, Pluto is far from unique. Evidence for present-day oceans inside icy moons such as Jupiter’s <a href="https://theconversation.com/new-water-plumes-from-jupiters-moon-europa-raise-hopes-of-detecting-microbial-life-66019">Europa</a>, and Saturn’s <a href="https://theconversation.com/titan-first-global-map-uncovers-secrets-of-a-potentially-habitable-moon-of-saturn-126985">Titan</a> and <a href="https://theconversation.com/nasa-saturn-moon-enceladus-is-able-to-host-life-its-time-for-a-new-mission-76102">Enceladus</a> is so strong that few scientists doubt the likelihood of an ocean inside Pluto for at least part of its history.</p>
<h2>Cracking time</h2>
<p>The insight offered by the new study comes from studying maps of Pluto’s shape and features. The researchers discovered that cracks in its surface are of all ages – right back to the most remote times we can see, soon after the surface formed, possibly 4.5 billion years ago.</p>
<p>Scientists have assumed that Pluto grew by slowly accumulating icy material that condensed when the outer solar system was forming. In such a scenario, no internal ocean could have formed until trapped heat generated by radioactive decay in the rocky core had built up sufficiently to melt the overlying ice. </p>
<p>In that situation, the oldest geological faults on the surface would have certain specific characteristics (dubbed <a href="https://earthquake.usgs.gov/learn/glossary/?term=compressional%20stress">compressional features</a>). This is because turning the lower part of the ice into liquid water, which is denser and occupies less volume, would have placed the overlying ice into compression.</p>
<p>Other types of fractures interpreted as “<a href="https://www.usgs.gov/media/images/extensional-crack">extensional cracks</a>” could begin to form only when the top of this ocean began to freeze as its heat escaped to space. The pressure of the ice forced the interior to expand slightly, stretching and cracking the surface a little. However, Pluto’s surface is cut by what appear to be extensional cracks only, right back to the most ancient times. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/342657/original/file-20200618-41204-ox6o73.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/342657/original/file-20200618-41204-ox6o73.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/342657/original/file-20200618-41204-ox6o73.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=402&fit=crop&dpr=1 600w, https://images.theconversation.com/files/342657/original/file-20200618-41204-ox6o73.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=402&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/342657/original/file-20200618-41204-ox6o73.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=402&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/342657/original/file-20200618-41204-ox6o73.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=506&fit=crop&dpr=1 754w, https://images.theconversation.com/files/342657/original/file-20200618-41204-ox6o73.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=506&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/342657/original/file-20200618-41204-ox6o73.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=506&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Part of a map of Pluto’s topography (dark = low, bright = high). The dark (low) area in the east is part of Sputnik Planitia. Ancient north-south troughs run to its west; more obvious narrower and younger cracks run obliquely.</span>
<span class="attribution"><span class="source">Paul Schenk</span></span>
</figcaption>
</figure>
<p>The authors therefore argue that the young Pluto grew to its present size by accumulating tiny pieces of material in a so-called “pebble accretion” process that was energetic and rapid enough to cause melting at the base of the ice layer. This is termed a “hot start”, though all it means is “just warm enough for water-ice to melt”. </p>
<p>The crust, from the first moment that it became stable, never experienced compression. Instead, its surface suffered extension as liquid water at top of the ocean froze onto the base of the ice shell during Pluto’s first half billion years. </p>
<p>Ocean freezing may then have paused for about the next billion years because the build-up of radioactive heat was temporarily able to balance the rate of heat escape to space. But ever since then, as Pluto’s radioactive heat production dwindled over time, the roof of the ocean continued to freeze. The thickness of the ice shell has maybe doubled to about 180km. The surviving ocean is likely a 200km thick layer between the ice and the rock.</p>
<h2>Oceans and life</h2>
<p>Internal oceans are fascinating, not just because of how changes in volume can stretch or compress the surface, but because they are potential habitats for life. It is irrelevant that Pluto’s surface temperature is extremely low, because any internal ocean would be warm enough for life.</p>
<p>This could not be life depending on sunlight for its energy, like most life on Earth, and it would have to survive on the probably very meagre chemical energy available within Pluto. So while we can’t rule out there could be life inside Pluto, Europa and Enceladus are likely to be better contenders, since they have <a href="https://theconversation.com/nasa-saturn-moon-enceladus-is-able-to-host-life-its-time-for-a-new-mission-76102">more chemical energy available</a>.</p>
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<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/stunning-crystal-clear-images-of-pluto-but-what-do-they-mean-47517">Stunning, crystal-clear images of Pluto – but what do they mean?</a>
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<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/nasa-mission-brings-pluto-into-sharp-focus-but-its-still-not-a-planet-40495">NASA mission brings Pluto into sharp focus – but it's still not a planet</a>
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<img src="https://counter.theconversation.com/content/140976/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Rothery is Professor of Planetary Geosciences at the Open University. He is co-leader of the European Space Agency's Mercury Surface and Composition Working Group, and a Co-Investigator on MIXS (Mercury Imaging X-ray Spectrometer) that is now on its way to Mercury on board the European Space Agency's Mercury orbiter BepiColombo. He has received funding from the UK Space Agency and the Science & Technology Facilities Council for work related to Mercury BepiColombo, and is currently funded by the European Commission under its Horizon 2020 programme for work on planetary geological mapping (776276 Planmap). He is author of Planet Mercury - from Pale Pink Dot to Dynamic World (Springer, 2015), Moons: A Very Short Introduction (Oxford University Press, 2015) and Planets: A Very Short Introduction (Oxford University Press, 2010). He is Educator on the Open University's free learning Badged Open Course (BOC) on Moons and its equivalent FutureLearn Moons MOOC, and chair of the Open University's level 2 course on Planetary Science and the Search for Life.</span></em></p>Pluto began hot inside, study of its surface fractures suggestsDavid Rothery, Professor of Planetary Geosciences, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1155672019-04-23T20:12:49Z2019-04-23T20:12:49ZWhy Pluto is losing its atmosphere: winter is coming<figure><img src="https://images.theconversation.com/files/269935/original/file-20190418-28113-wpxk2p.jpg?ixlib=rb-1.1.0&rect=138%2C81%2C2898%2C1714&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The spectacular layers of blue haze in Pluto's atmosphere, captured by NASA's New Horizons spacecraft.</span> <span class="attribution"><a class="source" href="https://solarsystem.nasa.gov/resources/637/blue-rays-new-horizons-high-res-farewell-to-pluto/?category=planets/dwarf-planets_pluto">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</a></span></figcaption></figure><p>The ominous warning – “winter is coming”, popularised by fantasy series Game of Thrones – applies equally well to Pluto.</p>
<p>The dwarf planet’s tenuous atmosphere appears to be on the verge of a stunning collapse due to a change in the seasons and approaching colder conditions, according to <a href="https://arxiv.org/abs/1903.02315" title="Pluto's lower atmosphere and pressure evolution from ground-based stellar occultations, 1988-2016">research to be published in the journal Astronomy & Astrophysics</a>.</p>
<p>Discovered in 1930, it was only around 1980 that astronomers began to suspect Pluto might have an atmosphere. <a href="https://solarsystem.nasa.gov/planets/dwarf-planets/pluto/in-depth/#atmosphere_otp">That atmosphere</a> was <a href="http://www.cbat.eps.harvard.edu/iauc/04000/04097.html#Item2">tentatively discovered</a> in 1985 and fully <a href="https://www.space.com/29885-pluto-atmosphere-to-be-revealed-by-nasa-new-horizons-spacecraft.html">confirmed by independent observations in 1988</a>.</p>
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Read more:
<a href="https://theconversation.com/ive-always-wondered-how-do-we-know-what-lies-at-the-heart-of-pluto-101327">I've Always Wondered: How do we know what lies at the heart of Pluto?</a>
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<p>At the time, astronomers had no way of knowing what dramatic changes were in store for the little world’s thin envelope of nitrogen, methane and hydrocarbons.</p>
<h2>A cosmic coincidence</h2>
<p>By a cosmic coincidence, the last decades of the 20th century and first decades of the 21st also saw a lucky alignment of Earth, Pluto and the dense stellar fields of the distant centre of the Milky Way.</p>
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<img src="https://cdn.theconversation.com/static_files/files/563/pluto_through_years_full.gif?1555809969" width="100%">
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<span class="caption">This animation combines various observations of Pluto over the course of several decades.</span>
<span class="attribution"><span class="source">NASA</span></span>
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<p>This coincidence means Pluto passes relatively often between us and a background star. When this happens, its shadow falls on Earth, an event astronomers refer to as an <a href="https://www.space.com/33946-occultations.html">occultation</a>.</p>
<p>During an occultation, any observatory that happens to lie within the path of the shadow can watch the star seem to disappear as Pluto passes in front of it, and then to reappear as the planetary alignments shift. For any given place on Earth’s surface, a Pluto occultation lasts a couple of minutes at most. </p>
<p>The technique of occultations has been widely used to study the orbits, rings, moons, shapes and atmospheres of the worlds of the outer Solar System, including asteroids, comets, planets and dwarf planets.</p>
<p>By comparing what observers see at different locations on Earth, the size and shape of the occulting world can be worked out. If the object has an atmosphere, then for a few brief seconds as the starlight winks out and then comes back on, the starlight can be altered by absorption and refraction as it passes through the planetary atmosphere.</p>
<p>Since the first successful occultation measurements in the 1980s, a succession of observations have established increasingly precise measures of Pluto’s radius, as well as continually sharpening our understanding of the temperature and pressure of its atmosphere.</p>
<h2>Long orbit and seasons</h2>
<p>Like Earth, Pluto has a seasonal cycle due to the <a href="https://solarsystem.nasa.gov/planets/dwarf-planets/pluto/in-depth/#orbit_and_rotation_otp">inclination of its poles</a> to the plane of its orbit. Over the course of Pluto’s long year – equivalent to 248 Earth years – first the north pole and then the south pole are angled toward the distant Sun.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/270196/original/file-20190421-28090-1me0otf.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/270196/original/file-20190421-28090-1me0otf.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/270196/original/file-20190421-28090-1me0otf.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=337&fit=crop&dpr=1 600w, https://images.theconversation.com/files/270196/original/file-20190421-28090-1me0otf.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=337&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/270196/original/file-20190421-28090-1me0otf.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=337&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/270196/original/file-20190421-28090-1me0otf.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=423&fit=crop&dpr=1 754w, https://images.theconversation.com/files/270196/original/file-20190421-28090-1me0otf.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=423&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/270196/original/file-20190421-28090-1me0otf.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=423&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">A drawing of the Solar System shows Pluto’s tilted orbit, which is also more elliptical than that of the planets.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-pluto-58.html">NASA (modified)</a></span>
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<p>But unlike Earth, Pluto’s orbit is stretched into an extreme elliptical shape. Its orbit is so elongated that its distance from the Sun varies from 4.4 to 7.4 billion kilometres (30 to 50 times as far as the Earth-Sun distance).</p>
<p>By contrast, Earth’s distance from the Sun varies by only 3.4% over a year. Pluto’s atmosphere was discovered just before Pluto reached its closest approach to the Sun, which happened in 1989. </p>
<p>Since 1989, Pluto has been retreating from the Sun. The temperatures have been decreasing accordingly. </p>
<h2>Under pressure</h2>
<p>At the time Pluto started moving away from the sun, astronomers expected that this would cause its atmospheric pressure to drop, in much the same way that the pressure in an automobile tyre decreases with cold weather and increases in the heat. On the contrary, observations from 1988-2016 have shown a steady increase in the atmospheric pressure. </p>
<p>Immediately before the arrival of <a href="https://www.nasa.gov/mission_pages/newhorizons/main/index.html">NASA’s New Horizons probe</a> in 2015, occultation measurements <a href="https://iopscience.iop.org/article/10.3847/2041-8205/819/2/L38/meta">discovered</a> the atmospheric pressure on Pluto has tripled since 1988 (the equivalent on Earth would be to compare the pressure at the top of Mt Everest to that at sea level).</p>
<p>What is the cause of the discrepancy? Any thought that the occultation measurements were in error was banished by the Radio Science Experiment (<a href="https://www.nasa.gov/mission_pages/newhorizons/spacecraft/index.html">REX</a>) aboard New Horizons, which returned direct measurements in agreement with the Earthbound observers.</p>
<p>The new research has solved the mystery using a seasonal model for the transport of gas and ice around the surface of the planet.</p>
<p>Even though Pluto is moving farther from the Sun every year, its north pole is continuously sunlit during this part of its orbit, causing its nitrogen ice cap to revert to the gas phase. </p>
<p>This explains the rapid increase of atmospheric pressure over the past three decades. </p>
<p>But the climate modelling shows this trend will not continue.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/270193/original/file-20190421-28087-1ll5i3g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/270193/original/file-20190421-28087-1ll5i3g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/270193/original/file-20190421-28087-1ll5i3g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=370&fit=crop&dpr=1 600w, https://images.theconversation.com/files/270193/original/file-20190421-28087-1ll5i3g.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=370&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/270193/original/file-20190421-28087-1ll5i3g.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=370&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/270193/original/file-20190421-28087-1ll5i3g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=465&fit=crop&dpr=1 754w, https://images.theconversation.com/files/270193/original/file-20190421-28087-1ll5i3g.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=465&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/270193/original/file-20190421-28087-1ll5i3g.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=465&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">The frozen canyons of Pluto’s north pole captured by NASA’s New Horizons spacecraft.</span>
<span class="attribution"><a class="source" href="https://solarsystem.nasa.gov/resources/639/the-frozen-canyons-of-plutos-north-pole/">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</a></span>
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<h2>Winter really is coming</h2>
<p>Pluto will continue to move farther from the Sun until the year 2113, and the weak sunlight will not be sufficient to similarly warm the southern polar regions.</p>
<p>During the long northern autumn and winter, Pluto’s atmosphere is expected to collapse, frosting out onto the surface like ice on a car windscreen on a clear and cold winter night.</p>
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Read more:
<a href="https://theconversation.com/planet-or-dwarf-planet-all-worlds-are-worth-investigating-74682">Planet or dwarf planet: all worlds are worth investigating</a>
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<p>At its lowest ebb, the atmosphere is predicted to have less than 5% of its current pressure. The combination of Pluto’s close approach to the Sun and northern hemisphere spring won’t recur until the year 2237.</p>
<p>Until then, it will be of critical importance to test our understanding of planetary atmospheric models under extreme low-temperature and low-pressure conditions through continued occultation measurements.</p>
<p>But these opportunities will become less frequent as <a href="https://theskylive.com/pluto-tracker">Pluto’s orbit</a> takes its apparent position farther from the dense starfields of the galactic centre that helped us make the observations.</p><img src="https://counter.theconversation.com/content/115567/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andrew A. Cole receives funding from l'Agence Nationale de la Recherche. The University of Tasmania Greenhill Observatory is supported in part by contributions from the SearchLight Observatory Network and Dr David Warren. </span></em></p>The dwarf planet Pluto is heading away from the Sun and that’s having a devastating impact on its atmosphere.Andrew A. Cole, Senior Lecturer in Astrophysics, University of TasmaniaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1124362019-02-28T19:42:54Z2019-02-28T19:42:54ZThere are missing objects at the fringe of the solar system – new study puzzles astronomers<figure><img src="https://images.theconversation.com/files/261286/original/file-20190227-150698-64ge6x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Where are the smallest of the icy worlds we thought resided in the Kuiper belt?</span> <span class="attribution"><span class="source">ESO/Flickr</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>In the dimly lit spaces of our solar system beyond Neptune’s orbit lies the <a href="https://theconversation.com/after-pluto-theres-still-plenty-of-the-solar-system-left-to-explore-45002">Kuiper Belt</a>. This a region between about 35 and 50 times further from the sun than the Earth, populated by icy bodies so sparsely distributed that they never had the chance to collide and merge into planet sized objects. </p>
<p>Pluto is the largest that we know of, but only just. And over the past two decades, telescope surveys <a href="https://theconversation.com/how-we-discovered-840-minor-planets-beyond-neptune-and-what-they-can-tell-us-96431">have found</a> a couple of thousand more ranging down in size to only a few tens of kilometres across. The trouble is that most of the objects of that size or smaller are too faint to be spotted by telescopes. So it will be difficult to ever work out how many small but unseen bodies there actually are in the Kuiper belt. Now a new paper, <a href="http://science.sciencemag.org/cgi/doi/10.1126/science.aap8628">published in Science</a>, has used an ingenious method to help us find out.</p>
<p>This is important, because scientists believe Kuiper Belt objects are survivors from the solar system’s birth, developing from a primordial cloud of dust and gas. That means that their size distribution could have a lot to tell us about how the material from which the planets grew was initially assembled.</p>
<h2>Counting craters</h2>
<p>Instead of counting the small Kuiper belt objects directly, the researchers behind the new study counted the craters made by the random sample of objects that have impacted the surfaces of Pluto and its largest moon, <a href="https://theconversation.com/images-of-plutos-moon-charon-show-huge-fractures-and-hints-of-icy-lava-flows-48533">Charon</a>. There, craters 13km across would have been made by objects only 1km-2km in size. That is already way below the telescopic detection limit for Kuiper belt objects themselves, but images from <a href="https://theconversation.com/new-horizons-finally-gets-up-close-with-pluto-for-15-minutes-44603">the flyby of NASA’s New Horizons mission in 2015</a> allow craters as small as 1.4km to be mapped. Those must have been made by impacts of Kuiper Belt objects not much bigger than 100 metres in size.</p>
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<img alt="" src="https://images.theconversation.com/files/260674/original/file-20190225-26171-1u9e0w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/260674/original/file-20190225-26171-1u9e0w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=360&fit=crop&dpr=1 600w, https://images.theconversation.com/files/260674/original/file-20190225-26171-1u9e0w.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=360&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/260674/original/file-20190225-26171-1u9e0w.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=360&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/260674/original/file-20190225-26171-1u9e0w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=453&fit=crop&dpr=1 754w, https://images.theconversation.com/files/260674/original/file-20190225-26171-1u9e0w.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=453&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/260674/original/file-20190225-26171-1u9e0w.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=453&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Details of the ancient cratered surface of Charon’s Vulcan Planitia.</span>
<span class="attribution"><span class="source">: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/K. Singer</span></span>
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<p>The researchers’ analysis shows that for craters of 13km or larger, on both Pluto and Charon, the frequency of impacts of various sizes seems to match with what would be expected from the known size distribution for Kuiper belt objects. However, for smaller craters the abundance falls off dramatically, and so by implication must the abundance of the Kuiper Belt objects capable of making those craters. The same does not happen for the well-documented asteroids that collide with the bodies in the region of Jupiter, Mars and Earth, nor is it consistent with theoretical models.</p>
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<a href="https://images.theconversation.com/files/260748/original/file-20190225-26177-o5p85w.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/260748/original/file-20190225-26177-o5p85w.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/260748/original/file-20190225-26177-o5p85w.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=363&fit=crop&dpr=1 600w, https://images.theconversation.com/files/260748/original/file-20190225-26177-o5p85w.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=363&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/260748/original/file-20190225-26177-o5p85w.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=363&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/260748/original/file-20190225-26177-o5p85w.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=457&fit=crop&dpr=1 754w, https://images.theconversation.com/files/260748/original/file-20190225-26177-o5p85w.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=457&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/260748/original/file-20190225-26177-o5p85w.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=457&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">800km wide view of part of Cthulhu Regio, extracted from the most detailed colour map of Pluto.</span>
