tag:theconversation.com,2011:/uk/topics/planetary-formation-17928/articlesPlanetary formation – The Conversation2019-05-23T00:39:05Ztag:theconversation.com,2011:article/1120672019-05-23T00:39:05Z2019-05-23T00:39:05ZCurious Kids: how was the Earth made?<figure><img src="https://images.theconversation.com/files/274810/original/file-20190516-69195-162c6fm.jpg?ixlib=rb-1.1.0&rect=3%2C6%2C2041%2C2038&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Earth is really ancient, and humans have only been around for a tiny part of that time. </span> <span class="attribution"><a class="source" href="https://www.nasa.gov/sites/default/files/1-bluemarble_west.jpg">NASA</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p><em><a href="https://theconversation.com/au/topics/curious-kids-36782">Curious Kids</a> is a series for children. If you have a question you’d like an expert to answer, send it to curiouskids@theconversation.edu.au You might also like the podcast <a href="http://www.abc.net.au/kidslisten/imagine-this/">Imagine This</a>, a co-production between ABC KIDS listen and The Conversation, based on Curious Kids.</em> </p>
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<p><strong>How was the Earth made? - Audrey, age 5.</strong></p>
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<p>More than <a href="https://science.sciencemag.org/content/338/6107/651">4,500,000,000 years ago</a> – before even the dinosaurs existed, before even the Earth existed – there was space.</p>
<p>And in one part of space, there was a huge collection of stars mixed in with massive clouds of gas and dust, that today we call the Milky Way galaxy. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/274794/original/file-20190516-69199-13uc9qz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/274794/original/file-20190516-69199-13uc9qz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/274794/original/file-20190516-69199-13uc9qz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=495&fit=crop&dpr=1 600w, https://images.theconversation.com/files/274794/original/file-20190516-69199-13uc9qz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=495&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/274794/original/file-20190516-69199-13uc9qz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=495&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/274794/original/file-20190516-69199-13uc9qz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=622&fit=crop&dpr=1 754w, https://images.theconversation.com/files/274794/original/file-20190516-69199-13uc9qz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=622&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/274794/original/file-20190516-69199-13uc9qz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=622&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 Milky Way is just one of many galaxies. This is Galaxy NGC 4414, a spiral galaxy just like our own Milky Way.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/File:NGC_4414_(NASA-med).jpg">NASA</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
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Read more:
<a href="https://theconversation.com/curious-kids-the-milky-way-is-huge-but-just-how-huge-117023">Curious Kids: The Milky Way is huge. But just how huge?</a>
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<p>In a small corner of that huge galaxy, in an area that would later become our solar system, there was a big cloud of gas that had been swirling around since the <a href="https://theconversation.com/curious-kids-what-started-the-big-bang-79845">Big Bang</a>. There were also some dusty remains of an old star that had exploded long ago. </p>
<p>The gas and dust were floating, swirling and spinning past each other - but they were all quite far apart. But then… a nearby star exploded, in what we call a supernova.</p>
<p>This supernova sent a shockwave of light and energy rippling across space, pushing some of the gas and dust towards each other. This gas and dust soon became a ball, which started to get bigger and bigger because of gravity. </p>
<p>Gravity makes everything in the universe move towards everything else - and when things get really big (like, planet-size big), they start to pull all nearby things towards it. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/274806/original/file-20190516-69195-15ftlvs.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/274806/original/file-20190516-69195-15ftlvs.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/274806/original/file-20190516-69195-15ftlvs.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=592&fit=crop&dpr=1 600w, https://images.theconversation.com/files/274806/original/file-20190516-69195-15ftlvs.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=592&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/274806/original/file-20190516-69195-15ftlvs.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=592&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/274806/original/file-20190516-69195-15ftlvs.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=744&fit=crop&dpr=1 754w, https://images.theconversation.com/files/274806/original/file-20190516-69195-15ftlvs.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=744&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/274806/original/file-20190516-69195-15ftlvs.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=744&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 Eagle Nebula, filled with gas and dust, and currently the birthplace of lots of new stars.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/File:Eagle_nebula_pillars.jpg">Hubble Telescope/NASA</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
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<p>As the ball of gas and dust got bigger, the gas and dust started to crush in on itself until something called a “nuclear reaction” happened right in the middle of the ball. A nuclear reaction is super powerful, and this particular one turned our Sun into a brilliantly shining star, throwing light across the rest of the gas and dust that was still spinning around it.</p>
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Read more:
<a href="https://theconversation.com/curious-kids-why-has-nobody-found-any-life-outside-of-earth-105128">Curious Kids: why has nobody found any life outside of Earth?</a>
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<h2>Gas and dust started clumping together to form planets</h2>