<span class="attribution"><span class="source">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</span></span>
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<p>Interpretation of the most heavily cratered terrains led the researchers to rule out that the small craters have been erased by geological resurfacing such as cryovolcanic activity (eruptions of icy fluids) during the past four billion years. This reinforces the conclusion that smaller craters were never made in the expected numbers, so there must be a mysterious corresponding deficit of Kuiper belt objects less than about 1-2km in size.</p>
<h2>Blorping and flomping</h2>
<p>When the researchers, led by Kelsi Singer of the Southwest Research Institute (Boulder, Colorado), wrote their paper no one had yet seen a small Kuiper Belt object in detail. However, New Horizons recently flew past a 30km long object known as 2014 MU₆₉ <a href="https://gizmodo.com/new-horizons-scientists-double-down-on-ultima-thule-nic-1831439791">(more controversially nicknamed “Ultima Thule”</a>) on January 1, and has now transmitted probably the best images we are going to get. </p>
<p>Sometimes described as “snowman-shaped”, it is a two-lobed “contact binary”, almost certainly formed by a merger of two round objects that happened so slowly and gently that neither component was deformed in the process. But what happened before that? If you look at the larger of the two lobes, in particular, you can make out what looks like traces of component parts that merged vigorously enough to squish together into an approximate sphere, but with insufficient violence to smash each other apart.</p>
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<a href="https://images.theconversation.com/files/260705/original/file-20190225-26181-17snz3t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/260705/original/file-20190225-26181-17snz3t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/260705/original/file-20190225-26181-17snz3t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=490&fit=crop&dpr=1 600w, https://images.theconversation.com/files/260705/original/file-20190225-26181-17snz3t.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=490&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/260705/original/file-20190225-26181-17snz3t.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=490&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/260705/original/file-20190225-26181-17snz3t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=616&fit=crop&dpr=1 754w, https://images.theconversation.com/files/260705/original/file-20190225-26181-17snz3t.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=616&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/260705/original/file-20190225-26181-17snz3t.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=616&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Kuiper belt object 2014 MU₆₉. The two-lobed object is about 30km from end to end.</span>
<span class="attribution"><span class="source">NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute, National Optical Astronomy Observatory</span></span>
</figcaption>
</figure>
<p>These ideas have inspired <a href="https://www.hou.usra.edu/meetings/lpsc2019/pdf/1248.pdf">quirky new terms</a>. “Blorping” refers to the collisional merging of material to assemble each of the lobes, and “flomping” describes the coming together when two lobes meet without causing any deformation. More importantly, this could offer an insight into the processes that robbed the Kuiper belt of the smaller objects that would otherwise have impacted to make small craters on Pluto and Charon.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/260770/original/file-20190225-26174-1qfnbjm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/260770/original/file-20190225-26174-1qfnbjm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=549&fit=crop&dpr=1 600w, https://images.theconversation.com/files/260770/original/file-20190225-26174-1qfnbjm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=549&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/260770/original/file-20190225-26174-1qfnbjm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=549&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/260770/original/file-20190225-26174-1qfnbjm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=690&fit=crop&dpr=1 754w, https://images.theconversation.com/files/260770/original/file-20190225-26174-1qfnbjm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=690&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/260770/original/file-20190225-26174-1qfnbjm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=690&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Detail of Charon’s Vulcan Planitia, where small craters are deficient in numbers.</span>
<span class="attribution"><a class="source" href="http://pluto.jhuapl.edu/Galleries/Featured-Images/image.php?page=12&gallery_id=2&image_id=324">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</a></span>
</figcaption>
</figure>
<p>The relative lack of small Kuiper Belt objects may be because, instead of breaking each other apart in collisions, they tended to merge by blorping – eventually growing into objects like 2014 MU₆₉. If this is correct, then when we try to count them, we see a record of growth rather than collisional fragmentation. </p>
<p>Orbital speeds are slower the further you get from the sun, so we would expect collisions to be less violent in the Kuiper Belt than in the inner solar system. But even so, a “blorp” event to fuse two lumps together rather than break them apart probably requires the ices that make up the bulk of their substance to be a lot less brittle and more squishy than we might have expected. That is crucial information, as these lumps are made from the raw material that the solar system formed from, shedding important light on its evolution. </p>
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Read more:
<a href="https://theconversation.com/images-of-plutos-moon-charon-show-huge-fractures-and-hints-of-icy-lava-flows-48533">Images of Pluto's moon Charon show huge fractures and hints of icy 'lava flows'</a>
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Read more:
<a href="https://theconversation.com/stunning-crystal-clear-images-of-pluto-but-what-do-they-mean-47517">Stunning, crystal-clear images of Pluto – but what do they mean?</a>
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Read more:
<a href="https://theconversation.com/nasa-mission-brings-pluto-into-sharp-focus-but-its-still-not-a-planet-40495">NASA mission brings Pluto into sharp focus – but it's still not a planet</a>
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<img src="https://counter.theconversation.com/content/112436/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Rothery is co-leader of the European Space Agency's Mercury Surface and Composition Working Group, and a Co-Investigator on MIXS (Mercury Imaging X-ray Spectrometer) that is now on its way to Mercury on board the European Space Agency's Mercury orbiter BepiColombo. He has received funding from the UK Space Agency and the Science & Technology Facilities Council for work related to Mercury BepiColombo, and is currently funded by the European Commission under its Horizon 2020 programme for work on planetary geological mapping (776276 Planmap). He is author of Planet Mercury - from Pale Pink Dot to Dynamic World (Springer, 2015), Moons: A Very Short Introduction (Oxford University Press, 2015) and Planets: A Very Short Introduction (Oxford University Press, 2010). He is Educator on the Open University's free learning Badged Open Course (BOC) on Moons and its equivalent FutureLearn Moons MOOC, and chair of the Open University's level 2 course on Planetary Science and the Search for Life.</span></em></p>There’s a mysterious lack of small bodies beyond Neptune, but a ‘snowman-shaped’ object may help explain why.David Rothery, Professor of Planetary Geosciences, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1043252018-10-18T19:12:05Z2018-10-18T19:12:05ZA Goblin could guide us to a mystery planet thought to exist in the Solar system<figure><img src="https://images.theconversation.com/files/239258/original/file-20181004-52681-hqw9wt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Pluto's ghoulish cousin, 2015 TG387, lurks in the distant reaches of our own Solar System.</span> <span class="attribution"><a class="source" href="https://carnegiescience.edu/news/new-extremely-distant-solar-system-object-found-during-hunt-planet-x">Illustration by Roberto Molar Candanosa and Scott Sheppard, courtesy of Carnegie Institution for Science.</a></span></figcaption></figure><p>Out in the depths of the Solar system, astronomers recently discovered a small, icy object, <a href="https://carnegiescience.edu/news/new-extremely-distant-solar-system-object-found-during-hunt-planet-x">named 2015 TG387</a>. </p>
<p><a href="https://www.minorplanetcenter.net/mpec/K18/K18T05.html">First observed in October 2015</a>, it has been nicknamed “The Goblin” by its discoverers. It is currently almost 12 billion kilometres from the Sun – about 80 times the distance between Earth and the Sun (or 80au, where 1au is the distance from Earth to the Sun).</p>
<p>“The Goblin” is thought to be about 300km in diameter, and moves on a highly elongated orbit, far beyond the realm of the eight planets (Mercury to Neptune). It is so distant that it takes more than 34,000 years to orbit the Sun. </p>
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Read more:
<a href="https://theconversation.com/discovered-a-huge-liquid-water-lake-beneath-the-southern-pole-of-mars-100523">Discovered: a huge liquid water lake beneath the southern pole of Mars</a>
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<p>What’s most exciting about the new discovery, though, is that it might hold the key to helping astronomers discover an unseen planet that some believe lurks in the Solar system’s icy depths.</p>
<h2>The history of Planet X</h2>
<p>In 1984, <a href="http://www.pnas.org/content/pnas/81/3/801.full.pdf">research suggested</a> that, over the past 250 million years, mass extinctions had happened on Earth every 26 million years. But what could cause such periodic extinctions?</p>
<p>A hypothetical red dwarf star, known as Nemesis, <a href="https://www.nature.com/articles/308715a0">was proposed to orbit the Sun</a> at a great distance. Every 26 million years, as it passed through its closest approach to the Sun, the star would scatter a deluge of comets towards the inner Solar system. The result? One or more of those <a href="https://www.space.com/31001-earth-mass-extinctions-comet-strikes.html">comets would collide with Earth</a>, triggering a mass extinction. </p>
<p>But over the years the evidence waned, and no companion star was found. Nemesis passed into history.</p>
<p>At the turn of the millennium, a new “Planet X” was proposed - <a href="https://www.space.com/9612-giant-stealth-planet-explain-rain-comets-solar-system-edge.html">nicknamed Tyche</a>. Where Nemesis was the bringer of death, Tyche’s influence was more subtle, resulting in a slight increase in the number of inbound comets from certain regions of the sky, and explaining observed peculiarities in the distribution of those comets. </p>
<p>Once again, observations soon <a href="https://arxiv.org/abs/astro-ph/0205150">weakened the case for Tyche</a>. The final nail came with NASA’s Wide-field Infrared Survey Explorer (<a href="https://www.nasa.gov/mission_pages/WISE/main/index.html">WISE</a>), which looked at the entire sky at infrared wavelengths. If Tyche existed, <a href="http://iopscience.iop.org/article/10.1088/0004-637X/781/1/4/meta">WISE would almost certainly have found it</a>. </p>
<p>Still, the idea of an unseen planet beyond Neptune’s orbit rears its head every few years. In 2008 <a href="https://45170ad8-a-62cb3a1a-s-sites.googlegroups.com/site/patryksofialykawka/database/Patryk-PAPER_Planetoid.pdf">an unseen, distant Earth-mass planet</a> was proposed to explain the distribution of small, icy bodies beyond Neptune.</p>
<p>Other researchers pointed out that <a href="https://arxiv.org/abs/1109.2949">planet-mass objects could have formed</a> along with the Solar system’s outer planets, before being scattered outward, but never ejected.</p>
<p>All this brings us to Planet X’s <a href="https://arxiv.org/abs/1601.05438">latest incarnation</a> - known as “<a href="http://www.caltech.edu/news/caltech-researchers-find-evidence-real-ninth-planet-49523">Planet Nine</a>”.</p>
<h2>Planet X reborn: Planet Nine</h2>
<p>The story of Planet Nine begins with Sedna, a <a href="http://web.gps.caltech.edu/%7Embrown/sedna/">dwarf planet discovered beyond Pluto in November 2003</a>. Sedna has a highly unusual orbit, with its orbital distance varying between 76au and 936au from the Sun. This poses an obvious question: how did Sedna get captured in that orbit?</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/239633/original/file-20181008-72121-hiys1v.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/239633/original/file-20181008-72121-hiys1v.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/239633/original/file-20181008-72121-hiys1v.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=449&fit=crop&dpr=1 600w, https://images.theconversation.com/files/239633/original/file-20181008-72121-hiys1v.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=449&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/239633/original/file-20181008-72121-hiys1v.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=449&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/239633/original/file-20181008-72121-hiys1v.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=564&fit=crop&dpr=1 754w, https://images.theconversation.com/files/239633/original/file-20181008-72121-hiys1v.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=564&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/239633/original/file-20181008-72121-hiys1v.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=564&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Artist’s impression of the view from Sedna, looking back towards the Sun.</span>
<span class="attribution"><span class="source">NASA, ESA and Adolf Schaller</span></span>
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</figure>
<p>At its closest, Sedna is too distant for the planets to perturb it - they cannot be responsible. At its farthest, Sedna is still just one 1/400th of the distance to the nearest star – so close to the Sun that it is very unlikely a passing star could do the deed.</p>
<p>To explain the odd orbit, several theorists suggested that Sedna could have been placed on its orbit <a href="https://arxiv.org/abs/astro-ph/0404456">when the Solar system was young</a>. At that time, the Sun would have been embedded within a stellar cluster, and close encounters with other stars would have been more frequent. </p>
<p>Now, if Sedna’s orbit <em>was</em> the result of capture during the Solar system’s youth, one might expect other objects to have shared the same fate. The theory therefore predicted that Sedna could be one of a population of Sednoids, all moving on similar orbits.</p>
<p>In the past decade, <a href="https://home.dtm.ciw.edu/users/sheppard/inner_oort_cloud/sednoids.html">several more Sednoids have been found</a>. Peculiarly, all of their orbits seem to align, roughly, in space. In other words, the long axes of their orbits all point in roughly the same direction. This is not what you would expect of a population born of the Sun’s birth cluster. So what could be the cause?</p>
<p>To explain Sedna’s unusual orbit, several researchers invoked a variety of Planets X. As new objects were discovered, these theories were continually revisited, until US researchers Konstantin Batygin and Mike Brown came up with <a href="https://arxiv.org/abs/1601.05438">the current version</a>, which they nicknamed “<a href="http://www.caltech.edu/news/caltech-researchers-find-evidence-real-ninth-planet-49523">Planet Nine</a>”.</p>
<p>Batygin and Brown propose that Planet Nine is comparable in mass to Neptune, moving on a highly eccentric orbit, with a period of around 15,000 years.</p>
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<a href="https://images.theconversation.com/files/239129/original/file-20181003-52688-mby0m1.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/239129/original/file-20181003-52688-mby0m1.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/239129/original/file-20181003-52688-mby0m1.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/239129/original/file-20181003-52688-mby0m1.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/239129/original/file-20181003-52688-mby0m1.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/239129/original/file-20181003-52688-mby0m1.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/239129/original/file-20181003-52688-mby0m1.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/239129/original/file-20181003-52688-mby0m1.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The orbits of six ‘detached’ trans-Neptunian objects roughly align in space. Perhaps their similar orbits are the result of Planet Nine’s influence?</span>
<span class="attribution"><span class="source">nagualdesign/Wikimedia</span></span>
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<p>With this theory comes a prediction: as more Sednoids are found, the evidence for the planet will grow, as those objects will also have been sculpted to their current orbits by the hidden planet’s influence.</p>
<p>But astronomers are not universally convinced that an unseen planet is to blame for the alignment of the unusual objects. Extraordinary claims require extraordinary evidence, and there has been much debate over whether the theory stands up to scrutiny. </p>
<p><a href="https://arxiv.org/abs/astro-ph/0205150">As with Tyche</a>, proposed at the turn of the millennium, the apparent clustering of these distant objects could instead be the <a href="https://arxiv.org/abs/1706.05348">result of observational biases</a>. </p>
<p>Simply put, we are more likely to find such faint objects in some parts of the night sky than in others. Since we find those objects at or near <a href="https://astronomy.swin.edu.au/cms/astro/cosmos/P/Perihelion">perihelion</a> – when they are nearest to the Sun, so at their brightest – that would naturally create just such a cluster of orbits from the first discoveries.</p>
<p>How to break the impasse? More discoveries are needed. Which brings us to our newly discovered friend, “The Goblin”.</p>
<h2>Discovering our ghoulish friend</h2>
<p>2015 TG387 was first observed on October 13, 2015, with follow-up observations carried out over the past three years. The extended observations show TG387’s orbit is even more extreme than Sedna’s.</p>
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<figcaption>
<span class="caption">The orbit of 2015 TG387 is even more extreme than Sedna’s - but is oriented in roughly the same direction, adding weight to the Planet Nine hypothesis.</span>
<span class="attribution"><span class="source">Robert Molar Candanosa & Scott Sheppard, Carnegie Institution for Science</span></span>
</figcaption>
</figure>
<p>TG387’s orbit brings it in to around 65au, but stretches all the way out to 2,027au, and one lap takes more than 34,000 years to complete. </p>
<p>Like the other objects that hint at the presence of Planet Nine, TG387’s orbit is oriented just right to add weight to that hypothesis. </p>
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Read more:
<a href="https://theconversation.com/planet-or-dwarf-planet-all-worlds-are-worth-investigating-74682">Planet or dwarf planet: all worlds are worth investigating</a>
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<p>So does this mean that Planet Nine is real? </p>
<p>While this could be an extra piece of evidence for the planet’s existence, it is far from definitive. It could equally be the case that TG387 is further evidence that observational biases drive the clustering. </p>
<p>We need to find more objects before we can be sure, either way. Fortunately, there are likely to be millions of similar objects like TG387 – indeed, its discoverers predict that it is just one of several million bodies, all moving on similar orbits in the Solar system’s icy depths.</p><img src="https://counter.theconversation.com/content/104325/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jake Clark is supported by an Australian Government Research Training Program (RTP) Scholarship.</span></em></p><p class="fine-print"><em><span>Jonti Horner does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Whether you call it Planet X or Planet Nine, talk of another planet lurking in our Solar system won’t go away. So what does the discovery of a new object – nicknamed “The Goblin” – add to the debate?Jonti Horner, Professor (Astrophysics), University of Southern QueenslandJake Clark, PhD Candidate, University of Southern QueenslandLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1013272018-09-10T20:12:58Z2018-09-10T20:12:58ZI’ve Always Wondered: How do we know what lies at the heart of Pluto?<figure><img src="https://images.theconversation.com/files/233760/original/file-20180828-75987-xj2pza.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Pluto in enhanced color, to illustrate differences in the composition and texture of its surface.</span> <span class="attribution"><a class="source" href="https://www.nasa.gov/image-feature/the-rich-color-variations-of-pluto">NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute</a></span></figcaption></figure><p><em>This is an article from <a href="https://theconversation.com/au/topics/ive-always-wondered-43449">I’ve Always Wondered</a>, a series where readers send in questions they’d like an expert to answer. Send your question to alwayswondered@theconversation.edu.au</em></p>
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<p><strong>I’ve always wondered: how do astronomers determine what comprises the core and layers of distant planetary bodies like Pluto when we’ve never been there? – Brian, Darwin</strong></p>
<p>Its not just astronomers that get to answer this question, though they do play a key role. Like many issues in planetary science, it takes a village of different specialists to solve these planet-sized problems.</p>
<p>To build up a picture of each planet’s interior has required the merging of keenly observed astronomy, complex theoretical calculations, and the most elegant of experiments. And it is very much ongoing work; only this year our idea of what’s inside Jupiter <a href="https://www.space.com/37005-jupiter-fuzzy-core-nasa-juno.html">changed completely</a>.</p>
<h2>Let’s start with Earth</h2>
<p>The deepest hole that’s been dug (well, drilled) into Earth is the <a href="https://www.atlasobscura.com/places/kola-superdeep-borehole">Kola super deep borehole</a>. Cutting through the Siberian peninsula it is 12.6 km deep,only a fraction of the 6,400km to the centre of Earth. Despite this we do know quite a bit about the interior of our own planet.</p>
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Read more:
<a href="https://theconversation.com/ive-always-wondered-why-are-the-volcanoes-on-earth-active-but-the-ones-on-mars-are-not-99831">I've Always Wondered: Why are the volcanoes on Earth active, but the ones on Mars are not?</a>
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<p>We know Earth has layers of minerals that increase in density as you delve deeper and the pressure increases, until we reach the core. We also know that the very centre of Earth, its core, is made of two components: a surprising liquid outer part, and a solid inner. Both parts of the core are made of super-dense iron and nickel mixture, with some other mystery element in the mix. </p>
<p>Our knowledge of Earth’s interior has come from listening to earthquakes that send sound waves right through our planet. These sound waves are affected by the density changes, and this can be unwrapped by having a network of siesmometers that can pick up signals from each quake.</p>
<p>The density changes have been followed by <a href="https://www.nature.com/news/earth-science-crystallography-s-journey-to-the-deep-earth-1.14755">extensive laboratory studies</a> that have recreated the conditions and come up with a great picture of the mineral changes as you delve towards Earth’s core.</p>
<p>Sadly, however, there is no other planet with a seismometer on it. There will be soon though, as <a href="https://mars.nasa.gov/insight/">NASA’s Insight mission</a> is on its way to plant one on Mars. Yet, like Earth, we do have some good theories about the centre of Mars, Pluto and indeed all of the planetary bodies in our solar system.</p>
<h2>How dense is your planet?</h2>
<p>A big clue to a planet’s interior is its average density. This can be calculated from its mass (which you can measure as soon as you have anything orbiting it) and its radius (which can be found from telescope observations). Once you have that, you can relate this average density to that of a similar material.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/233771/original/file-20180828-75978-1l7yu0i.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/233771/original/file-20180828-75978-1l7yu0i.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/233771/original/file-20180828-75978-1l7yu0i.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=453&fit=crop&dpr=1 600w, https://images.theconversation.com/files/233771/original/file-20180828-75978-1l7yu0i.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=453&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/233771/original/file-20180828-75978-1l7yu0i.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=453&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/233771/original/file-20180828-75978-1l7yu0i.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=569&fit=crop&dpr=1 754w, https://images.theconversation.com/files/233771/original/file-20180828-75978-1l7yu0i.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=569&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/233771/original/file-20180828-75978-1l7yu0i.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=569&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Average density of a few planetary bodies.</span>
<span class="attribution"><span class="source">Helen Maynard-Casely</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>I’ve plotted a few of them (above) and you can see that rocky planets such as <a href="https://solarsystem.nasa.gov/planets/earth/overview/">Earth</a> have an average density close to that of rock (about 5,000 kg/m<sup>3</sup>), whereas gas giants have a much lower density.</p>
<p>Even the difference between two gas giants can be quite big. The change between <a href="https://solarsystem.nasa.gov/planets/saturn/overview/">Saturn</a> and <a href="https://solarsystem.nasa.gov/planets/uranus/overview/">Uranus</a> tells us that Saturn is mainly made of the light gases hydrogen and helium, whereas Uranus is made of heavier molecules such as water.</p>
<p><a href="https://solarsystem.nasa.gov/planets/dwarf-planets/pluto/overview/">Pluto</a>, like many icy worlds, has a density between that of rock and ice – but closer to ice. So that immediately suggests it is a mixture of both.</p>
<p>As a planet evolves, heavier materials sink towards its centre. So it is safe to assume that, in Pluto’s case, the rock will sit at its core and the ice and lighter materials will make up its surface and subsurface.</p>
<p>But can we tell any more than that? We can, by examining the detail of a planet’s gravity field. </p>
<h2>Looking for wobbles</h2>
<p>Slight wobbles in how spacecraft orbit planets can tell us how density is distributed beneath the surface. For gas giants such as Jupiter, this can extend right through the planet.</p>
<p>The <a href="https://www.nasa.gov/mission_pages/juno/main/index.html">Juno spacecraft</a> is currently measuring Jupiter’s gravity field in more detail than ever before – and has already revolutionised what we know of the gas giant’s interior. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/233773/original/file-20180828-75999-1nwkgot.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/233773/original/file-20180828-75999-1nwkgot.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/233773/original/file-20180828-75999-1nwkgot.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=532&fit=crop&dpr=1 600w, https://images.theconversation.com/files/233773/original/file-20180828-75999-1nwkgot.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=532&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/233773/original/file-20180828-75999-1nwkgot.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=532&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/233773/original/file-20180828-75999-1nwkgot.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=668&fit=crop&dpr=1 754w, https://images.theconversation.com/files/233773/original/file-20180828-75999-1nwkgot.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=668&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/233773/original/file-20180828-75999-1nwkgot.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=668&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The inside story of Pluto and its largest moon Charon.</span>
<span class="attribution"><a class="source" href="http://pluto.jhuapl.edu/Participate/learn/What-We-Know.php?link=The-Inside-Story">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</a>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>This does work for the smaller rocky planetary bodies – but gives us a less complete picture. For instance, small wobbles in Cassini’s orbit (only milimetres) around Saturn that were observed all the way back on Earth gave us evidence that there is a ocean under the south pole of <a href="https://theconversation.com/waterworld-cassini-spots-the-motion-of-enceladuss-ocean-25069">Saturn’s tiny moon Enceladus</a>.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/planet-or-dwarf-planet-all-worlds-are-worth-investigating-74682">Planet or dwarf planet: all worlds are worth investigating</a>
</strong>
</em>
</p>
<hr>
<p>With Pluto, evidence from the flyby suggests it also has a <a href="https://theconversation.com/why-pluto-may-have-a-large-ocean-beneath-its-icy-surface-68878">liquid ocean under its icy surface</a>. But gravity field data from a flyby, like that of NASA’s <a href="https://www.nasa.gov/mission_pages/newhorizons/main/index.html">New Horizons</a>, is never as good as having a spacecraft in orbit - so we’ll have to wait until we return to Pluto to know more. </p>
<p>You can watch me here explaining in a bit more detail how we’ve followed these observations with lab work to discover yet more about the insides of our planetary neighbours. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/HdSp5nvny-g?wmode=transparent&start=430" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Helen takes us on a journey to get to know the planets of our solar system, filmed at Science Academy.</span></figcaption>
</figure><img src="https://counter.theconversation.com/content/101327/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Helen Maynard-Casely does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Pluto has a density between that of rock and ice – so that immediately suggests the dwarf planet is made of a mix of both. But how do we know?Helen Maynard-Casely, Instrument Scientist, Australian Nuclear Science and Technology OrganisationLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/964312018-05-25T09:21:55Z2018-05-25T09:21:55ZHow we discovered 840 minor planets beyond Neptune – and what they can tell us<figure><img src="https://images.theconversation.com/files/218467/original/file-20180510-5968-7lw5y4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The Canada-France-Hawaii Telescope (CFHT) at sunset, which observed the OSSOS survey.</span> <span class="attribution"><span class="source">wikipedia</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Our solar system is a tiny but wonderfully familiar corner of the vast, dark universe – we have even been able to land spacecraft on our celestial neighbours. Yet its outer reaches are still remarkably unmapped. Now we <a href="https://ui.adsabs.harvard.edu/#abs/2018ApJS..236...18B/abstract">have discovered</a> 840 small worlds in the distant and hard-to-explore region beyond Neptune. This is the largest set of discoveries ever made, increasing the number of distant objects with well known paths around the sun by 50%.</p>
<p>These little icy worlds are important as they help us tell the solar system’s history. They can also help us <a href="https://theconversation.com/our-discovery-of-a-minor-planet-beyond-neptune-shows-there-might-not-be-a-planet-nine-after-all-75656">test the idea</a> that there’s a yet unseen planet lurking in the outer solar system. </p>
<p>Our planetary system as we see it today is not as it formed. When the sun was newborn, it was surrounded by a massive disk of material. Encounters with tiny, growing planets – including some of the worlds we’ve just discovered – moved the giant planets outward from the sun until they settled into their present locations. The growing planets, on the other hand, went everywhere, scattering both inward and outward. </p>
<p>Planetary migration also happened in far away systems around many other stars. Fortunately, the celestial bodies in our own planetary system are comparatively close by, making it the only place where we can see the intricate details of how migration happened. Mapping the minor planet populations that are left over from the disk lets us reconstruct the history of how the big planets were pushed into place.</p>
<h2>Mapping the sky</h2>
<p>The new discoveries were made as part of a five year project called the <a href="http://www.ossos-survey.org/">Outer Solar System Origins Survey</a> (OSSOS). The observations, conducted in 2013-2017, used the imaging camera of one of the world’s major telescopes – the <a href="http://www.cfht.hawaii.edu/">Canada-France-Hawaii Telescope</a> on Maunakea in Hawaii. The survey looked for faint, slow-moving points of light within eight big patches of sky near the plane of the planets and away from the dense star fields of the Milky Way. </p>
<p>With 840 discoveries made at distances between six and 83 astronomical units (au) – one such unit is the distance between the sun and the Earth – the survey gives us a very good overview of the many sorts of orbits these “trans-Neptunian objects” have.</p>
<p>Earlier surveys have suffered from losing some of their distant discoveries – when too few observations occur, the predicted path of a minor planet in the sky will be so uncertain that a telescope can’t spot it again, and it is considered “lost”. This happens more to objects with highly tilted and elongated orbits, producing a bias in what’s currently known about these populations.</p>
<p>Our new survey successfully tracked all its distant discoveries. The frequent snapshots we made of the 840 objects over several years meant that each little world’s orbit could be determined very precisely. In total, more than 37,000 hand-checked measurements of the hundreds of discoveries precisely pinned down their arcs across the sky.</p>
<p>We also created an accompanying software “simulator” (a computer model), which provides a powerful tool for testing the inventory and history of our solar system. This lets theorists <a href="https://arxiv.org/abs/1802.00460">test out their models</a> of how the solar system came to be in the shape we see it today, comparing them with our real discoveries.</p>
<h2>Strange new worlds</h2>
<p>The new icy and rocky objects fall into two main groups. One includes those that reside on roundish orbits in the Kuiper belt, which extends from 37au to approximately 50au from the sun. The other consists of worlds that orbit in a careful dance of avoidance with Neptune as it travels around the sun. These “resonant” trans-Neptunian objects, which include Pluto, were pushed into their current elongated orbits during Neptune’s migration outwards. </p>
<p>In the Kuiper belt, we found 436 small worlds. Their orbits confirm that a concentrated “kernel” of the population nestles on almost perfectly round, flat orbits at 43 to 45au. These quiet orbits may have been undisturbed since the dawn of the solar system, a leftover fraction of the original disk. Soon, we will see a member of this group up close: the <a href="https://theconversation.com/new-horizons-is-an-old-spacecraft-but-it-will-transform-our-knowledge-of-pluto-44524">New Horizons spacecraft</a>, which <a href="https://theconversation.com/new-horizons-finally-gets-up-close-with-pluto-for-15-minutes-44603">visited Pluto in 2015</a>, will be flying by a world that’s about the size of London on New Year’s Day 2019.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/218465/original/file-20180510-185500-4w73js.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/218465/original/file-20180510-185500-4w73js.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=379&fit=crop&dpr=1 600w, https://images.theconversation.com/files/218465/original/file-20180510-185500-4w73js.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=379&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/218465/original/file-20180510-185500-4w73js.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=379&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/218465/original/file-20180510-185500-4w73js.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=477&fit=crop&dpr=1 754w, https://images.theconversation.com/files/218465/original/file-20180510-185500-4w73js.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=477&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/218465/original/file-20180510-185500-4w73js.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=477&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The dwarf planet candidate 2015 RR245 is on an exceptionally distant orbit, but is one of the few dwarf planets that could one day be reached by a spacecraft mission.</span>
<span class="attribution"><span class="source">Alex Parker/OSSOS</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>We found 313 resonant trans-Neptunian objects, with the survey showing that they exist <a href="https://arxiv.org/abs/1802.05805">as far out as an incredible 130au</a> – and are <a href="https://arxiv.org/abs/1604.08177">far more abundant</a> than previously thought. Among these discoveries is the dwarf planet 2015 RR245, which is about half the size of Britain. It may have hopped onto its current orbit at 82au <a href="https://arxiv.org/abs/1607.06970">after an encounter with Neptune</a> hundreds of millions of years ago. It was once among the <a href="https://arxiv.org/abs/1803.07521">90,000 scattered objects</a> of smaller size that we estimate currently exist. </p>
<h2>Are there more planets?</h2>
<p>Among the most unusual of the discoveries are nine little worlds on incredibly distant orbits, never coming closer to the sun than Neptune’s orbit, and taking as long as 20,000 years to travel around our star. Their existence implies an unseen population of hundreds of thousands of trans-Neptunian objects on similar orbits.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/220427/original/file-20180525-51141-1jeqew.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/220427/original/file-20180525-51141-1jeqew.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=357&fit=crop&dpr=1 600w, https://images.theconversation.com/files/220427/original/file-20180525-51141-1jeqew.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=357&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/220427/original/file-20180525-51141-1jeqew.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=357&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/220427/original/file-20180525-51141-1jeqew.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=449&fit=crop&dpr=1 754w, https://images.theconversation.com/files/220427/original/file-20180525-51141-1jeqew.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=449&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/220427/original/file-20180525-51141-1jeqew.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=449&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Artist’s concept of Planet Nine.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech/Robert Hurt</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>How these objects got on their present paths is unclear — some orbit so far out that, even at their closest approach, they are barely tugged by Neptune’s gravity. One explanation that has been put forward is that a yet unseen large planet, sometimes called “Planet Nine”, could be causing them to cluster in space. However, our nine minor planets all seem to be <a href="https://arxiv.org/abs/1706.05348">spread out smoothly</a>, rather than clustering. Perhaps the shepherding of such a large planet is more subtle – or these orbits instead <a href="https://theconversation.com/our-discovery-of-a-minor-planet-beyond-neptune-shows-there-might-not-be-a-planet-nine-after-all-75656">formed in a different way</a>.</p>
<p>The history of our solar system is just beginning to be told. We hope this new set of discoveries will help piece together the story.</p><img src="https://counter.theconversation.com/content/96431/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Michele Bannister receives funding from the STFC, and has previously been funded by Canada's NRC and NSERC. She serves on the committee of the AAS's Division of Planetary Sciences.</span></em></p>Discovery of many icy worlds helps unravel the solar system’s history.Michele Bannister, Research Fellow, planetary astronomy, Queen's University BelfastLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/756562017-04-18T12:37:32Z2017-04-18T12:37:32ZOur discovery of a minor planet beyond Neptune shows there might not be a ‘Planet Nine’ after all<figure><img src="https://images.theconversation.com/files/165592/original/image-20170418-32700-1fxdsgo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The Canada-France-Hawaii Telescope has spotted a new dwarf planet.</span> <span class="attribution"><span class="source">Michele Banister</span>, <span class="license">Author provided</span></span></figcaption></figure><p>Ever since <a href="https://theconversation.com/a-stolen-exoplanet-that-will-kill-us-all-heres-what-we-do-know-about-planet-nine-60407">enthusiasm started growing</a> over the possibility that there could be a ninth major planet orbiting the sun beyond Neptune, astronomers have been busy hunting it. One group is investigating <a href="http://www.independent.co.uk/news/science/planet-nine-9-found-new-astronomers-four-unknown-objects-citizen-neptune-pluto-a7668411.html">four new moving objects</a> found by members of the public to see if they are potential new solar system discoveries. As exciting as this is, researchers are also making discoveries that question the entire prospect of a ninth planet.</p>
<p>One such finding is <a href="https://arxiv.org/abs/1704.01952">our discovery of a minor planet</a> in the outer solar system: <a href="http://www.minorplanetcenter.net/mpec/K17/K17G55.html">2013 SY99</a>. This small, icy world has an orbit so distant that it takes 20,000 years for one long, looping passage. We found SY99 with the <a href="http://www.cfht.hawaii.edu/">Canada-France-Hawaii Telescope</a> as part of the <a href="http://www.ossos-survey.org/">Outer Solar System Origins Survey</a>. SY99’s great distance means it travels very slowly across the sky. Our measurements of its motion show that its orbit is a very stretched ellipse, with the closest approach to the sun at 50 times that between the Earth and the sun (a distance of 50 “astronomical units”). </p>
<p>The new minor planet loops even further out than previously discovered dwarf planets such as <a href="https://www.universetoday.com/37479/dwarf-planet-sedna/">Sedna</a> and 2013 VP113. The long axis of its orbital ellipse is 730 astronomical units. Our observations with other telescopes show that SY99 is a small, reddish world, some 250 kilometres in diameter, or about the size of Wales in the UK.</p>
<p>SY99 is one of only seven known small icy worlds that orbit beyond Neptune at remarkable distances. How these “extreme trans-Neptunian objects” were placed on their orbits is uncertain: their distant paths are isolated in space. Their closest approach to the sun is so far beyond Neptune that they are thought to be <a href="https://ui.adsabs.harvard.edu/#abs/2007Icar..189..213L/abstract">“detached”</a> from the strong gravitational influence of the giant planets in our solar system. But at their furthest points, they are still too close to be nudged around by the slow tides of the galaxy itself.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/108837/original/image-20160121-9766-aqpwmc.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/108837/original/image-20160121-9766-aqpwmc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/108837/original/image-20160121-9766-aqpwmc.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/108837/original/image-20160121-9766-aqpwmc.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/108837/original/image-20160121-9766-aqpwmc.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/108837/original/image-20160121-9766-aqpwmc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=425&fit=crop&dpr=1 754w, https://images.theconversation.com/files/108837/original/image-20160121-9766-aqpwmc.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=425&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/108837/original/image-20160121-9766-aqpwmc.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=425&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Planet Nine could explain why the few known extreme trans-Neptunian objects seem to be clustered together in space. The diagram was created using WorldWide Telescope.</span>
<span class="attribution"><span class="source">Caltech/R. Hurt (IPAC)</span></span>
</figcaption>
</figure>
<p>It’s been suggested that the extreme trans-Neptunian objects could be clustered in space by the gravitational influence of a “Planet Nine” that orbits much further out than Neptune. This planet’s gravity could lift out and detach their orbits – constantly changing their tilt. But this planet is far from proven. </p>
<p>In fact, its existence is based on the orbits of only six objects, which are very faint and hard to discover even with large telescopes. They are therefore prone to odd biases. It’s a bit like looking down into the deep ocean at a school of fish. The fish swimming near the surface are clearly visible. But the ones even only a meter down are fainter and murky, and take quite a lot of peering to be certain. The great bulk of the school, in the depths, is completely invisible. But the fish at the surface and their behaviour betray the existence of a whole school.</p>
<p>The biases mean SY99’s discovery can’t prove or disprove the existence of a Planet Nine. However, <a href="https://ui.adsabs.harvard.edu/?#abs/2017AJ....153...63S">computer models</a> do show that a Planet Nine would be an unfriendly neighbour to tiny worlds like SY99: its gravitational influence would starkly change its orbit – throwing it from the solar system entirely, or poking it into an orbit so highly inclined and distant that we wouldn’t be able to see it. SY99 would have to be one of an utterly vast throng of small worlds, continuously being sucked in and cast out by the planet. </p>
<h2>The alternative explanation</h2>
<p>But it turns out that there are other explanations. Our study based on <a href="https://arxiv.org/abs/1704.01952">computer modelling</a>, accepted for publication in the Astronomical Journal, hint at the influence of an idea from everyday physics called <a href="http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011/cells/cells3.shtml">diffusion</a>. This is a very common type of behaviour in the natural world. Diffusion typically explains the random movement of a substance from a region of higher concentration to one of lower concentration – such as the way perfume drifts across a room.</p>
<p>We showed that a related form of diffusion can cause the orbits of minor planets to change from an ellipse that is initially only 730 astronomical units on its long axis to one that is as big as 2,000 astronomical units or bigger – and change it back again. In this process, the size of each orbit would vary by a random amount.