<p>Some of those other swirling, twirling chunks of gas and dust (that hadn’t been sucked into the Sun) were bumping and clumping into each other. Soon, those clumps got big enough that gravity started pulling in all the other gas and dust around it, all while still going round and round the giant shining Sun. </p>
<p>Some of these twirling bits clumped together to make our Earth. Others clumped together to make Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune – and all of their moons too.</p>
<p>All these baby planets swirled and spun, and pulled in all the nearby matter. They squished together to become super-tight big giant hot balls of spinning stuff. </p>
<p>Our own Earth was getting hit by rocks that were falling towards it. It kept getting bigger and hotter until it was a giant ball of melted rock. </p>
<p>Then, a <em>really</em> huge rock smashed into Earth and made it even bigger. And a little bit of <em>that</em> rock flew off and floated into space to make the Moon.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/274809/original/file-20190516-69178-18t2acm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/274809/original/file-20190516-69178-18t2acm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/274809/original/file-20190516-69178-18t2acm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=358&fit=crop&dpr=1 600w, https://images.theconversation.com/files/274809/original/file-20190516-69178-18t2acm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=358&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/274809/original/file-20190516-69178-18t2acm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=358&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/274809/original/file-20190516-69178-18t2acm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=450&fit=crop&dpr=1 754w, https://images.theconversation.com/files/274809/original/file-20190516-69178-18t2acm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=450&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/274809/original/file-20190516-69178-18t2acm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=450&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">Early on, a big bit of rock hit Earth. And a little bit of it flew off and floated into space to make the Moon.</span>
<span class="attribution"><a class="source" href="https://astrobiology.nasa.gov/news/support-for-a-catastrophic-formation-of-the-moon/">NASA/JPL-CALTECH/T. PYLE</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
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<p>
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<strong>
Read more:
<a href="https://theconversation.com/curious-kids-is-there-anything-hotter-than-the-sun-105748">Curious Kids: Is there anything hotter than the Sun?</a>
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<p>So the Earth was just out there floating in space, near the Sun. But it looked totally different to the Earth we live on today. There were volcanoes all over the place, with hot lava and gas everywhere. </p>
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<a href="https://images.theconversation.com/files/274808/original/file-20190516-69174-1pteuj2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/274808/original/file-20190516-69174-1pteuj2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/274808/original/file-20190516-69174-1pteuj2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/274808/original/file-20190516-69174-1pteuj2.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/274808/original/file-20190516-69174-1pteuj2.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/274808/original/file-20190516-69174-1pteuj2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/274808/original/file-20190516-69174-1pteuj2.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/274808/original/file-20190516-69174-1pteuj2.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>
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<span class="caption">An artist’s impression of a hot planet.</span>
<span class="attribution"><a class="source" href="https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA15808">NASA/JPL-Caltech</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
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<h2>Cooling down</h2>
<p>But slowly over many years, Earth started to cool down. Some rocks full of ice and gas hit it and melted to make the sea. </p>
<p>This is continuing today - every year more than three tonnes of space rocks hit the Earth.</p>
<p>But slowly, over many years, the top layer of the Earth was cool enough to harden. This is the ground we walk on today. We call it the Earth’s crust, like a crust of bread. Deep down underground, the Earth is still full of melted hot rock. </p>
<p>And gradually, over a long time, plants started to grow, bugs started to live and life on Earth began to form (which is a whole story on its own).</p>
<p>Earth is really ancient, and humans have only been around for a tiny part of that. All the buildings and the cars and the restaurants, and the phones and even everything that’s inside of you… it all started with an exploding star, billions of years ago. </p>
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<em>
<strong>
Read more:
<a href="https://theconversation.com/curious-kids-what-existed-before-the-big-bang-did-something-have-to-be-there-to-go-boom-103742">Curious Kids: What existed before the Big Bang? Did something have to be there to go boom?</a>
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<p><em>Hello, curious kids! Have you got a question you’d like an expert to answer? Ask an adult to send your question to curiouskids@theconversation.edu.au</em></p>
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<img alt="" src="https://images.theconversation.com/files/168011/original/file-20170505-21620-huq4lj.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/168011/original/file-20170505-21620-huq4lj.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=376&fit=crop&dpr=1 600w, https://images.theconversation.com/files/168011/original/file-20170505-21620-huq4lj.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=376&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/168011/original/file-20170505-21620-huq4lj.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=376&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/168011/original/file-20170505-21620-huq4lj.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=472&fit=crop&dpr=1 754w, https://images.theconversation.com/files/168011/original/file-20170505-21620-huq4lj.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=472&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/168011/original/file-20170505-21620-huq4lj.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=472&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="attribution"><a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