When SY99 comes to its closest approach every 20,000 years, Neptune will often be in a different part of its orbit on the opposite side of the solar system. But at encounters where both SY99 and Neptune are close, Neptune’s gravity will subtly nudge SY99, minutely changing its velocity. As SY99 travels out away from the sun, the shape of its next orbit will be different. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/164426/original/image-20170407-29365-1illepf.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/164426/original/image-20170407-29365-1illepf.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/164426/original/image-20170407-29365-1illepf.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/164426/original/image-20170407-29365-1illepf.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/164426/original/image-20170407-29365-1illepf.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/164426/original/image-20170407-29365-1illepf.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/164426/original/image-20170407-29365-1illepf.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">New discovery 2013 SY99 (red) and the orbits of the other known trans-Neptunian minor planets with orbits larger than 250 astronomical units (grey). All their orbits are far outside the outermost known planet, Neptune (blue), even at their closest approach to the sun.</span>
<span class="attribution"><span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>The long axis of SY99’s ellipse will alter, becoming either larger or smaller, in what physicists call a “random walk”. The orbit change takes place on truly astronomical time scales. It diffuses over the space of tens of millions of years. The long axis of SY99’s ellipse would change by hundreds of astronomical units over the 4.5 billion-year history of the solar system.</p>
<p>Several other extreme trans-Neptunian objects with smaller orbits also show diffusion, on a smaller scale. Where one goes, more can follow. It’s <a href="https://arxiv.org/abs/1704.01952">entirely plausible</a> that the gradual effects of diffusion act on the tens of millions of tiny worlds orbiting in the near fringe of the <a href="https://solarsystem.nasa.gov/planets/oort">Oort cloud</a> (a shell of icy objects at the edge of the solar system). This gentle influence would slowly lead some of them to randomly shift their orbits closer to us, where we see them as extreme trans-Neptunian objects. </p>
<p>However, diffusion won’t explain the distant orbit of Sedna, which has its closest point too far out from Neptune for it to change its orbit’s shape. Perhaps Sedna gained its orbit from a passing star, aeons ago. But diffusion could certainly be bringing in extreme trans-Neptunian objects from the inner Oort cloud – without the need for a Planet Nine. To find out for sure, we’ll need to make more discoveries in this most distant region using our largest telescopes.</p><img src="https://counter.theconversation.com/content/75656/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Michele Bannister receives funding from the STFC, and has previously been funded by Canada's NRC and NSERC. </span></em></p>It turns out that a common physical process called diffusion can explain the orbits of faraway minor planets – no need for a Planet Nine.Michele Bannister, Research Fellow, planetary astronomy, Queen's University BelfastLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/746822017-03-19T19:27:32Z2017-03-19T19:27:32ZPlanet or dwarf planet: all worlds are worth investigating<figure><img src="https://images.theconversation.com/files/161149/original/image-20170316-10898-18gv4z8.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Pluto is a dwarf planet but that doesn't make it any less worthy of our attention.</span> <span class="attribution"><a class="source" href="https://www.nasa.gov/feature/one-year-later-new-horizons-top-10-discoveries-at-pluto">NASA/JHUAPL/SwRI</a></span></figcaption></figure><p>Pluto’s status as a “dwarf planet” is once again stirring debate. This comes as some planetary scientists are trying to have <a href="http://www.lpi.usra.edu/meetings/lpsc2017/pdf/1448.pdf">Pluto reclassified as a planet</a> – a wish that’s not likely to come true. </p>
<p>Pluto has been known as a <a href="https://www.iau.org/public/themes/pluto/">dwarf planet</a> for more than a decade. Back in August 2006 <a href="http://www.space.com/2791-pluto-demoted-longer-planet-highly-controversial-definition.html">astronomers voted to shake up the Solar System</a>, and the number of planets dropped from nine to eight. Pluto was the one cast aside.</p>
<p>There was some outcry that Pluto had been destroyed in an instant and was no longer important, and the reverberations were most keenly felt across America. </p>
<p>After all, Pluto was “their planet”, discovered in 1930 through the meticulous observations of American astronomer <a href="https://lowell.edu/in-depth/pluto/the-discovery-of-pluto/">Clyde Tombaugh at the Lowell Observatory</a> in Arizona. </p>
<p>At the time of the vote, NASA’s <a href="https://www.nasa.gov/mission_pages/newhorizons/main/index.html">New Horizons spacecraft</a> was only seven months into its nine-year journey to Pluto. There was concern that when it finally arrived, would people even care about a dwarf planet?</p>
<p>For many astronomers, the demotion of Pluto was a defining moment. It wasn’t a gesture of destruction and it wasn’t aimed specifically at Pluto. What it signalled was a major leap forward. </p>
<p>In that moment the world’s astronomers acknowledged significant progress in our understanding of the Solar System, an achievement to be proud of – even if everyone was not entirely happy. </p>
<h2>What’s in a name?</h2>
<p>The first step to understanding a group of objects is to classify them. We group like with like to examine the aligned characteristics or any significant differences between groups. With this insight comes a deeper understanding of how things work, form or evolve. </p>
<p>The planets were originally grouped together because the <a href="https://en.wikipedia.org/wiki/Ancient_Greek_astronomy">ancient Greeks</a> saw them as “the wanderers”, travelling across the sky. Five bright objects – Mercury, Venus, Mars, Jupiter and Saturn – may have looked like stars, but while stars stayed fixed within their constellations, these planets moved independently from them.</p>
<p>The cause of this planetary motion was eventually established by the Polish astronomer <a href="https://www.britannica.com/biography/Nicolaus-Copernicus">Nicolaus Copernicus</a> in the 16th century, bringing with it a new revelation. Planets were more than wanderers, they were objects in orbit about the Sun and with this understanding Earth became a planet too. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/161151/original/image-20170316-10902-fbxyf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/161151/original/image-20170316-10902-fbxyf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/161151/original/image-20170316-10902-fbxyf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/161151/original/image-20170316-10902-fbxyf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/161151/original/image-20170316-10902-fbxyf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/161151/original/image-20170316-10902-fbxyf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/161151/original/image-20170316-10902-fbxyf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/161151/original/image-20170316-10902-fbxyf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Earth became a planet too, once the ‘wanderers’ were understood.</span>
<span class="attribution"><span class="source">NASA/Reid Wiseman</span></span>
</figcaption>
</figure>
<h2>Defining a planet in the 21st century</h2>
<p>More than 400 years and many discoveries later, a new storm began brewing in our understanding of the Solar System. </p>
<p>Since 1992, astronomers had begun to find <a href="http://solarsystem.nasa.gov/galleries/1992-qb1">objects orbiting the Sun</a> out in the realm of Pluto. Were they planets too? </p>
<p>Conversely, <a href="http://solarsystem.nasa.gov/planets/pluto">Pluto</a> was a bit of an oddball. It was smaller than several moons of other planets, and it had a highly inclined orbit that made it stand out from the others. Was it truly a planet or was it part of a much larger family of objects?</p>
<p>With the discovery of <a href="http://solarsystem.nasa.gov/planets/eris">Eris</a> (originally known by its designation 2003 UB313) in 2003, a decision could no longer be avoided. Eris was about the size of Pluto and certainly more massive. Was Eris a planet? And if not, where did that leave Pluto?</p>
<p>Astronomers have a forum for such deliberations via the International Astronomical Union (<a href="https://www.iau.org/">IAU</a>). Representing astronomers worldwide, the IAU is the recognised authority responsible for naming and classifying planetary bodies and their satellites.</p>
<p>The IAU formed a <a href="https://www.iau.org/public/images/detail/iau0601g/">Planet Definition Committee</a> to consider the scientific, cultural and historical issues at hand. A draft proposal was put forward, and during the 2006 IAU General Assembly in Prague, with the world’s astronomers gathered together, the Committee’s proposal was vigorously debated. </p>
<p>A revised proposal was presented to the IAU membership on the final day of the General Assembly and <a href="https://www.iau.org/news/pressreleases/detail/iau0603/">was passed with a large majority</a>. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/161158/original/image-20170316-10902-6oi2qj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/161158/original/image-20170316-10902-6oi2qj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/161158/original/image-20170316-10902-6oi2qj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/161158/original/image-20170316-10902-6oi2qj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/161158/original/image-20170316-10902-6oi2qj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/161158/original/image-20170316-10902-6oi2qj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=501&fit=crop&dpr=1 754w, https://images.theconversation.com/files/161158/original/image-20170316-10902-6oi2qj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=501&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/161158/original/image-20170316-10902-6oi2qj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=501&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Astronomers raise their yellow cards and Pluto becomes a dwarf planet.</span>
<span class="attribution"><span class="source">Martin George</span></span>
</figcaption>
</figure>
<p>For the first time, <a href="https://www.iau.org/news/pressreleases/detail/iau0603/">a planet was formally recognised</a> as being “a celestial body that”: </p>
<blockquote>
<p>(a) is in orbit around the Sun</p>
<p>(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape</p>
<p>(c) has cleared the neighbourhood around its orbit.</p>
</blockquote>
<p>Since Pluto had not “cleared the neighbourhood around its orbit”, it was not a planet but would be recognised as a “dwarf planet”. </p>
<p>A colleague of mine, Martin George, director of the <a href="http://www.qvmag.tas.gov.au/qvmag/index.php?c=23">Launceston Planetarium</a>, was there when the vote was taken and captured the excitement and the nuance of the event.</p>
<blockquote>
<p>There was quite a buzz in the room and we knew we were about to make history. Did everyone agree on the exact wording? Perhaps not. However, I think it would have been worse to see media headlines reading ‘Astronomers cannot decide what a planet is’.</p>
</blockquote>
<h2>Size matters and location too</h2>
<p>The distinction of planet and dwarf planet brings a consistency to how objects are named across the universe. On the grand scale, there are galaxies and there are dwarf galaxies. </p>
<p>Within our Milky Way Galaxy, the Sun is a yellow dwarf star that in billions of years will evolve to become a red giant before ending its life as a white dwarf.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/161159/original/image-20170316-10890-fwfouy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/161159/original/image-20170316-10890-fwfouy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/161159/original/image-20170316-10890-fwfouy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/161159/original/image-20170316-10890-fwfouy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/161159/original/image-20170316-10890-fwfouy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/161159/original/image-20170316-10890-fwfouy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/161159/original/image-20170316-10890-fwfouy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/161159/original/image-20170316-10890-fwfouy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Milky Way and its neighbouring dwarf galaxies, the Large and Small Magellanic Clouds seen in the lower left.</span>
<span class="attribution"><span class="source">ESO/C. Malin</span></span>
</figcaption>
</figure>
<p>These distinctions among galaxies and stars helps astronomers interpret and understand them, tracing their evolution. </p>
<p>Planets and dwarf planets are distinct because of their size and their location in the solar system. It provides a way to examine how planets and dwarf planets may have originated and evolved differently.</p>
<h2>Planetary resemblance</h2>
<p>At present, the IAU has officially recognised five dwarf planets. They are <a href="http://solarsystem.nasa.gov/planets/pluto">Pluto</a>, <a href="http://solarsystem.nasa.gov/planets/eris">Eris</a>, <a href="http://solarsystem.nasa.gov/planets/makemake">Makemake</a> and <a href="http://solarsystem.nasa.gov/planets/haumea">Haumea</a>, which orbit the Sun beyond Neptune, and <a href="http://solarsystem.nasa.gov/planets/ceres">Ceres</a>, which is the only object in the asteroid belt massive enough to be spherical. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/161173/original/image-20170316-10929-16bauqe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/161173/original/image-20170316-10929-16bauqe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/161173/original/image-20170316-10929-16bauqe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=388&fit=crop&dpr=1 600w, https://images.theconversation.com/files/161173/original/image-20170316-10929-16bauqe.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=388&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/161173/original/image-20170316-10929-16bauqe.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=388&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/161173/original/image-20170316-10929-16bauqe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=488&fit=crop&dpr=1 754w, https://images.theconversation.com/files/161173/original/image-20170316-10929-16bauqe.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=488&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/161173/original/image-20170316-10929-16bauqe.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=488&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The dwarf planets compared to Earth.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>Detractors and also supporters of the standing planet definition can point to problems with it. For instance, it only applies to objects orbiting the Sun. But what about exoplanets? And what is meant by “cleared its neighbourhood”? If Earth was located farther away from the Sun, would it be able to clear its orbit? </p>
<p>But, as astrophysicist Ethan Seigal explains, <a href="https://www.forbes.com/sites/startswithabang/2017/03/08/the-science-has-spoken-pluto-will-never-be-a-planet-again/#63c5112968d9">minor qualifications to the planet definition</a> can bring it in line with exoplanets and allows the definition to work with renewed clarity.</p>
<p>Whereas the latest proposal to reinstate Pluto, advocates a geophysical definition of planet. Namely, that a planet should be large enough to be round, but not so big that it is a star. This broad definition casts the net wide, and not only Pluto, but also the Moon and more than 100 other Solar System objects would become planets. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/161178/original/image-20170316-10895-18l697i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/161178/original/image-20170316-10895-18l697i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/161178/original/image-20170316-10895-18l697i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=212&fit=crop&dpr=1 600w, https://images.theconversation.com/files/161178/original/image-20170316-10895-18l697i.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=212&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/161178/original/image-20170316-10895-18l697i.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=212&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/161178/original/image-20170316-10895-18l697i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=266&fit=crop&dpr=1 754w, https://images.theconversation.com/files/161178/original/image-20170316-10895-18l697i.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=266&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/161178/original/image-20170316-10895-18l697i.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=266&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">There are many Solar System objects smaller than Earth.</span>
<span class="attribution"><span class="source">primefac</span></span>
</figcaption>
</figure>
<p>Now wouldn’t that be a leap backwards in regards to structuring and understanding our Solar System? How much of it is driven by the notion that nothing but a planet is worth exploration?</p>
<p>There’s a plethora of “<a href="http://www.planetary.org/blogs/emily-lakdawalla/2015/07141338-the-not-planets.html">not-planets</a>” in our Solar System that are worlds worthy of attention. This includes the fiery volcanoes of <a href="https://www.nasa.gov/topics/solarsystem/features/io-volcanoes-displaced.html">Io</a>, the icy geysers of <a href="https://www.jpl.nasa.gov/news/news.php?feature=4759">Enceladus</a>, the reddish surface of <a href="https://www.nasa.gov/feature/goddard/2016/hubble-discovers-moon-orbiting-the-dwarf-planet-makemake">Makemake</a>, the crazy spin of <a href="http://web.gps.caltech.edu/%7Embrown/2003EL61/">Haumea</a> and the mystery of hundreds of worlds unknown orbiting beyond Neptune. </p>
<p>So let the official word on planets and dwarf planets be as passed in 2006 and let our exploration of the Solar System continue to amaze us.</p><img src="https://counter.theconversation.com/content/74682/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Tanya Hill does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>A group of astronomers are trying to reclassify Pluto as full ‘planet’. But there are good reasons to leave our classification system alone, and this doesn’t mean Pluto is any less interesting.Tanya Hill, Honorary Fellow of the University of Melbourne and Senior Curator (Astronomy), Museums Victoria Research InstituteLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/646272016-09-02T10:42:21Z2016-09-02T10:42:21ZCeres asteroid may have an ‘ice volcano’ and other signs of water, NASA mission reveals<figure><img src="https://images.theconversation.com/files/136403/original/image-20160902-20217-4yfkat.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Ceres' Haulani Crater shows evidence of landslides from its crater rim</span> <span class="attribution"><span class="source">NASA/JPL-Caltech/UCLA/MPS/DLR/IDA</span></span></figcaption></figure><p>The arrival of <a href="https://theconversation.com/dawn-breaks-over-distant-ceres-and-perhaps-reveals-signs-of-habitability-38967">NASA’s Dawn mission</a> at the huge asteroid “1 Ceres” in early 2015 has turned out to have been well worth waiting for. This dwarf planet is the largest body in the asteroid belt between Mars and Jupiter and was the first to be discovered. But, until recently, we have only had information from ground and space-based telescopes, which have given us tantalising glimpses of a dark, possibly water-rich object.</p>
<p>Now the Dawn space probe has sent back a bumper harvest of findings, summarised in six new research papers published in a special issue of the journal Science. We now have a map of Ceres that reveals unusual minerals, a surface peppered with craters, and water in the form of ice and possibly an outer atmosphere of vapour. There’s also enough uncertainty in the results to sow the seeds for future research.</p>
<p>The data provides a global geological map of the asteroid showing that its entire surface appears to be <a href="http://science.sciencemag.org/cgi/doi/10.1126/science.aaf4279">covered in phyllosilicates</a>, an important group of clay minerals. Two specific clays are identified: one that is magnesium-rich, the second an ammonium-rich species. There seems to be little or no pattern to the distribution of the two minerals – they are both almost everywhere.</p>