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<p><em>Please tell us your name, age and which city you live in. We won’t be able to answer every question but we will do our best.</em></p><img src="https://counter.theconversation.com/content/112067/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Dr Niraj Lal is a Visiting Fellow at the Australian National University Centre for Sustainable Energy Systems, Director of First Principles Consulting, Senior Manager for Technology at Solar Victoria in the Victorian Government, and presenter of ABC Sciencey on iView. He is a member of the Australian Institute of Physics, Australian Science Communicators, IEEE, and the Australian Greens Party, and his consultancy has received funding from the ABC.</span></em></p>All the buildings and the cars and the restaurants, and the phones and even everything that’s inside of you… it all started with an exploding star, billions of years ago.Niraj Lal, Visiting Fellow at the ANU Centre for Sustainable Energy Systems, Australian National UniversityLicensed 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>
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<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>
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<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/1001602018-07-25T04:35:50Z2018-07-25T04:35:50ZJupiter’s new moons: an irregular bunch with an extra oddball that’s the smallest discovered so far<figure><img src="https://images.theconversation.com/files/228790/original/file-20180723-189335-bt90kx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A moon shadow on Jupiter, the red planet now has a dozen more moons added to the list or such orbiting bodies.</span> <span class="attribution"><a class="source" href="https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA21969">NASA/JPL-Caltech/SwRI/MSSS</a></span></figcaption></figure><p><a href="https://solarsystem.nasa.gov/planets/jupiter/overview/">Jupiter</a> is the largest planet in the Solar system and has been <a href="https://solarsystem.nasa.gov/planets/jupiter/exploration/">studied intensively for hundreds of years</a>, so you might think there would be little left to find.</p>
<p>But earlier this month, researchers announced that another <a href="https://carnegiescience.edu/news/dozen-new-moons-jupiter-discovered-including-one-%E2%80%9Coddball%E2%80%9D">12 moons have been added</a> to the number of such bodies orbiting the giant planet.</p>
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<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/the-latest-from-juno-as-jupiter-appears-bright-in-the-night-sky-96108">The latest from Juno as Jupiter appears bright in the night sky</a>
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</p>
<hr>
<p>That brings the tally for Jupiter to a whopping 79, the most moons for any known planet. But where did these newly discovered moons come from, and what do they tell us about Jupiter and its place in the Solar system?</p>
<h2>Moons: regular and irregular</h2>
<p>The Solar system’s giant planets have two types of moon: regular and irregular. </p>
<p>Regular moons orbit close to their host, follow nearly circular paths, and move in the same plane as the planet’s equator. In some ways, these moons resemble miniature planetary systems, and we think that they formed in much the same manner <a href="https://theconversation.com/pluto-and-its-collision-course-place-in-our-solar-system-43404">as the planets around the Sun</a>. </p>
<p>As the giant planets gathered material from the disk of gas and dust that surrounded the young Sun – a process known as accretion – they were surrounded by their own miniature disks. Within those disks, the regular moons grew, all in the planet’s equatorial plane.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/228522/original/file-20180720-142432-h0y9mz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/228522/original/file-20180720-142432-h0y9mz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/228522/original/file-20180720-142432-h0y9mz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/228522/original/file-20180720-142432-h0y9mz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/228522/original/file-20180720-142432-h0y9mz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/228522/original/file-20180720-142432-h0y9mz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/228522/original/file-20180720-142432-h0y9mz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/228522/original/file-20180720-142432-h0y9mz.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">Artist’s impression of a protoplanetary disk - a place where planets are born. Around young giant planets, similar disks give birth to regular moons.</span>
<span class="attribution"><span class="source">ESO/L. Calçada</span></span>
</figcaption>
</figure>
<p>But the irregular moons are another story.</p>
<p>Their orbits are highly eccentric (elliptical) and inclined relative to the plane of their host planet’s equator. Many even move on retrograde orbits, travelling in the opposite direction to the spin and orbital motion of their hosts. And they are located much farther from their planet than their regular cousins. </p>
<h2>Where do the irregulars come from?</h2>
<p>Because of their wild orbits, the irregular moons cannot have formed in the same way as their regular cousins. Instead, they are thought to have been <a href="http://home.dtm.ciw.edu/users/sheppard/pub/Nicholson2008KBOBook.pdf">captured by their host planets as the process of planet formation came to an end</a>. </p>
<p>We think that each giant planet captured just a handful of irregular moons – a number far smaller than we see today. Over the billions of years since, those moons were pummelled and destroyed by passing asteroids and comets, and collisions with other members of their swarm.</p>
<p>The shattered fragments of those ancient satellites form families of smaller moons - the irregulars we see today. For example, among Jupiter’s satellites we see <a href="https://arxiv.org/pdf/1706.01423.pdf">at least four distinct families</a> of irregular moons, each named after their largest member. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/228674/original/file-20180721-142435-1j9s0vv.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/228674/original/file-20180721-142435-1j9s0vv.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/228674/original/file-20180721-142435-1j9s0vv.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=571&fit=crop&dpr=1 600w, https://images.theconversation.com/files/228674/original/file-20180721-142435-1j9s0vv.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=571&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/228674/original/file-20180721-142435-1j9s0vv.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=571&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/228674/original/file-20180721-142435-1j9s0vv.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=718&fit=crop&dpr=1 754w, https://images.theconversation.com/files/228674/original/file-20180721-142435-1j9s0vv.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=718&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/228674/original/file-20180721-142435-1j9s0vv.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=718&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 motion of Jupiter’s irregular moons around the giant planet. The main plot (bottom, left) shows the orbits looking top-down, while the other (right and top) plots show the movement out of the plane of the system. Moons of the same colour are members of the same family.</span>
<span class="attribution"><span class="source">Christopher Tylor</span></span>
</figcaption>
</figure>
<h2>What does the new discovery add to our understanding?</h2>
<p>If we consider Jupiter’s moons in terms of their orbital distance, and the direction in which they move, we can break them into three distinct groups. </p>
<p>The first consists of the inner eight moons, including the famous <a href="https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA01299">Galilean moons</a> Io, Europa, Ganymede and Callisto, whose orbits lie in the plane of Jupiter’s equator, at distances less than 2 million kilometres. </p>
<p>The second group lies significantly farther from the planet, and move on orbits tilted by between 25° and 56° relative to Jupiter’s equator. These are the prograde irregulars - ten moons orbiting at distances between 7 million and 19 million km. Two of the new discoveries are members of this group.</p>
<p>The final and most populous group is the retrograde irregulars - 60 moons located between 19 million and 29 million kilometres from Jupiter, all moving on orbits inclined by between about 140° and 170° to Jupiter’s equator.</p>
<p>In other words, they orbit backwards, in the opposite direction to everything else. Nine of the new discoveries fall into this category.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/228382/original/file-20180719-142420-17w9qtk.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/228382/original/file-20180719-142420-17w9qtk.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/228382/original/file-20180719-142420-17w9qtk.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=375&fit=crop&dpr=1 600w, https://images.theconversation.com/files/228382/original/file-20180719-142420-17w9qtk.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=375&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/228382/original/file-20180719-142420-17w9qtk.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=375&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/228382/original/file-20180719-142420-17w9qtk.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=471&fit=crop&dpr=1 754w, https://images.theconversation.com/files/228382/original/file-20180719-142420-17w9qtk.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=471&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/228382/original/file-20180719-142420-17w9qtk.png?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">Plot showing the three groups of moons orbiting Jupiter.</span>
<span class="attribution"><span class="source">Carnegie Institution for Science/Roberto Molar-Candamosa</span></span>
</figcaption>
</figure>
<p>So that covers 11 of our new moons. What of the 12th? Well, it turns out that the most exciting of the new moons is an oddball - an object that does not fit into any of the groups mentioned above. </p>
<h2>The oddball: Valetudo</h2>
<p>The 12th new moon has tentatively been named Valetudo, after Jupiter’s mythological great granddaughter.</p>
<p>Valetudo is the dimmest of the newly discovered moons. At just a kilometre in diameter (or less), it is the smallest Jovian moon found to date. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/228519/original/file-20180720-142432-1u1ipnd.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/228519/original/file-20180720-142432-1u1ipnd.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/228519/original/file-20180720-142432-1u1ipnd.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=737&fit=crop&dpr=1 600w, https://images.theconversation.com/files/228519/original/file-20180720-142432-1u1ipnd.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=737&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/228519/original/file-20180720-142432-1u1ipnd.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=737&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/228519/original/file-20180720-142432-1u1ipnd.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=926&fit=crop&dpr=1 754w, https://images.theconversation.com/files/228519/original/file-20180720-142432-1u1ipnd.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=926&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/228519/original/file-20180720-142432-1u1ipnd.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=926&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 yellow lines point to the tiny moving speck of light, the newly discovered moon Valetudo.</span>
<span class="attribution"><span class="source">Carnegie Institute for Science</span></span>
</figcaption>
</figure>
<p>In terms of its orbital distance, Valetudo lies bang in the middle of the retrograde irregulars - some 24 million kilometres from the giant planet. But its orbit is prograde - meaning that it moves in the direction of Jupiter’s rotation, and in the opposite direction to all other satellites in its vicinity.</p>
<p>Valetudo’s size and unusual orbit pose interesting questions.</p>
<p>How did something so small survive in the celestial firing range around Jupiter? </p>
<p>Could Valetudo be the final surviving remnant of a previously uncharted family, whittled to nothing by aeons of headlong flight into the retrograde irregulars? </p>