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<img alt="" src="https://images.theconversation.com/files/136405/original/image-20160902-20232-vcgvuq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/136405/original/image-20160902-20232-vcgvuq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=708&fit=crop&dpr=1 600w, https://images.theconversation.com/files/136405/original/image-20160902-20232-vcgvuq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=708&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/136405/original/image-20160902-20232-vcgvuq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=708&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/136405/original/image-20160902-20232-vcgvuq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=890&fit=crop&dpr=1 754w, https://images.theconversation.com/files/136405/original/image-20160902-20232-vcgvuq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=890&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/136405/original/image-20160902-20232-vcgvuq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=890&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Dawn over Ceres.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech/UCLA/MPS/DLR/IDA</span></span>
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</figure>
<p>This ubiquity is what is important. The minerals could not have been formed in a local event, such as an impact into an ice-filled crater. They must have been produced by planet-wide alteration, presumably implying there must have been volumes of water. It is clear that enormous quantities of liquid water are not present on Ceres now. But the signal of water-ice <a href="http://science.sciencemag.org/cgi/doi/10.1126/science.aaf3010">has been detected</a> in at least one crater.</p>
<p>Because the temperature of Ceres is relatively warm (between -93°C and -33°C), water-ice exposed at the surface would rapidly convert into a gas in such a low-pressure environment. So the discovered traces of water-ice suggest some underground ice was recently exposed and that there must be some mechanism to explain how the surface was disturbed in this way. Some researchers <a href="http://science.sciencemag.org/cgi/doi/10.1126/science.aaf4286">think that the answer</a> is cryovolcanism, where subsurface layers of mixed ice and minerals percolate slowly to the surface through cracks and fractures, or more swiftly following an impact. If the minerals are chlorides, then a low temperature brine can keep the subsurface layer mobile.</p>
<h2>Ice flows</h2>
<p>As well as a geological map of Ceres, we also have a picture of Ceres’ <a href="http://science.sciencemag.org/cgi/doi/10.1126/science.aaf4332">global geomorphology</a> (its surface features). This shows that the surface of Ceres is peppered with impact craters, although the craters <a href="http://science.sciencemag.org/cgi/doi/10.1126/science.aaf4759">are not distributed evenly</a> over the surface. Much more interesting are the three distinct types of mineral flow across the landscape, produced by the movement of ice-rich material, landslides or blankets of ejected particles following impact into ice-rich material. The distribution of the flow types varies with latitude – and the researchers think this means different surface layers of the asteroid contain different amounts of ice.</p>
<p>One of the <a href="http://science.sciencemag.org/cgi/doi/10.1126/science.aaf4219">most remarkable results</a> is the detection of a sudden burst of highly energetic electrons over a period of around a week in June 2015, coinciding with a solar proton storm. The researchers think the protons fired out by the sun interacted with particles in Ceres’ weak atmosphere, creating a shock wave that accelerated the electrons. Based on <a href="http://onlinelibrary.wiley.com/doi/10.1002/2015GL067451/full">observations by the Hubble Space Telescope</a>, Ceres is believed to have a tenuous exosphere (outer atmosphere) of water vapour. <a href="http://science.sciencemag.org/cgi/doi/10.1126/science.aaf4219">The results</a> from Dawn suggest that this may indeed be the case.</p>
<p>Together, this new set of information shows that Ceres is a world that has been shaped by a series of events, with a strong crust of magnesium- and ammonium-bearing phyllosilicates overlying an interior of briny ice and hydrated minerals. What other hidden secrets will be revealed as research continues on the trove of data from Ceres? Questions still remain about the variety of mineral deposits, the depth of the subsurface ice-rock layer, and, of course, the potential for organic material on the minor planet. The harvest from Ceres so far has been rich and promises to keep us busy for years to come.</p><img src="https://counter.theconversation.com/content/64627/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Monica Grady is a Trustee of Lunar Mission One and receives funding from the STFC and the EU.</span></em></p>The latest data from the Dawn space probe points to underground ice flows and a water vapour atmosphere.Monica Grady, Professor of Planetary and Space Sciences, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/492742015-12-22T20:15:45Z2015-12-22T20:15:45ZAfter eight years, NASA’s Dawn probe brings dwarf planet Ceres into closest focus<figure><img src="https://images.theconversation.com/files/106933/original/image-20151222-27863-xoz6fr.jpg?ixlib=rb-1.1.0&rect=0%2C136%2C1024%2C763&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Ceres, as seen by NASA's Dawn spacecraft on December 10, around a crater chain called Gerber Catena.</span> <span class="attribution"><a class="source" href="http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA20186">NASA/JPL-Caltech/UCLA/MPS/DLR/IDA</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>More than a thousand times farther from Earth than the moon, farther even than the sun, an extraordinary extraterrestrial expedition is taking place. <a href="http://dawn.jpl.nasa.gov">NASA’s Dawn spacecraft</a> is exploring dwarf planet Ceres, which orbits the sun between Mars and Jupiter. The probe has just reached the closest point it ever will, and is now beginning to collect its most detailed pictures and other measurements on this distant orb.</p>
<p>Ceres is a remnant from the <a href="http://www.jpl.nasa.gov/blog/2014/12/ceres-curiosities-the-mysterious-world-comes-into-view">dawn of our solar system</a> nearly 4.6 billion years ago. All the data Dawn is now sending back will provide insight into Ceres’ history and geology, including the presence of water, past or present. Scientists believe that by studying Ceres, we can unlock some of the secrets of the epoch in which planets, including our own, formed.</p>
<p>But this mission isn’t only for scientists. Discovering the nature of an uncharted world is a thrill that can be shared by anyone who has ever gazed up at the night sky in wonder, been curious about the universe and Earth’s place in it, or felt the lure of a bold adventure into the unknown. </p>
<p>I happen to fall into all those categories. I fell in love with space at the age of four, and I knew by the fourth grade that I wanted to earn a doctorate in physics. (It was a few more years before I did.) My passion for the exploration of space and the grandeur of scientific discovery and understanding has never wavered. It’s a dream come true for me to be the mission director and chief engineer on Dawn at JPL.</p>
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<figcaption><span class="caption">False color video of Ceres from distance of 2,700 miles, courtesy of Dawn.</span></figcaption>
</figure>
<h2>Ceres before Dawn</h2>
<p>Named for the <a href="https://en.wikipedia.org/wiki/Ceres_(mythology)">Roman goddess of agriculture and grain</a>, Ceres was the <a href="http://dawnblog.jpl.nasa.gov/2014/12/29/dawn-journal-december-29">first dwarf planet discovered</a>, in 1801. That’s 129 years before Pluto – and in fact, both were originally considered planets, only later to be designated dwarf planets.</p>
<p>Although Ceres appeared as little more than a fuzzy blob of light amidst the stars, scientists determined that it’s the behemoth of the main asteroid belt between Mars and Jupiter – nearly 600 miles in diameter. Its surface area is more than a third of the area of the continental US. Before Dawn’s arrival, Ceres was the largest object between the sun and Pluto that a spacecraft had not visited.</p>
<p>Since well before Dawn, we’ve had telescopic evidence that Ceres harbors water. While it’s mostly in the form of ice, scientists have good reason to believe an underground ocean once circulated. The question of whether reservoirs still lurk beneath the alien surface remains open. Dawn’s studies of Ceres may even provide hints about how Earth acquired its own supply of that precious liquid billions of years ago.</p>
<h2>Dawn en route to Ceres</h2>
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<figcaption>
<span class="caption">Dawn launches at dawn on September 27 2007, headed for the asteroid belt.</span>
<span class="attribution"><span class="source">NASA</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>In 2007, we launched Dawn from Cape Canaveral, and it will never again visit its erstwhile planetary home. In 2011, it became the only spacecraft ever to orbit an object in the main asteroid belt, devoting 14 months to scrutinizing <a href="http://www.jpl.nasa.gov/blog/2013/1/the-giant-asteroid-a-retrospective">protoplanet Vesta</a>. Dawn showed us this second most massive resident of the belt is more closely related to the terrestrial planets (including Earth) than to the much smaller chunks of rock that are typical of asteroids.</p>
<p>The unique capability to travel to worlds beyond Mars, enter orbit and maneuver extensively and then depart for yet another destination is achieved with advanced ion propulsion. The technology spent much of its history in the domain of sci-fi, including Star Trek and Star Wars. (Darth Vader’s TIE Fighter is named for its twin ion engines.) But what may have seemed only science fiction is science fact. Without its three ion engines (note that Dawn does the TIE Fighters one better), Dawn’s mission wouldn’t be possible. </p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/0Kl-vromzaQ?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">A gridded ion thruster uses electrical energy to create, accelerate and neutralize positively charged ions to generate thrust.</span></figcaption>
</figure>
<p>The ion engines use xenon gas, a chemical cousin of helium and neon. With electrical power from Dawn’s large solar panels, the xenon is given an electrical charge in a process called ionization. The engines use high voltage to accelerate the ions. They’re then shot out of the engines at up to 90,000 mph. When the ions leave the spacecraft at this fantastically high speed, it’s pushed in the opposite direction. Dawn’s ion propulsion system is exceptionally efficient – 10 times as efficient as conventional spacecraft propulsion. It’s comparable to your car getting 250 miles per gallon.</p>
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<a href="https://images.theconversation.com/files/105532/original/image-20151211-14647-1okz4ce.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/105532/original/image-20151211-14647-1okz4ce.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/105532/original/image-20151211-14647-1okz4ce.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/105532/original/image-20151211-14647-1okz4ce.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/105532/original/image-20151211-14647-1okz4ce.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/105532/original/image-20151211-14647-1okz4ce.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/105532/original/image-20151211-14647-1okz4ce.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/105532/original/image-20151211-14647-1okz4ce.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Artist’s conception of the Dawn spacecraft arriving at Ceres. The engine’s xenon ions glow with blue light.</span>
<span class="attribution"><a class="source" href="http://photojournal.jpl.nasa.gov/catalog/PIA18921">NASA/JPL-Caltech</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Dawn drops into Cerean orbit</h2>
<p>Finally, after a journey of more than seven years and three billion miles, our interplanetary ambassador reached Ceres on March 6 2015, and <a href="http://www.jpl.nasa.gov/blog/2015/3/we-did-it-dawn-arrives-at-dwarf-planet-ceres">gracefully entered</a> the dwarf planet’s permanent gravitational embrace.</p>
<p>Mission controllers at JPL then piloted the craft to three orbits at successively lower altitudes, so we could first obtain an overview and then gain better and better views of this vast unexplored territory. And Dawn has just performed the penultimate act in its grand celestial choreography. It’s spent the last seven weeks maneuvering to its lowest altitude. Orbiting now about 240 miles above the exotic terrain of rock and ice, Dawn is closer to Ceres than the International Space Station is to Earth.</p>
<h2>Dawn brings Ceres into focus</h2>
<p>Included in the spacecraft’s suite of sophisticated sensors is a camera that has already taken 10,000 pictures of alien landscapes on Ceres. Following from Ceres’ own name, features Dawn discovers are named for agricultural deities and festivals from around the world.</p>
<p>We see rugged terrain and smooth areas, sometimes with streaks of material that’s flowed across it. There are craters large and small, created by billions of years of assaults in the rough-and-tumble neighborhood of the asteroid belt. We see mountains and valleys, huge fissures in the ground and <a href="http://dawn.jpl.nasa.gov/news/news-detail.html?id=4785">bright spots that glow</a> with a mysterious luster, reflecting much more sunlight than most of the dark surface.</p>
<p>The most striking of these shining regions, inside the 55-mile-wide Occator Crater (named for the Roman deity of harrowing), is so bright that the Hubble Space Telescope detected a hint of it a decade ago. Dawn’s pictures to date have been more than 200 times sharper than Hubble’s. The images we’re starting to get back now will be even better, revealing 850 times the detail that Hubble had provided.</p>
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<figcaption>
<span class="caption">Dawn took this image in its low-altitude mapping orbit from an approximate distance of 240 miles (385 kilometers) from Ceres on December 10.</span>
<span class="attribution"><a class="source" href="http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA20185">NASA/JPL-Caltech/UCLA/MPS/DLR/IDA</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Dawn has shown us a mountain named Ahuna Mons that towers more than 20,000 feet in an otherwise unremarkable area, comparable to the elevation of North America’s tallest peak, Mt Denali. (Ahuna is a celebration of thanksgiving for the harvest among the Sumis of northeast India.) Bright streaks seem to suggest some unidentified material once flowed down the steep slopes of Ahuna Mons. While scientists have not yet determined what forces and processes shaped this conical mountain, it doesn’t take a geologist to notice its resemblance to terrestrial volcanic cones. Imagine what it might have been like to witness an eruption of some strange combination of water and other chemicals on this cold, distant world.</p>
<p>Beyond photos, Dawn will take a great many other measurements from its new orbital perch before its mission concludes in 2016. It will measure radiation to help scientists determine what types of atoms are present on Ceres. It will use infrared light to identify the minerals on Ceres’ surface. And it will gauge subtle variations in the gravitational field to reveal the interior structure of the dwarf planet.</p>
<p>Once the spacecraft exhausts the small supply of conventional rocket propellant it squirts through thrusters to control its orientation in the zero-gravity, frictionless conditions of spaceflight, it will no longer be able to point its solar arrays at the sun, its antenna at Earth, its sensors at Ceres or its ion engines in the direction needed to travel elsewhere. But the ship will remain in orbit around Ceres as surely as the moon remains in orbit around Earth and Earth remains in orbit around the sun. Its legacy in the history of our efforts to reach out from our humble home to touch the stars is secure. Dawn will become an inert celestial monument to humankind’s creativity, ingenuity, and passion for exploring the cosmos.</p>
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<figcaption>
<span class="caption">This part of Ceres, near the south pole, has such long shadows because, from the perspective of this location, the sun is near the horizon. At the time When Dawn took this image on December 10, the sun was 4 degrees north of the equator. If you were standing this close to Ceres’ south pole, the sun would never get high in the sky during the course of a nine-hour Cerean day.</span>
<span class="attribution"><a class="source" href="http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA20188">NASA/JPL-Caltech/UCLA/MPS/DLR/IDA</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure><img src="https://counter.theconversation.com/content/49274/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Marc D Rayman receives funding from NASA through Caltech for the Dawn mission at JPL. </span></em></p>Dawn’s mission director and chief engineer describes his ‘dream come true’ job – and how the new data coming back from Ceres could unlock some of the secrets of the earliest days of our solar system.Marc D Rayman, Dawn Chief Engineer and Mission Director at JPL, NASALicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/521052015-12-09T18:08:10Z2015-12-09T18:08:10ZCeres reveals its salty secrets – and blurs the line between comets and asteroids<figure><img src="https://images.theconversation.com/files/105065/original/image-20151209-15556-1c6g1dn.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Ceres: a bright spot in planetary exploration.</span> <span class="attribution"><span class="source">NASA/JPL-Caltech/UCLA/MPS/DLR/IDA</span></span></figcaption></figure><p>When <a href="http://dawn.jpl.nasa.gov/DawnCommunity/flashbacks/fb_04.asp">Guiseppe Piazzi</a> reported his observations of a minor planet in 1801, he originally thought it might be a comet. But follow-up observations by fellow astronomers suggested that Ceres was actually an asteroid. So it’s somewhat ironic that the latest results from <a href="http://dawn.jpl.nasa.gov/">NASA’s Dawn mission</a> suggest this asteroid is confusingly similar to a comet.</p>
<p>Dawn has found a number of mysterious features on Ceres so far, including <a href="https://theconversation.com/dawn-breaks-over-distant-ceres-and-perhaps-reveals-signs-of-habitability-38967">bright white spots</a> on its surface. Its latest results suggest that these are salts left behind as ice vaporised from the surface by sublimation – a process often seen in comets. They also suggest Ceres may have formed far away from its current location in orbit between Mars and Jupiter. This would be surprising as many astronomers believe that a key <a href="http://www.universetoday.com/33006/what-is-the-difference-between-asteroids-and-comets/">difference between comets and asteroids</a> is that asteroids form closer to the sun.</p>
<h2>Mysterious spots</h2>
<p>Ceres is the largest asteroid we know of – it is also classified as a <a href="https://theconversation.com/nasa-missions-may-re-elevate-pluto-and-ceres-from-dwarf-planets-to-full-on-planet-status-36081">dwarf planet</a>. Its bright spots were first discovered when Dawn started orbiting Ceres in 2014, the largest at a latitude of around 25°N. There was intense speculation about what these features were, as they had the characteristics of ice. The Herschel Space Observatory later found that <a href="http://www.bbc.co.uk/news/science-environment-25849871">water vapour</a> was being produced at specific locations on Ceres. </p>
<p>It therefore seemed that Ceres was acting like a comet, with ice-rich regions releasing dust and vapour during daylight hours. If that were the case, then ice might be a major component of the asteroid, buried below a surface of dust and rubble. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/YNjktBvsGA0?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<p>But the two new studies (see <a href="http://nature.com/articles/doi:10.1038/nature15754">here</a> and <a href="http://nature.com/articles/doi:10.1038/nature16172">here</a>), using information from different instruments on the Dawn spacecraft, did not record any ice on the surface. However, one article speculates that ice may still be buried just below the surface while the other suggests that water bound in minerals is abundant.</p>
<p>The researchers also investigated the bright feature at the bottom of <a href="https://www.nasa.gov/jpl/dawn/pia19617/occator-crater-enhanced-view">Occator Crater</a>, the brightest of the white spots, and concluded that they may be hydrated magnesium salts. The salts are deposits left behind from recent sublimation of water ice that have not yet been covered by soil. Other bright spots, although not as prominent, may also be salt deposits, but that material is likely to be older.</p>