<p>Are there are other members of the Valetudo family out there, awaiting discovery?</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/water-water-everywhere-in-our-solar-system-but-what-does-that-mean-for-life-76315">Water, water, everywhere in our Solar system but what does that mean for life?</a>
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</p>
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<p>Beyond these questions, Valetudo’s small size offers an important clue to the origin of the Jovian satellite system. Had Valetudo been on its current orbit while Jupiter was still accreting, it would have been too small to resist the drag of the inflowing gas. Like a ping pong ball in a gale, it would have been dragged inwards, to be devoured by the giant planet.</p>
<p>In other words, tiny Valetudo tells us that the process that created the irregular satellite families continued long after the formation of Jupiter was complete. In fact, that process likely continues even now, with occasional collisions tearing moons asunder, to birth new families of irregular worlds. </p>
<p>Who knows? The next such collision might come when Valetudo runs into one of the retrograde irregulars. Given that their orbits cross, it may only be a matter of time.</p><img src="https://counter.theconversation.com/content/100160/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Jupiter now has at least 79 moons, the most for any known planet. But where did these newly discovered moons come from?Jonti Horner, Professor (Astrophysics), University of Southern QueenslandChristopher C.E. Tylor, PhD Candidate, Adjunct Lecturer, Assistant Examiner, University of Southern QueenslandLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/961082018-05-08T05:25:13Z2018-05-08T05:25:13ZThe latest from Juno as Jupiter appears bright in the night sky<figure><img src="https://images.theconversation.com/files/218016/original/file-20180508-46359-jrtpun.jpg?ixlib=rb-1.1.0&rect=1036%2C0%2C2067%2C1264&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Time to peer below the swirling clouds of Jupiter.</span> <span class="attribution"><a class="source" href="https://photojournal.jpl.nasa.gov/catalog/PIA21974">NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>Now is a great time to see Jupiter in the night sky, as the planet reaches opposition on Wednesday, May 9.</p>
<p>Opposition means that Jupiter sits opposite the Sun in the sky. So tonight, as the Sun sets in the west, Jupiter can be found rising in the east. It’s lovely and bright, outshining all the stars of the night sky.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/218018/original/file-20180508-46347-1q0cjvz.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/218018/original/file-20180508-46347-1q0cjvz.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/218018/original/file-20180508-46347-1q0cjvz.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=443&fit=crop&dpr=1 600w, https://images.theconversation.com/files/218018/original/file-20180508-46347-1q0cjvz.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=443&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/218018/original/file-20180508-46347-1q0cjvz.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=443&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/218018/original/file-20180508-46347-1q0cjvz.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=557&fit=crop&dpr=1 754w, https://images.theconversation.com/files/218018/original/file-20180508-46347-1q0cjvz.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=557&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/218018/original/file-20180508-46347-1q0cjvz.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=557&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"></span>
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</figure>
<p>In fact, opposition also means that Jupiter is at its closest to Earth, making the planet shine even more brilliantly than usual. So be sure to look east over the next few weeks to catch Jupiter at its best.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/218019/original/file-20180508-46364-18ohfar.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/218019/original/file-20180508-46364-18ohfar.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/218019/original/file-20180508-46364-18ohfar.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=392&fit=crop&dpr=1 600w, https://images.theconversation.com/files/218019/original/file-20180508-46364-18ohfar.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=392&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/218019/original/file-20180508-46364-18ohfar.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=392&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/218019/original/file-20180508-46364-18ohfar.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=493&fit=crop&dpr=1 754w, https://images.theconversation.com/files/218019/original/file-20180508-46364-18ohfar.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=493&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/218019/original/file-20180508-46364-18ohfar.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=493&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Jupiter will be seen throughout May, rising in the east at sunset, ahead of the constellation Scorpius.</span>
<span class="attribution"><span class="source">Museums Victoria/stellarium</span></span>
</figcaption>
</figure>
<h2>Jupiter like we’ve never seen it before</h2>
<p>Also catching Jupiter at its best will be NASA’s spacecraft, <a href="https://www.missionjuno.swri.edu/">Juno</a>. After a five-year journey, Juno entered orbit around Jupiter in mid-2016. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/early-images-of-the-closest-look-at-jupiters-great-red-spot-80808">Early images of the closest look at Jupiter's Great Red Spot</a>
</strong>
</em>
</p>
<hr>
<p>It’s the second spacecraft to orbit Jupiter (after <a href="https://www.jpl.nasa.gov/missions/galileo/">Galileo</a> in 1995), but importantly it’s the first to orbit Jupiter’s poles, allowing us to see a part of the planet that can’t be seen from Earth.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/217983/original/file-20180507-46350-ib2s4u.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/217983/original/file-20180507-46350-ib2s4u.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/217983/original/file-20180507-46350-ib2s4u.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=770&fit=crop&dpr=1 600w, https://images.theconversation.com/files/217983/original/file-20180507-46350-ib2s4u.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=770&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/217983/original/file-20180507-46350-ib2s4u.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=770&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/217983/original/file-20180507-46350-ib2s4u.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=967&fit=crop&dpr=1 754w, https://images.theconversation.com/files/217983/original/file-20180507-46350-ib2s4u.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=967&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/217983/original/file-20180507-46350-ib2s4u.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=967&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 new look at Jupiter, where multiple images have been combined to show the south pole in full sunlight.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech/SwRI/MSSS/Betsy Asher Hall/Gervasio Robles</span></span>
</figcaption>
</figure>
<p>Juno has shown us that Jupiter’s colourful bands – the clearly defined belts and zones (the dark and light bands, respectively) circling the bulk of the planet – give way to a striking configuration of cyclones at each of Jupiter’s poles. </p>
<p>Discovering cyclones at the top and bottom of Jupiter is not completely unexpected, but what’s surprising is their stability and the patterns they have formed.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/217988/original/file-20180507-46335-cval8e.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/217988/original/file-20180507-46335-cval8e.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/217988/original/file-20180507-46335-cval8e.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/217988/original/file-20180507-46335-cval8e.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/217988/original/file-20180507-46335-cval8e.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/217988/original/file-20180507-46335-cval8e.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/217988/original/file-20180507-46335-cval8e.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/217988/original/file-20180507-46335-cval8e.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"></a>
<figcaption>
<span class="caption">Jupiter’s northern cyclones in infrared, which captures the radiating heat. In this original image, darker regions are colder and cloudier, while brighter regions are relatively cloud-free, allowing us to look deeper.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM</span></span>
</figcaption>
</figure>
<p>At the north pole, one central cyclone is surrounded by eight outer cyclones. In the south, the central cyclone has five others encircling it.</p>
<p>These cyclones are huge – the southern ones range from 5,600km to 7,000km in diameter; that’s about as <a href="https://solarsystem.nasa.gov/planets/mars/by-the-numbers/">wide as Mars</a>. The northern ones are slightly smaller, with diameters of around 4,000km to 4,600km. The wind speeds are as great as 350km per hour.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/217991/original/file-20180507-46359-1h7epum.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/217991/original/file-20180507-46359-1h7epum.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/217991/original/file-20180507-46359-1h7epum.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=465&fit=crop&dpr=1 600w, https://images.theconversation.com/files/217991/original/file-20180507-46359-1h7epum.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=465&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/217991/original/file-20180507-46359-1h7epum.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=465&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/217991/original/file-20180507-46359-1h7epum.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=585&fit=crop&dpr=1 754w, https://images.theconversation.com/files/217991/original/file-20180507-46359-1h7epum.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=585&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/217991/original/file-20180507-46359-1h7epum.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=585&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Jupiter’s southern cyclones. Note this enhanced image shows an inverted view, the darker regions are deep, while the higher, thicker clouds are white. This view aims to match the way we see clouds in space images of Earth.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM</span></span>
</figcaption>
</figure>
<p>Over the seven months of <a href="https://www.nature.com/articles/nature25491#f4">observations analysed so far</a>, the cyclones have remained surprisingly distinct, with no signs that they might merge together. Yet most of the cyclones are so tightly packed that their spiral arms are touching. (You can see the movement of the cyclones in this <a href="http://junocam.pictures/gerald/uploads/20170424/anim/jnc_pj05_N_089_to_105_blend4_enh.html">raw footage</a>.) </p>
<p>Also, the pattern itself is highly stable and shows barely any motion. Even though there’s a central cyclone churning around the pole, its motion doesn’t seem to be pushing the outer cyclones to circle around it (a la “Ring a Ring o’ Roses”). If they are circling the pole, then they must be drifting very slowly. </p>
<h2>Juno - please drive safely</h2>
<p>The other exciting thing about Juno is that it was built to probe the inner depths of Jupiter. One way it does this is by intricately <a href="https://www.nature.com/articles/nature25776">mapping Jupiter’s gravitational field</a> to a precision 100 times better than ever before.</p>
<p>Every 53 days, Juno carries out a stunning Jupiter flyby. The probe takes two hours to travel from one pole to the other, zipping past at more than 200,000km/h and skimming just 4,000km above Jupiter’s cloud tops.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/NsCirkzmfmk?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The latest Jupiter flyby (from April, 2018) shown in 70 seconds. NASA/JPL/SwRI/MSSS/SPICE/Gerald Eichstädt.</span></figcaption>
</figure>
<p>As Juno races by the planet it feels the gravitational tug of Jupiter. It speeds up slightly when flying over regions of high mass and slows down wherever the mass drops off. </p>