<h2>A Kuiper Belt Object?</h2>
<p>The researchers also identified a mixture of minerals on the surface of Ceres, which they think are ammonia-bearing clay minerals and <a href="http://pubchem.ncbi.nlm.nih.gov/compound/magnesium_carbonate">magnesium carbonate</a>. The clay minerals could have been produced by <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/geophys/silicate.html">silicates</a> reacting with ammonia ice. However, if Ceres had formed where it is now, it would not have been able to pick up any ammonia ice to enable such a reaction, because the ice would not be stable.</p>
<p>This means that Ceres may have originally formed in the <a href="http://solarsystem.nasa.gov/planets/kbos">Kuiper Belt</a> on the outskirts of the solar system and then scattered inwards as the giant planets migrated outwards. Alternatively, Ceres could have formed more or less where it is, and incorporated nitrogen-containing organic molecules, which, like the water ice, were transported inwards from beyond Neptune.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/105064/original/image-20151209-15567-14qwjcc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/105064/original/image-20151209-15567-14qwjcc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/105064/original/image-20151209-15567-14qwjcc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=404&fit=crop&dpr=1 600w, https://images.theconversation.com/files/105064/original/image-20151209-15567-14qwjcc.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=404&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/105064/original/image-20151209-15567-14qwjcc.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=404&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/105064/original/image-20151209-15567-14qwjcc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=507&fit=crop&dpr=1 754w, https://images.theconversation.com/files/105064/original/image-20151209-15567-14qwjcc.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=507&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/105064/original/image-20151209-15567-14qwjcc.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=507&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Did Ceres form in the main belt and incorporate ammonia from the outer solar system or did Ceres itself form there?</span>
<span class="attribution"><span class="source">L.Giacomini</span></span>
</figcaption>
</figure>
<p>While this might not sound all that significant, it does have quite profound ramifications for our understanding of how material has been mixed to form planets, minor planets, comets and Kuiper Belt Objects. </p>
<p>This year has been an amazing one for small icy bodies. <a href="https://theconversation.com/stunning-crystal-clear-images-of-pluto-but-what-do-they-mean-47517">Images from the New Horizons mission</a> to Pluto have shown us the variety of landscapes that can be sculpted on an icy surface. Similarly, pictures of the surface of <a href="https://theconversation.com/building-blocks-of-life-found-among-organic-compounds-on-comet-67p-what-philae-discoveries-mean-45379">comet 67P Churyumov Gerasimenko </a> taken by Rosetta have revealed canyons and pits probably caused by fracturing and ice sublimation. </p>
<p>Now we can add a third small body where a combination of ice, water and salts have left behind an environment in which there is the potential for an active, sub-surface chemistry that might, eventually, result in formation of complex molecules. It is also becoming more clear than ever that strict division between comets and asteroids is no longer realistic, and that they represent a spectrum of objects of varying activity and orbit.</p>
<p>Just one last word about the surface of Ceres. I might not be much of a farmer – but I’m fairly certain that magnesium salts and nitrogen-bearing clays are important ingredients in a good, rich soil for raising crops. So naming Ceres after a <a href="http://www.ceresva.org/Goddess/Ceres.htm">harvest deity</a> was more appropriate than Piazzi could have imagined!</p><img src="https://counter.theconversation.com/content/52105/count.gif" alt="The Conversation" width="1" height="1" />
<h4 class="border">Disclosure</h4><p class="fine-print"><em><span>Monica Grady receives funding from the STFC and is a Trustee of Lunar Mission One</span></em></p>Is it a dwarf planet, an asteroid or a comet? All of the above? Well, with the latest results about Ceres, researchers aren’t entirely sure anymore.Monica Grady, Professor of Planetary and Space Sciences, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/446962015-07-15T18:39:40Z2015-07-15T18:39:40ZA team member’s view of all the work on Earth it took to get New Horizons to Pluto<figure><img src="https://images.theconversation.com/files/88545/original/image-20150715-26277-ujyla8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">It's all happening! New Horizons' science team members react to crisp shots of Pluto.</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/nasahqphoto/19686046915">NASA/Bill Ingalls</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc/4.0/">CC BY-NC</a></span></figcaption></figure><p>Science is hard and good science is harder – it takes persistence and tons of patience. When we began planning a mission to Pluto over 15 years ago, we knew it was going to be, as they say, a long haul. But we also knew it had a huge potential payoff. New Horizons would be the first closeup look at a world that we’d known about only distantly for 70 years. That kind of challenge is hard to pass by. And I didn’t.</p>
<h2>Earliest rumblings</h2>
<p>The first serious discussion of a mission to Pluto occurred in the late 1980s. That’s when telescope observations of distant stars going behind Pluto (known as <a href="http://occult.mit.edu/research/stellarOccultations.php">stellar occultations</a>) showed that it <a href="http://dx.doi.org/10.1006/icar.1997.5709">had a methane atmosphere</a>, probably rapidly evaporating to space. Considering Pluto’s small size, it was a surprise to see that its gravity was strong enough to hold on to an atmosphere at all.</p>
<p>Several planetary scientists began working on Pluto in earnest around that time. Throughout the 1990s, it was pretty obvious who the enthusiastic contributors to Pluto science were, and we naturally congregated at scientific meetings to speculate about a mission to the ninth planet. Several <a href="http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=PLUTOKE">attempts at a Pluto mission</a> were formulated by NASA, but for various reasons none came to fruition, usually because of insufficient funding. By the time NASA announced the opportunity for a Pluto mission in 2000, everyone was ready with some pretty clear plans for how to optimize the science that could be done via a flyby.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/88553/original/image-20150715-26334-l1f92w.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/88553/original/image-20150715-26334-l1f92w.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/88553/original/image-20150715-26334-l1f92w.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/88553/original/image-20150715-26334-l1f92w.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/88553/original/image-20150715-26334-l1f92w.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/88553/original/image-20150715-26334-l1f92w.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/88553/original/image-20150715-26334-l1f92w.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/88553/original/image-20150715-26334-l1f92w.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Checking New Horizons’ electronics, pre-launch.</span>
<span class="attribution"><a class="source" href="http://pluto.jhuapl.edu/Multimedia/Images/Spacecraft-Images.php">NASA/JHUAPL/SwRI</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Pluto’s mysterious atmosphere</h2>
<p>I became interested in Pluto as a graduate student at Caltech in 1984. So little was known about the atmosphere at the time that you could get a computer model to predict just about anything. But after those stellar occultations revealed its unexpected atmosphere, I was hooked on Pluto atmosphere research. I focused on the planet’s chemistry. When Alan Stern, the <a href="http://www.nasa.gov/mission_pages/newhorizons/main/index.html">New Horizons</a> principal investigator, started pulling together <a href="http://pluto.jhuapl.edu/Mission/The-Team.php">researchers to work on the mission</a>, he asked me to join the atmosphere science team. Other mission members are focused on surface geology, interior structure, and formation of the Pluto–Charon system of five moons, among other things.</p>
<p>My main interest is understanding the chemistry and stability of Pluto’s atmosphere. How could Pluto hold onto its extended layer of gases over the 4.5 billion year age of the solar system? It should have long ago escaped to space because of tiny Pluto’s low gravity. But apparently it hasn’t.</p>
<p>Also, I’m interested to see whether the methane in its atmosphere produced complex hydrocarbons that would condense onto aerosols, settle downward, and be deposited on Pluto’s surface. Over the age of the solar system, these molecules should accumulate so that now we’d expect to see tens of meters of the stuff on Pluto’s surface.</p>
<p>And in fact, New Horizons does see large very dark regions on Pluto that might be made up of these photochemical products, generated when the methane in Pluto’s atmosphere absorbs ultraviolet sunlight. Similar types of chemically processed hydrocarbons are seen elsewhere in the outer solar system. They are generally known as “tholins” – a generic term to describe this reddish material. We have further observations planned that will probe Pluto’s atmosphere and map the distributions of hydrocarbon gases such as ethane, acetylene and ethylene that condense to form the aerosols.</p>
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<a href="https://images.theconversation.com/files/88549/original/image-20150715-26284-19li3yf.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/88549/original/image-20150715-26284-19li3yf.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/88549/original/image-20150715-26284-19li3yf.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/88549/original/image-20150715-26284-19li3yf.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/88549/original/image-20150715-26284-19li3yf.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/88549/original/image-20150715-26284-19li3yf.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/88549/original/image-20150715-26284-19li3yf.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/88549/original/image-20150715-26284-19li3yf.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Pluto, in its full-color glory, taken by New Horizons from only 476,000 miles away.</span>
<span class="attribution"><a class="source" href="http://pluto.jhuapl.edu/Multimedia/Science-Photos/image.php?page=1&gallery_id=2&image_id=225">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>The long haul behind a long-haul mission</h2>
<p>Of course we had to do tons of prep work – deciding what scientific questions we’d try to tackle, which instruments to include, how to effectively steer and communicate with the probe over decades. And our work didn’t end with the launch nine-and-a-half years ago – there was still plenty left for us to do here on Earth.</p>
<p>Maybe it’s surprising, but this project has been exciting all the way through. The science team meetings were always interesting and animated, and at every step our conversations were laced with wild speculation about Pluto. Even the zillions of teleconferences usually had doses of humor sprinkled throughout.</p>
<p>While New Horizons was plugging away on its journey, my work mostly consisted of trying to prepare computer models of Pluto’s atmosphere to help us understand our eventual observations. This was much more difficult that I had thought it would be. We’d never encountered a planetary atmosphere like Pluto’s: gases escaping at supersonic speeds from such a frigid planet are a completely new situation. And that challenge continues to complicate interpreting the observations that are rolling in right now. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/88546/original/image-20150715-26334-1a9hgu1.gif?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/88546/original/image-20150715-26334-1a9hgu1.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/88546/original/image-20150715-26334-1a9hgu1.gif?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=240&fit=crop&dpr=1 600w, https://images.theconversation.com/files/88546/original/image-20150715-26334-1a9hgu1.gif?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=240&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/88546/original/image-20150715-26334-1a9hgu1.gif?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=240&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/88546/original/image-20150715-26334-1a9hgu1.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=302&fit=crop&dpr=1 754w, https://images.theconversation.com/files/88546/original/image-20150715-26334-1a9hgu1.gif?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=302&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/88546/original/image-20150715-26334-1a9hgu1.gif?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=302&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The four smaller known moons of Pluto in orbit.</span>
<span class="attribution"><a class="source" href="http://www.nasa.gov/image-feature/nasa-s-new-horizons-spots-pluto-s-faintest-known-moons">NASA</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Cracking Pluto’s mysteries</h2>
<p>Pluto’s intrigue seemed only to increase, even after we launched New Horizons, when <a href="http://www.planetsedu.com/moon/nix/">two more moons</a> were discovered in addition to its known large moon Charon. And later <a href="http://www.sci-news.com/space/science-kerberos-styx-pluto-moons-names-01192.html">two more were discovered</a>! People were drawn to Pluto’s mysteries and each new revelation seemed to captivate more of their interest. And the International Astronomical Union’s demotion of Pluto <a href="https://www.iau.org/public/themes/pluto/">from planet to dwarf planet status</a> in 2006 seemed to make it even more interesting to the public. The political wrangling over Pluto’s status, the bizarre discoveries and New Horizons’ continuing quest worked via synergy to increase public interest in and support for our mission, which in turn energized the team.</p>
<p>This was truly a once-in-a-lifetime opportunity to be involved in such a history-making event. It’s gratifying to know that for decades to come, the New Horizons mission will be known as the endeavor that gave us our biggest leap forward in understanding Pluto, and that I was a part of it.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/88547/original/image-20150715-26296-ymipcd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/88547/original/image-20150715-26296-ymipcd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/88547/original/image-20150715-26296-ymipcd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=422&fit=crop&dpr=1 600w, https://images.theconversation.com/files/88547/original/image-20150715-26296-ymipcd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=422&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/88547/original/image-20150715-26296-ymipcd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=422&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/88547/original/image-20150715-26296-ymipcd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=530&fit=crop&dpr=1 754w, https://images.theconversation.com/files/88547/original/image-20150715-26296-ymipcd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=530&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/88547/original/image-20150715-26296-ymipcd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=530&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Principal Investigator Alan Stern celebrates with New Horizons flight controllers after they received confirmation from the spacecraft that it had successfully completed the flyby of Pluto.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/nasahqphoto/19710637771">NASA/Bill Ingalls</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc/4.0/">CC BY-NC</a></span>
</figcaption>
</figure>
<p>As I write, this it’s the day after the encounter. We are getting back results that are astonishing! Discoveries left and right. We’ll be working on understanding the brand new data for years. And this is nowhere near the end for the New Horizons team. We’ll stay together to plan an Extended Mission for New Horizons to fly by another dwarf ice planet – either the cryptically named PT1 or PT3 – hopefully in mid-2018.</p><img src="https://counter.theconversation.com/content/44696/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Mike Summers receives research funding from NASA.</span></em></p>New Horizons mission members have worked on the project for even longer than it’s taken the spacecraft to get to Pluto. They’ve planned, built and researched – and now their efforts are paying off.Mike Summers, Professor of Planetary Science and Astronomy, George Mason UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/389672015-03-18T12:44:38Z2015-03-18T12:44:38ZDawn breaks over distant Ceres … and perhaps reveals signs of habitability<figure><img src="https://images.theconversation.com/files/75137/original/image-20150317-2175-1q3yp4o.jpg?ixlib=rb-1.1.0&rect=0%2C115%2C1150%2C845&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">What is the bright spot of Ceres? Not long till we find out.</span> <span class="attribution"><a class="source" href="http://commons.wikimedia.org/wiki/File:PIA18920-Ceres-DwarfPlanet-20150219.jpg">NASA</a></span></figcaption></figure><p>NASA’s <a href="http://dawn.jpl.nasa.gov/mission/">Dawn spacecraft</a> is about to start its investigation of the largest member of the asteroid belt, <a href="http://space-facts.com/ceres/">1 Ceres</a>. It will take detailed images of the dwarf planet, and produce a geological map of its entire surface. But even before the spacecraft has reached its optimum orbit, the preliminary results just released are already surprising and delighting planetary scientists.</p>
<p>Up until February 2015, the best images taken of Ceres were from the <a href="http://hubblesite.org/newscenter/archive/releases/2005/27/image/e">Hubble space telescope</a>, showing a near-spherical body with one area that was much brighter than the rest of the surface. As Dawn approached Ceres, its camera acquired some remarkable images, at about three times the resolution of those from Hubble. The pictures verified that there was indeed a brighter region. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/75133/original/image-20150317-2165-y1j1mt.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/75133/original/image-20150317-2165-y1j1mt.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/75133/original/image-20150317-2165-y1j1mt.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=300&fit=crop&dpr=1 600w, https://images.theconversation.com/files/75133/original/image-20150317-2165-y1j1mt.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=300&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/75133/original/image-20150317-2165-y1j1mt.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=300&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/75133/original/image-20150317-2165-y1j1mt.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=376&fit=crop&dpr=1 754w, https://images.theconversation.com/files/75133/original/image-20150317-2165-y1j1mt.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=376&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/75133/original/image-20150317-2165-y1j1mt.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=376&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Exploded map of Ceres showing ‘bright spot’.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>Even better, close examination of the images showed that the area varied in brightness over the course of Ceres’ day (which is only about nine hours long), growing dimmer as the dwarf planet moved into darkness. It is interpretation of this variability that has planetary scientists buzzing.</p>
<p>As if that were not enough, a further series of pictures appear to show a plume emanating from the surface. Is Ceres active? Does it have a layer of water or ice below a thin crust of rock? Could it be a ball of mud, overlain by a muddy ocean, on top of which is another thin muddy crust? The exact structure of Ceres is not yet known, although it is clear that it’s not rocky all the way through – its density is too low, so there must be at least some water or ice present.