<p>These tiny changes in Juno’s speed are measured using a kind of interplanetary radar gun; Juno transmits a radio signal of a certain frequency and when it arrives here on Earth, any change to that frequency alerts us to Juno’s changing speed.</p>
<h2>Feeling the pressure</h2>
<p>If we think about Earth, there is a clear distinction between the clouds, the atmosphere, and the rocky planet itself. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/218020/original/file-20180508-46341-1gd4s0b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/218020/original/file-20180508-46341-1gd4s0b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/218020/original/file-20180508-46341-1gd4s0b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=225&fit=crop&dpr=1 600w, https://images.theconversation.com/files/218020/original/file-20180508-46341-1gd4s0b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=225&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/218020/original/file-20180508-46341-1gd4s0b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=225&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/218020/original/file-20180508-46341-1gd4s0b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=282&fit=crop&dpr=1 754w, https://images.theconversation.com/files/218020/original/file-20180508-46341-1gd4s0b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=282&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/218020/original/file-20180508-46341-1gd4s0b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=282&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Swirling cloud belts of Jupiter’s northern hemisphere.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech/SwRI/MSSS/Kevin M Gill</span></span>
</figcaption>
</figure>
<p>But being made of gas, Jupiter is essentially all atmosphere. By definition, the planet begins when the atmospheric pressure of its gas equals 1 bar. That’s equivalent to the pressure we feel at sea level on Earth. </p>
<p>This provides a kind of surface for Jupiter, as such, although it is biased by our Earthling viewpoint. Juno is already giving us much better insights into how Jupiter is truly structured.</p>
<p>Previously what we’ve seen of Jupiter, the banded belts and zones, are the cloud tops sitting just above the planet’s “surface”. They circle around the planet, with alternate bands moving in opposite directions.</p>
<p>Juno’s data has shown that this banding <a href="https://www.nature.com/articles/nature25793">continues deep into Jupiter</a>, appearing to be much more than just a thin layer of weather (that’s driven by the Sun). </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/rHwkdcppsuo?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Swirling clouds of Jupiter from Voyager 1 (1979). NASA/JPL/Björn Jónsson/Ian Regan.</span></figcaption>
</figure>
<h2>How low can you go?</h2>
<p>Juno’s gravity mapping was separated into two components: a static component, modelled as Jupiter’s gas rotating as one; and a dynamic component, arising from flows. </p>
<p>The dynamic component was revealed by a north-south asymmetry in Jupiter’s gravity field. What that means is that the way gravity varied from the equator up to the north pole was not consistent with how it changed from the equator down to the south pole.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/218027/original/file-20180508-46356-cnfc64.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/218027/original/file-20180508-46356-cnfc64.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/218027/original/file-20180508-46356-cnfc64.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/218027/original/file-20180508-46356-cnfc64.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/218027/original/file-20180508-46356-cnfc64.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/218027/original/file-20180508-46356-cnfc64.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/218027/original/file-20180508-46356-cnfc64.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/218027/original/file-20180508-46356-cnfc64.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"></a>
<figcaption>
<span class="caption">Jupiter is turbulent above and below.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech/SwRI/MSSS/Gerald Eichstadt/Sean Doran</span></span>
</figcaption>
</figure>
<p>It also became clear that these changes in gravity tracked the banded structure of Jupiter’s cloud layer. </p>
<p>As a result, the cloud tops must extend into Jupiter, becoming swirling jet streams that reach depths of 3,000km. The amount of mass swirling around was calculated to be 1% of Jupiter’s total mass – more than triple the mass of the Earth.</p>
<h2>Is there a ‘planet’ deep within?</h2>
<p>By analysing the <a href="https://www.nature.com/articles/nature25775">static component of Jupiter’s gravitational field</a>, it was found that there is a point where Jupiter’s gas starts to rotate in harmony, like a rigid sphere. </p>
<p>It sits below a wind depth of at least 2,000km but less than 3,500km, which is well consistent with the jet stream findings. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/launching-in-may-the-insight-mission-will-measure-marsquakes-to-explore-the-interior-of-mars-91080">Launching in May, the InSight mission will measure marsquakes to explore the interior of Mars</a>
</strong>
</em>
</p>
<hr>
<p>At this depth, the pressure is 100,000 times greater than what we feel at the Earth’s surface and temperatures soar. Electric currents flowing through the compressed hydrogen gas and constrained by Jupiter’s powerful magnetic field, are thought to slow the winds down and drag the gas into uniform motion. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/LPvfeOiKbm8?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">First views of Jupiter’s magnetic dynamo shows irregularities and intense magnetic hot spots. NASA Goddard Space Flight Center.</span></figcaption>
</figure>