Suggestions at the 46th <a href="http://www.hou.usra.edu/meetings/lpsc2015">Lunar and Planetary Science Conference</a> in Houston, Texas, of icy volcanism on Ceres have led to speculation that the dwarf planet could potentially be habitable. Although Ceres does not have an atmosphere, life might exist in a subsurface ocean, as has been suggested for <a href="https://theconversation.com/the-moon-was-a-first-step-mars-will-test-our-capabilities-but-europa-is-the-prize-37253">Europa</a> or <a href="https://theconversation.com/icy-plumes-bursting-from-saturns-moon-enceladus-suggest-it-could-harbour-life-38673">Enceladus</a>, moons orbiting Jupiter and Saturn respectively. </p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/75138/original/image-20150317-2172-7petmp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/75138/original/image-20150317-2172-7petmp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/75138/original/image-20150317-2172-7petmp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=482&fit=crop&dpr=1 600w, https://images.theconversation.com/files/75138/original/image-20150317-2172-7petmp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=482&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/75138/original/image-20150317-2172-7petmp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=482&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/75138/original/image-20150317-2172-7petmp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=605&fit=crop&dpr=1 754w, https://images.theconversation.com/files/75138/original/image-20150317-2172-7petmp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=605&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/75138/original/image-20150317-2172-7petmp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=605&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Is Ceres more slush than solid inside?</span>
<span class="attribution"><a class="source" href="http://commons.wikimedia.org/wiki/File:Ceres_Cutaway.jpg">NASA</a></span>
</figcaption>
</figure>
<p><a href="http://link.springer.com/article/10.1007%2FBF00613296#page-1">Cryovolcanism</a> – the presence of ice volcanoes – is not the only mechanism that can produce a plume of dust and ice from a planetary surface. The Rosetta mission has delivered amazing images of plumes coming from comet <a href="http://www.esa.int/spaceinimages/Images/2015/03/Comet_on_9_March_2015_NavCam">P/67 Churyumov-Gerasimenko</a>, caused by sublimation of ice that releases dust and gas trapped inside the ice. Could the bright spot be an icy plume caused by the vaporisation of Ceres’ surface as it turns towards the sun’s heat, and then dropping away as night falls? Corridor talk at the conference speculates that Ceres might be closer to a comet than the asteroid it is usually regarded as.</p>
<p>Fortunately, we won’t have to wait much longer before we get some more definitive answers to questions of Ceres’ physical structure and heritage. By the beginning of April, the Dawn spacecraft will be much closer and will start its imaging campaign in earnest, at which point we will start seeing craters and other surface features at better resolution.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/75134/original/image-20150317-2147-q2t4d3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/75134/original/image-20150317-2147-q2t4d3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/75134/original/image-20150317-2147-q2t4d3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=1053&fit=crop&dpr=1 600w, https://images.theconversation.com/files/75134/original/image-20150317-2147-q2t4d3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=1053&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/75134/original/image-20150317-2147-q2t4d3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=1053&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/75134/original/image-20150317-2147-q2t4d3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1323&fit=crop&dpr=1 754w, https://images.theconversation.com/files/75134/original/image-20150317-2147-q2t4d3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1323&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/75134/original/image-20150317-2147-q2t4d3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1323&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">This is not the Ceres you are looking for.</span>
<span class="attribution"><a class="source" href="http://commons.wikimedia.org/wiki/File:Ceres_statue.jpg">Borghese Collection, Louvre</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>In preparation for descriptions of such features, and bearing in mind that Ceres was the Roman goddess of the harvest, the International Astronomical Union has <a href="http://astrogeology.usgs.gov/news/nomenclature/two-themes-approved-for-ceres">ruled</a> that craters on Ceres should be named after international deities of agriculture and vegetation, while other features will be named after agricultural festivals of the world. </p>
<p>I’m not sure just how many of these there are, or how memorable their names will turn out to be. But as the Dawn mission’s principal investigator Chris Russell pointed out, there is one Mayan deity named Yum (<a href="http://www.godchecker.com/pantheon/mayan-mythology.php?deity=YUM-KAAX">Yum Kaax</a>, god of agriculture and the jungle), who should readily be remembered. One can only hope the mission scientists find a suitably delicious feature on Ceres to give that name.</p><img src="https://counter.theconversation.com/content/38967/count.gif" alt="The Conversation" width="1" height="1" />
NASA’s Dawn spacecraft hasn’t reached optimum orbit around Ceres but the data it’s returning has already got scientists excited.Monica Grady, Professor of Planetary and Space Sciences, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/360812015-02-25T10:56:46Z2015-02-25T10:56:46ZNASA missions may re-elevate Pluto and Ceres from dwarf planets to full-on planet status<figure><img src="https://images.theconversation.com/files/72668/original/image-20150220-21899-1mcd7yx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Two views of Ceres acquired by NASA's Dawn spacecraft ten hours apart on Feb. 12, 2015, from a distance of about 52,000 miles as the dwarf planet rotated.</span> <span class="attribution"><a class="source" href="http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA19056">NASA/JPL-Caltech/UCLA/MPS/DLR/IDA</a></span></figcaption></figure><p>Ceres is the largest object in the asteroid belt, and NASA’s <a href="http://dawn.jpl.nasa.gov">Dawn</a> spacecraft will arrive at this dwarf planet on March 6, 2015. </p>
<p>Pluto is the largest object in the Kuiper belt, and NASA’s <a href="http://pluto.jhuapl.edu">New Horizons</a> spacecraft will arrive at this dwarf planet on July 15, 2015. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/72670/original/image-20150220-21904-1mdve8p.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/72670/original/image-20150220-21904-1mdve8p.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/72670/original/image-20150220-21904-1mdve8p.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=915&fit=crop&dpr=1 600w, https://images.theconversation.com/files/72670/original/image-20150220-21904-1mdve8p.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=915&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/72670/original/image-20150220-21904-1mdve8p.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=915&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/72670/original/image-20150220-21904-1mdve8p.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1149&fit=crop&dpr=1 754w, https://images.theconversation.com/files/72670/original/image-20150220-21904-1mdve8p.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1149&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/72670/original/image-20150220-21904-1mdve8p.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1149&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Launch of NASA’s Dawn spacecraft from Cape Canaveral Air Force Station, Florida, September 27, 2007. Get ready, Ceres!</span>
<span class="attribution"><a class="source" href="http://www.nasa.gov/mission_pages/dawn/launch/#.VOemf76JnzK">NASA</a></span>
</figcaption>
</figure>
<p>These two events will make 2015 an exciting year for solar system exploration and discovery. But there is much more to this story than mere science. I expect 2015 will be the year when general consensus, built upon our new knowledge of these two objects, will return Pluto and add Ceres to our family of solar system planets.</p>
<p>The efforts of a very small clique of Pluto-haters within the International Astronomical Union (IAU) plutoed Pluto in 2006. Of the approximately 10,000 internationally registered members of the IAU in 2006, only 237 voted in favor of the <a href="http://www.iau.org/news/pressreleases/detail/iau0603/">resolution redefining Pluto</a> as a “dwarf planet” while 157 voted against; the other 9,500 members were not present at the closing session of the IAU General Assembly in Prague at which the vote to demote Pluto was taken. Yet Pluto’s official planetary status was snatched away. </p>
<p>Ceres and Pluto are both spheroidal objects, like Mercury, Earth, Jupiter and Saturn. That’s part of the agreed upon definition of a planet. They both orbit a star, the Sun, like Venus, Mars, Uranus and Neptune. That’s also part of the widely accepted definition of a planet.</p>
<p>Unlike the larger planets, however, Ceres, like Pluto, according to the IAU definition, “has not cleared the neighborhood around its orbit.” The asteroid belt is, apparently, Ceres’ neighborhood while the Kuiper Belt is Pluto’s neighborhood – though no definition of a planet’s neighborhood exists, and no agreed upon understanding of what “clearing the neighborhood” yet exists. Furthermore, no broad-based agreement exists as to why “clearing the neighborhood” need be a requirement in order for an object to be considered a planet.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/72931/original/image-20150224-25698-m8p16z.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/72931/original/image-20150224-25698-m8p16z.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/72931/original/image-20150224-25698-m8p16z.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=459&fit=crop&dpr=1 600w, https://images.theconversation.com/files/72931/original/image-20150224-25698-m8p16z.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=459&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/72931/original/image-20150224-25698-m8p16z.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=459&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/72931/original/image-20150224-25698-m8p16z.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=577&fit=crop&dpr=1 754w, https://images.theconversation.com/files/72931/original/image-20150224-25698-m8p16z.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=577&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/72931/original/image-20150224-25698-m8p16z.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=577&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">An artist’s impression of the Dawn spacecraft traveling in the asteroid belt with its target Ceres on the right.</span>
<span class="attribution"><a class="source" href="https://solarsystem.nasa.gov/multimedia/display.cfm?Category=Spacecraft&IM_ID=183">JPL</a></span>
</figcaption>
</figure>
<p>Some planetary astronomers would argue that were the Earth placed in the Kuiper Belt, it would not be able to clear its neighborhood and thus would not be considered, by the IAU definition, a planet; apparently location matters. Here a planet, there not a planet. I’d argue that location shouldn’t matter; instead, the intrinsic properties of the objects themselves should matter more. And so we are led back to Ceres and Pluto.</p>
<p>Never before visited by human spacecraft, Ceres and Pluto, as we will soon bear witness, are both evolving, changing worlds. Yesterday, Ceres and Pluto were strangers, distant, barely known runt members of our solar system. By the end of this calendar year, however, we will have showered both objects with our passion and our attention, we will have welcomed them both into our embrace. And we almost certainly will once again call both of them planets.</p>
<h2>Ceres, temporarily a planet</h2>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/72424/original/image-20150218-20810-jd20fa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/72424/original/image-20150218-20810-jd20fa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/72424/original/image-20150218-20810-jd20fa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=763&fit=crop&dpr=1 600w, https://images.theconversation.com/files/72424/original/image-20150218-20810-jd20fa.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=763&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/72424/original/image-20150218-20810-jd20fa.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=763&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/72424/original/image-20150218-20810-jd20fa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=959&fit=crop&dpr=1 754w, https://images.theconversation.com/files/72424/original/image-20150218-20810-jd20fa.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=959&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/72424/original/image-20150218-20810-jd20fa.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=959&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Giovanni Piazzi, Italian astronomer who discovered Ceres on January 1, 1801.</span>
<span class="attribution"><a class="source" href="http://commons.wikimedia.org/wiki/File:Giuseppe_Piazzi.jpg">F. Bordiga</a></span>
</figcaption>
</figure>
<p>Ceres was discovered on New Year’s Day in 1801, by Italian astronomer Giuseppe Piazzi, a member of an international team of astronomers dubbed the Celestial Police, who were searching for a <a href="http://press.princeton.edu/titles/8247.html">supposedly missing planet</a> in between the orbits of Mars and Jupiter. When discovered, Ceres was immediately recognized as a planet, the eighth one known at the time (neither Neptune nor Pluto had been discovered yet).</p>
<p>But within a few years, other objects in the asteroid belt were discovered and Ceres no longer seemed to stand out as far from the crowd. In 1802, the great astronomer William Herschel suggested that Ceres and Pallas and any other smaller solar system objects should be called asteroids – meaning star-like. In telescope images, they were so tiny that they looked point-like, like stars, rather than disk-like, like planets. And so, more than a century before Pluto was discovered, Ceres was plutoed. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/71521/original/image-20150209-24704-ceh7d3.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/71521/original/image-20150209-24704-ceh7d3.gif?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=270&fit=crop&dpr=1 600w, https://images.theconversation.com/files/71521/original/image-20150209-24704-ceh7d3.gif?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=270&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/71521/original/image-20150209-24704-ceh7d3.gif?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=270&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/71521/original/image-20150209-24704-ceh7d3.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=339&fit=crop&dpr=1 754w, https://images.theconversation.com/files/71521/original/image-20150209-24704-ceh7d3.gif?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=339&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/71521/original/image-20150209-24704-ceh7d3.gif?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=339&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Animation of rotating Ceres, made from a series of images taken by NASA’s Dawn spacecraft on February 4, 2015, at a distance of about 90,000 miles from the planet.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>But Ceres does still stand out. It’s the largest asteroid, by far, nearly 1,000 kilometers across (twice as large in diameter as Vesta, the second largest asteroid), though not perfectly spherical in shape.</p>
<p>As happened inside Earth and other planets, planetary scientists think that long ago, the denser material in Ceres separated from the lighter material and sank to form a core.</p>
<p>Astronomers think Ceres is rich in water – as much as one-third of Ceres might be water – and may have a thin atmosphere. Bright, white spots on its surface might even be large frozen lakes. Ceres may, in fact, have as much fresh water as Earth, have Earth-like polar caps, and might even have a sub-surface liquid ocean layer, like Jupiter’s moon Europa and Saturn’s moon Enceladus. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/72661/original/image-20150220-21916-90yibz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/72661/original/image-20150220-21916-90yibz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/72661/original/image-20150220-21916-90yibz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=371&fit=crop&dpr=1 600w, https://images.theconversation.com/files/72661/original/image-20150220-21916-90yibz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=371&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/72661/original/image-20150220-21916-90yibz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=371&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/72661/original/image-20150220-21916-90yibz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=466&fit=crop&dpr=1 754w, https://images.theconversation.com/files/72661/original/image-20150220-21916-90yibz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=466&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/72661/original/image-20150220-21916-90yibz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=466&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Water geysers erupting from south polar region of Saturn’s moon Enceladus.</span>
<span class="attribution"><a class="source" href="http://photojournal.jpl.nasa.gov/catalog/PIA11688">NASA/JPL/Space Science Institute</a></span>
</figcaption>
</figure>
<p>Beginning this month, we’ll start to learn more about these tantalizing possibilities. With our increasing knowledge of and familiarity with Ceres, we will no longer be able to identify meaningful criteria that will allow us to continue to classify Ceres as not-a-planet. Ceres will continue to be a small planet, but in 2015 we will come to understand that dwarf planets are planets, too. </p>
<h2>Pluto’s short planetary reign</h2>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/72426/original/image-20150218-20818-fgi4zb.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/72426/original/image-20150218-20818-fgi4zb.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/72426/original/image-20150218-20818-fgi4zb.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=783&fit=crop&dpr=1 600w, https://images.theconversation.com/files/72426/original/image-20150218-20818-fgi4zb.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=783&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/72426/original/image-20150218-20818-fgi4zb.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=783&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/72426/original/image-20150218-20818-fgi4zb.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=984&fit=crop&dpr=1 754w, https://images.theconversation.com/files/72426/original/image-20150218-20818-fgi4zb.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=984&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/72426/original/image-20150218-20818-fgi4zb.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=984&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Clyde Tombaugh, discoverer of Pluto.</span>
<span class="attribution"><a class="source" href="http://commons.wikimedia.org/wiki/File:Clyde_W._Tombaugh.jpeg">NASA</a></span>
</figcaption>
</figure>
<p>When Pluto was discovered by Clyde Tombaugh in 1930, many astronomers were certain that a large planet orbited the Sun beyond Neptune. Instead they found Pluto, which turned out to be small compared to Earth and Neptune, though more than double the size of Ceres, with a diameter of 2,300 kilometers. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/72922/original/image-20150224-25707-1qna7kr.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/72922/original/image-20150224-25707-1qna7kr.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/72922/original/image-20150224-25707-1qna7kr.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=403&fit=crop&dpr=1 600w, https://images.theconversation.com/files/72922/original/image-20150224-25707-1qna7kr.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=403&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/72922/original/image-20150224-25707-1qna7kr.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=403&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/72922/original/image-20150224-25707-1qna7kr.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=507&fit=crop&dpr=1 754w, https://images.theconversation.com/files/72922/original/image-20150224-25707-1qna7kr.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=507&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/72922/original/image-20150224-25707-1qna7kr.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=507&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Size of Pluto (lower left) in comparison to Moon and Earth.</span>
<span class="attribution"><a class="source" href="http://commons.wikimedia.org/wiki/File:Pluto_Earth_Moon_Comparison.png">Tom.Reding</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Pluto also has an unusual orbit, as it crosses Neptune’s orbit, though it does so in such a way that it can never collide with Neptune. </p>
<p>Pluto’s modern-day troubles began in 1992, when astronomers <a href="http://www.cbat.eps.harvard.edu/iauc/05600/05611.html#Item1">David Jewitt and Jane Luu</a> discovered the first objects in the region of the solar system now known as the Kuiper Belt. Whereas the asteroid belt where Ceres resides is made mostly of house- and mountain-sized rocks that orbit the Sun in between the orbits of Mars and Jupiter, the Kuiper Belt is made mostly of house- and mountain-sized chunks of ice that orbit the Sun beyond the orbit of Neptune. Pluto, as it turns out, is one of the biggest objects in the Kuiper Belt. </p>
<p>So what is Pluto?</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/72665/original/image-20150220-21916-13mi8te.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/72665/original/image-20150220-21916-13mi8te.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/72665/original/image-20150220-21916-13mi8te.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=302&fit=crop&dpr=1 600w, https://images.theconversation.com/files/72665/original/image-20150220-21916-13mi8te.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=302&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/72665/original/image-20150220-21916-13mi8te.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=302&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/72665/original/image-20150220-21916-13mi8te.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=380&fit=crop&dpr=1 754w, https://images.theconversation.com/files/72665/original/image-20150220-21916-13mi8te.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=380&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/72665/original/image-20150220-21916-13mi8te.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=380&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Image of Pluto and its moon Charon, taken by NASA’s New Horizons spacecraft on January 25, 2015, from a distance of 125 million miles.</span>
<span class="attribution"><a class="source" href="http://pluto.jhuapl.edu/Multimedia/Science-Photos/image.php?page=1&gallery_id=2&image_id=103">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</a></span>
</figcaption>
</figure>
<p>Pluto is the last unexplored planet in our solar system. And the Kuiper Belt may contain hundreds of other planetary worlds like Pluto. These may be the most numerous worlds in the solar system; they may contain, together, the most total surface area of all the solid-surfaced planets.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/72666/original/image-20150220-21884-g51mbv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/72666/original/image-20150220-21884-g51mbv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/72666/original/image-20150220-21884-g51mbv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=668&fit=crop&dpr=1 600w, https://images.theconversation.com/files/72666/original/image-20150220-21884-g51mbv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=668&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/72666/original/image-20150220-21884-g51mbv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=668&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/72666/original/image-20150220-21884-g51mbv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=840&fit=crop&dpr=1 754w, https://images.theconversation.com/files/72666/original/image-20150220-21884-g51mbv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=840&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/72666/original/image-20150220-21884-g51mbv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=840&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Pluto and its five moons – as seen from the Hubble Space Telescope in July, 2012.</span>
<span class="attribution"><a class="source" href="http://www.spacetelescope.org/images/heic1212a/">NASA, ESA, and M. Showalter (SETI Institute)</a></span>
</figcaption>
</figure>
<p>Pluto has one large moon, Charon, and at least four small moons: Nix, Hydra, Kerberos and Styx. It has an atmosphere that expands and contracts as Pluto warms and cools during its 248 year orbit around the Sun. The surface is likely rich in water ice, enriched with methane and nitrogen and carbon monoxide frosts; these ices might contain complex organic molecules.</p>
<p>The New Horizons mission is poised to answer some of our myriad questions about Pluto. How did it form? What is the atmosphere made of? What is the surface like? Does Pluto have a magnetic field? What are the moons like? Does Pluto have a subsurface ocean? Is the surface of Pluto’s moon Charon pure water ice?</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/72669/original/image-20150220-21907-1vzn3rr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/72669/original/image-20150220-21907-1vzn3rr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/72669/original/image-20150220-21907-1vzn3rr.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/72669/original/image-20150220-21907-1vzn3rr.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/72669/original/image-20150220-21907-1vzn3rr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=501&fit=crop&dpr=1 754w, https://images.theconversation.com/files/72669/original/image-20150220-21907-1vzn3rr.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=501&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/72669/original/image-20150220-21907-1vzn3rr.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=501&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Launch of NASA’s New Horizons spacecraft from Cape Canaveral, Florida, on January 19, 2006. We’re on our way, Pluto.</span>
<span class="attribution"><a class="source" href="http://www.nasa.gov/mission_pages/newhorizons/launch/#.VOelYb6JnzK">NASA Kennedy</a></span>
</figcaption>
</figure>
<p>Pluto has guarded its secrets for four and half billion years. But in a few months, a few intrepid humans will pull back the curtain on Pluto and say “Hello, Pluto, we’re here.” And Pluto will begin to share her secrets with us. When she does, as with Ceres, our familiarity with Pluto will help us recognize that Pluto is, was, and has always been a planet, albeit a small one. </p>
<p>We only get to visit Ceres and Pluto for the very first time, once. This year. March 6 and July 15. In your lifetime. In this incredible year of the dwarf planet. Get ready to party. Ceres and Pluto are coming home.</p><img src="https://counter.theconversation.com/content/36081/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David A Weintraub has received funding from NASA and the National Science Foundation.</span></em></p>With increasing knowledge and familiarity, we’ll no longer be able to identify meaningful criteria to keep these good planets down.David Weintraub, Professor of Astronomy, Vanderbilt UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/353322014-12-18T19:46:49Z2014-12-18T19:46:49ZRise and shine! New Horizons awakes ahead of a date with Pluto<figure><img src="https://images.theconversation.com/files/67492/original/image-20141217-31021-1t4vqnr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Artist's impression of New Horizons as it swings past the dwarf planet Pluto, in July 2015. </span> <span class="attribution"><a class="source" href="http://commons.wikimedia.org/wiki/File:New_horizons_(NASA).jpg">NASA</a></span></figcaption></figure><p>While the <a href="https://theconversation.com/au/topics/mars-rover">Mars Rovers</a> and the <a href="https://theconversation.com/au/topics/rosetta">Rosetta</a> spacecraft will continue to make headlines in 2015, the stage is set for the solar system’s next great mission – the Pluto-bound <a href="http://www.nasa.gov/mission_pages/newhorizons/main/">New Horizons</a>. </p>
<p>Discovered in 1930, it was assumed that Pluto was a planet, with a mass comparable to that of the Earth. This was partially based on its brightness, along with the mistaken assumption that it was perturbing the orbits of Neptune and Uranus.</p>
<p>As the decades passed, and our observations of Pluto improved, its estimated mass dropped: smaller than Mars, then smaller still – smaller than our moon. Finally, with the discovery in 1978 of Pluto’s largest satellite, Charon, it became possible to precisely determine its mass – just over 0.2% of that of the Earth.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/67515/original/image-20141217-31031-3lgep1.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/67515/original/image-20141217-31031-3lgep1.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/67515/original/image-20141217-31031-3lgep1.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=480&fit=crop&dpr=1 600w, https://images.theconversation.com/files/67515/original/image-20141217-31031-3lgep1.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=480&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/67515/original/image-20141217-31031-3lgep1.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=480&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/67515/original/image-20141217-31031-3lgep1.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=603&fit=crop&dpr=1 754w, https://images.theconversation.com/files/67515/original/image-20141217-31031-3lgep1.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=603&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/67515/original/image-20141217-31031-3lgep1.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=603&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Pluto system – the dwarf planet itself, and its five moons; the giant Charon, along with Styx, Nix, Kerberos and Hydra, as imaged using the Hubble Space Telescope.</span>
<span class="attribution"><span class="source">NASA, ESA, and M Showalter (SETI Institute)</span></span>
</figcaption>
</figure>
<p>Despite its tiny mass and size (Pluto is significantly smaller than our moon), its planetary classification remained until 2006, when the International Astronomical Union created their official definition for the term “planet”. </p>
<p>Pluto, like Ceres before it, was relegated to minor planet status – becoming one of the founder members of the new “dwarf planet” class.</p>
<h2>Air today, gone tomorrow</h2>
<p>Despite its “demotion”, Pluto remains a fascinating object. Highly reflective, Pluto is far brighter than its fellow trans-Neptunian brethren. This contributed to its early discovery, resulting in the overestimates of its size and mass that led to its planetary longevity.</p>
<p>Pluto moves on a relatively eccentric orbit around the sun – one that brings it within the orbit of Neptune for 20 years out of every 248 year orbit. At its furthest from the sun, by contrast, Pluto lurks almost 49 times more distant than the Earth.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/67576/original/image-20141218-31031-jmknj9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/67576/original/image-20141218-31031-jmknj9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/67576/original/image-20141218-31031-jmknj9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=444&fit=crop&dpr=1 600w, https://images.theconversation.com/files/67576/original/image-20141218-31031-jmknj9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=444&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/67576/original/image-20141218-31031-jmknj9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=444&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/67576/original/image-20141218-31031-jmknj9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=558&fit=crop&dpr=1 754w, https://images.theconversation.com/files/67576/original/image-20141218-31031-jmknj9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=558&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/67576/original/image-20141218-31031-jmknj9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=558&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Pluto’s orbit, and the four giant planets, Jupiter, Saturn, Uranus and Neptune. The dwarf planet ranges widely, moving on an orbit far more eccentric than those of the giants.</span>
<span class="attribution"><span class="source">Wikimedia Commons</span></span>
</figcaption>
</figure>
<p>As a result, Pluto’s surface temperature varies dramatically – with the intensity of Solar radiation it receives varying by almost a factor of three through the course of the Plutonian year. </p>
<p>Herein, we think, lies the explanation for Pluto’s anomalous reflectivity. As it swung inwards for its last perihelion passage (which occurred back in 1989), the methane, nitrogen and carbon monoxide ices on its surface began to sublimate, building up a tenuous but measurable atmosphere. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/67512/original/image-20141217-31025-13rb21v.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/67512/original/image-20141217-31025-13rb21v.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/67512/original/image-20141217-31025-13rb21v.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/67512/original/image-20141217-31025-13rb21v.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/67512/original/image-20141217-31025-13rb21v.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/67512/original/image-20141217-31025-13rb21v.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/67512/original/image-20141217-31025-13rb21v.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/67512/original/image-20141217-31025-13rb21v.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Artist’s impression of Pluto’s surface, tenuous atmosphere, and giant satellite Charon.</span>
<span class="attribution"><span class="source">ESO/L. Calçada</span></span>
</figcaption>
</figure>
<p>As it recedes to the depths of the solar system, and the temperature falls once more, that atmosphere is predicted to freeze out, falling back to the surface as fresh snow. </p>
<h2>A whistle-stop tour of the solar system’s freezer</h2>
<p>The ephemeral nature of Pluto’s atmosphere was one of the driving factors behind the development and launch of the New Horizons mission. </p>
<p>The spacecraft, launched back in 2006, holds the record for the highest launch speed of any man-made object. And with good reason – the goal was to reach Pluto before its atmosphere collapsed back to its surface.</p>
<p>After nine and a half year’s travel, New Horizons will tear past Pluto on July 14 next year. Unlike Rosetta, which has the relative luxury of orbiting its target for months, collecting data all the while, Pluto and New Horizons will have the briefest of encounters. </p>
<p>The entire spacecraft is designed to make the most of a tiny window of opportunity – whizzing past the dwarf planet at a relative speed of almost 14km/s, it will hopefully approach to within around 10,000km of Pluto’s surface. At that speed, New Horizons will spend less than 40 hours within a million kilometres of Pluto.</p>
<h2>Awake, Sleeping Beauty</h2>
<p>In February 2007, New Horizons swung close to Jupiter, using the giant planet’s gravitational pull to get an extra kick, speeding it on its way to Pluto. The close encounter was also used to test the spacecraft’s instruments, and it returned spectacular images of the solar system’s biggest, baddest bully, and its satellite retinue.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/67495/original/image-20141217-31043-1k0u76m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/67495/original/image-20141217-31043-1k0u76m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/67495/original/image-20141217-31043-1k0u76m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=405&fit=crop&dpr=1 600w, https://images.theconversation.com/files/67495/original/image-20141217-31043-1k0u76m.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=405&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/67495/original/image-20141217-31043-1k0u76m.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=405&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/67495/original/image-20141217-31043-1k0u76m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=509&fit=crop&dpr=1 754w, https://images.theconversation.com/files/67495/original/image-20141217-31043-1k0u76m.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=509&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/67495/original/image-20141217-31043-1k0u76m.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=509&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A ‘family portrait’ of Jupiter’s four largest moons – the Galilean satellites – taken during New Horizon’s gravity assist flyby, in 2007. From left to right – Io, Europa, Ganymede and Callisto are imaged in striking detail, giving a hint to the spectacle the mission may reveal when it reaches its main target, Pluto.</span>
<span class="attribution"><span class="source">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</span></span>
</figcaption>
</figure>
<p>After that flyby, New Horizons entered a series of lengthy hibernations in order to conserve power for the main event. Occasionally, it awoke, and let everyone back home know that it was still fully functional.</p>
<p>The final wake-up call came just a couple of weeks ago, December 6, and New Horizons is now gearing up for its main mission.</p>
<h2>Early observations to check for plain sailing</h2>
<p>Now that it is awake, the plan is for New Horizons to begin observing Pluto in earnest this coming January.</p>
<p>By the start of May, it will be close enough to the Plutonian system to return images of higher quality than can be achieved with the Hubble Space Telescope. This will allow the mission team to check for any unknown satellites or rings that might lie in the spacecraft’s path. </p>
<p>As it swings closer to Pluto, it will continue to image the dwarf planet and its satellites. The key period for that imaging will be the final few days leading up to the closest approach, as the spacecraft attempts to store as much information as possible to squirt back to Earth over the months that follow. </p>
<h2>A celestial waltz, with a six day spin</h2>
<p>Pluto and its largest satellite, the behemoth Charon, are tidally locked to one another. Each spins with a period of just over six days and nine hours – the same period as it takes for them to orbit around their common centre of mass. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/67583/original/image-20141218-31040-fwx5v3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/67583/original/image-20141218-31040-fwx5v3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/67583/original/image-20141218-31040-fwx5v3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=375&fit=crop&dpr=1 600w, https://images.theconversation.com/files/67583/original/image-20141218-31040-fwx5v3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=375&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/67583/original/image-20141218-31040-fwx5v3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=375&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/67583/original/image-20141218-31040-fwx5v3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=471&fit=crop&dpr=1 754w, https://images.theconversation.com/files/67583/original/image-20141218-31040-fwx5v3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=471&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/67583/original/image-20141218-31040-fwx5v3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=471&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Artist’s impression of Pluto and its behemoth of a moon, Charon. The two are mutually tidally locked, keeping their faces pointed towards one another as they waltz around the sun.</span>
<span class="attribution"><span class="source">ESO</span></span>
</figcaption>
</figure>
<p>This means that Charon keeps one face pointed towards Pluto at all times – just as our moon keeps the same face pointed towards the Earth. But unlike our Earth-moon system, Pluto repays the favour – keeping one face permanently pointed towards Charon. </p>
<p>As New Horizons swings past the system, it will endeavour to produce maps of Pluto – but due to the very rapid flyby, the best images will only reveal one hemisphere of the dwarf planet. Imaging before and after the encounter will help scientists to fill in, to some extent, the rest of Pluto’s surface. </p>
<h2>Not just imaging …</h2>
<p>New Horizons is more than simply an expensive, fast-moving camera platform. In total, it carries seven science packages – each with a different role to play in the survey of Pluto and its satellites. </p>
<p>That said, the images returned at the time of closest approach will be exquisite, and will most likely be what captures the imagination of the viewing public. In addition to full colour maps of Pluto as a whole, there is the potential that features as small as 50 or 100m across will show up in images taken around the time of closest approach. </p>
<p>While the cameras carried by New Horizons are imaging Pluto and its satellites, other instruments will take spectroscopic observations of the dwarf planet’s atmosphere and surface, allowing their chemistry to be studied in detail. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/67585/original/image-20141218-31018-1gf1x43.gif?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/67585/original/image-20141218-31018-1gf1x43.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/67585/original/image-20141218-31018-1gf1x43.gif?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=493&fit=crop&dpr=1 600w, https://images.theconversation.com/files/67585/original/image-20141218-31018-1gf1x43.gif?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=493&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/67585/original/image-20141218-31018-1gf1x43.gif?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=493&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/67585/original/image-20141218-31018-1gf1x43.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=619&fit=crop&dpr=1 754w, https://images.theconversation.com/files/67585/original/image-20141218-31018-1gf1x43.gif?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=619&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/67585/original/image-20141218-31018-1gf1x43.gif?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=619&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The various instruments carried by New Horizons will obtain data that will keep astronomers busy for years, trying to understand Pluto and its satellites.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>At the same time, the SWAP and PEPSSI instruments will be sniffing the outer layers of Pluto’s very diffuse atmosphere, capturing particles and determining their composition, and measuring how the solar wind interacts with the material out-gassed during Pluto’s fleeting summer. </p>
<p>New Horizons will even allow us to learn something of Pluto’s interior structure. By measuring the precise deflection experienced by the spacecraft as it whooshes by the dwarf planet, astronomers will be able to determine, to some degree, the distribution of mass within. </p>
<p>This could reveal the degree to which Pluto is differentiated – whether it has a core, mantle and crust, like the Earth, or is more homogeneous in composition, like our moon. </p>
<h2>A wealth of data, returned in a slow trickle</h2>
<p>Transferring data back from Pluto is a challenging business. At the time of closest encounter, New Horizons will be an incredible five billion kilometres from the Earth – or more than four and a half light-hours. </p>
<p>So distant, the total bandwidth available for the spacecraft to communicate with the Earth will be extremely limited – of order 1kbit/s. Such slow speeds will be familiar to anyone who remembers the era of the dial-up modem – and receiving images back from New Horizons will feel somewhat like browsing the internet in the mid-1990s!</p>
<p>Due to the incredibly low bandwidth, although the core of New Horizons’ Pluto mission will take just around one day to complete, the data obtained is likely to take around nine months to be transmitted back to Earth. </p>
<p>So, rather than there being a glut of New Horizons images at the time of closest approach, we will instead see a slow trickle of new, exciting results, spreading the joy over almost a year. </p>
<h2>To infinity, and beyond …</h2>
<p>Once New Horizons leaves Pluto behind, its work will be far from complete. For the past few years, astronomers have been using the world’s largest telescopes and the Hubble Space Telescope to search for other objects, out beyond the orbit of Neptune, that could be visited by New Horizons during its one-way trip to interstellar space. </p>
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<a href="https://images.theconversation.com/files/67590/original/image-20141218-31040-u44bpe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/67590/original/image-20141218-31040-u44bpe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/67590/original/image-20141218-31040-u44bpe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=447&fit=crop&dpr=1 600w, https://images.theconversation.com/files/67590/original/image-20141218-31040-u44bpe.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=447&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/67590/original/image-20141218-31040-u44bpe.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=447&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/67590/original/image-20141218-31040-u44bpe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=562&fit=crop&dpr=1 754w, https://images.theconversation.com/files/67590/original/image-20141218-31040-u44bpe.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=562&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/67590/original/image-20141218-31040-u44bpe.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=562&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Trajectory followed by New Horizons on its whistle-stop tour of the solar system. After Pluto, it will fly through the Edgeworth-Kuiper belt, and hopefully visit at least one more icy wanderer before voyaging on to the stars.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>The final schedule remains to be set – but thanks to observations using Hubble, the New Horizons team has already identified three potential post-Pluto targets – which go by the imaginative names of PT1, PT2 and PT3. </p>
<p>For now, though, the focus remains on Pluto itself, and humanity’s first encounter with an object beyond the orbit of Neptune.</p><img src="https://counter.theconversation.com/content/35332/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jonti Horner does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>While the Mars Rovers and the Rosetta spacecraft will continue to make headlines in 2015, the stage is set for the solar system’s next great mission – the Pluto-bound New Horizons. Discovered in 1930…Jonti Horner, Vice Chancellor's Senior Research Fellow, University of Southern QueenslandLicensed as Creative Commons – attribution, no derivatives.