<p>As Juno continues to swing by Jupiter, scientists hope to better understand the <a href="https://www.nasa.gov/feature/jpl/nasa-s-juno-mission-provides-infrared-tour-of-jupiter-s-north-pole">dynamo powering Jupiter’s magnetic field</a> and ultimately to determine if Jupiter has a solid core, made of some kind of icy rock subjected to more than 50 million bars of pressure. Now that’s truly out of this world.</p><img src="https://counter.theconversation.com/content/96108/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>Now’s a great time to see Jupiter as it’s about to be the closest to Earth for some time. Time too to catch up with the latest on the Juno mission, exploring the largest planet in our Solar System.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/784592017-05-31T04:53:33Z2017-05-31T04:53:33ZJuno mission unveils Jupiter’s complex interior, weather and magnetism<figure><img src="https://images.theconversation.com/files/171379/original/file-20170530-25261-1tkieiz.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">This enhanced-color image of Jupiter’s south pole and its swirling atmosphere was created by citizen scientist Roman Tkachenko using data from the JunoCam imager on NASA’s Juno spacecraft.</span> <span class="attribution"><a class="source" href="https://www.nasa.gov/image-feature/jpl/pia21381/jupiter-from-below-enhanced-color">NASA/JPL-Caltech/SwRI/MSSS/Roman Tkachenko</a></span></figcaption></figure><p>The latest observations of the <a href="https://www.nasa.gov/mission_pages/juno/main/index.html">Juno spacecraft</a> are helping astronomers uncover the true nature of Jupiter in unprecedented detail. Many of the findings were unexpected.</p>
<p>Since July 2016, Juno has been revolving around Jupiter – the largest planet in our Solar System – in a highly elongated, 53-day orbit. This allows a clear view of its poles while the spacecraft ducks in and out of the strong radiation regions that surround the planet. </p>
<p>The first results of Juno observations were released in <a href="http://science.sciencemag.org/content/356/6340/821">two</a> <a href="http://science.sciencemag.org/content/356/6340/826">studies</a> published in Science last week. They reveal a very new picture of the Jovian interior, its atmosphere and magnetosphere.</p>
<p>Of course it’s not only the observations from Juno that are helping us better understand Jupiter. Simultaneous monitoring from ground based telescopes such as the ones on Mauna Kea in Hawaii, where I was recently, are also helping.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/171555/original/file-20170531-23667-13tse61.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/171555/original/file-20170531-23667-13tse61.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/171555/original/file-20170531-23667-13tse61.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=422&fit=crop&dpr=1 600w, https://images.theconversation.com/files/171555/original/file-20170531-23667-13tse61.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=422&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/171555/original/file-20170531-23667-13tse61.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=422&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/171555/original/file-20170531-23667-13tse61.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=530&fit=crop&dpr=1 754w, https://images.theconversation.com/files/171555/original/file-20170531-23667-13tse61.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=530&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/171555/original/file-20170531-23667-13tse61.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">Here I am on Mauna Kea in Hawaii.</span>
<span class="attribution"><span class="source">Marcel de Vriend</span></span>
</figcaption>
</figure>
<p>But first to the latest Juno discoveries.</p>
<h2>The atmosphere</h2>
<p>Juno’s multiple passes over polar regions of the planet revealed stunning images of swirling cyclones, some almost as large as Earth. </p>
<p>There is no banded structure visible in these images, in contrast to Jupiter’s equatorial regions. There is no hexagon or a central vortex in the southern polar region like the one that the <a href="https://www.nasa.gov/mission_pages/cassini/whycassini/cassini20130429.html">Cassini probe observed</a> in Saturn’s north polar atmosphere. </p>
<p>It also appears that a high-altitude thin cloud or a haze, of yet unknown composition, hovers over both poles of Jupiter. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/171418/original/file-20170530-16298-knsccb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/171418/original/file-20170530-16298-knsccb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/171418/original/file-20170530-16298-knsccb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=468&fit=crop&dpr=1 600w, https://images.theconversation.com/files/171418/original/file-20170530-16298-knsccb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=468&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/171418/original/file-20170530-16298-knsccb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=468&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/171418/original/file-20170530-16298-knsccb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=588&fit=crop&dpr=1 754w, https://images.theconversation.com/files/171418/original/file-20170530-16298-knsccb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=588&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/171418/original/file-20170530-16298-knsccb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=588&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Juno’s view of Jupiter’s south pole from an altitude of 52,000 kilometers. The oval features are cyclones, up to 1,000 kilometers in diameter.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/press-release/a-whole-new-jupiter-first-science-results-from-nasa-s-juno-mission">NASA/JPL-Caltech/SwRI/MSSS/Betsy Asher Hall/Gervasio Robles</a></span>
</figcaption>
</figure>
<p>Juno’s radiometry measurements can probe the atmosphere to the unprecedented depth of 350km. This takes it below the frozen ammonia cloud top that we usually see in visible light images of the planet, with atmospheric pressures up to 240 times greater than on Earth’s surface.</p>
<p>Astronomers have already studied the rich and dynamic Jovian weather system since the first space observations of its atmospheric composition and profiles from the <a href="https://voyager.jpl.nasa.gov/">Voyager probes</a>. But the previous deepest atmospheric measurements could be considered skin-deep when compared with Juno’s latest observations. </p>
<p>Only in one specific area of the planet was the atmosphere studied up to a depth of 100km. That was in 1995, when the Galileo probe descended into a so-called “hot spot”, a dark region between ammonia clouds that glows strongly in infrared light. Galileo’s probe measurements found this region surprisingly devoid of any water vapour clouds, as would have been expected below ammonia cloud. </p>
<p>Now, for the first time Juno’s radiometry allows a global view of deep atmosphere, showing that the banded pattern extends deeply below the visible tops of the clouds. </p>
<p>The measurements of ammonia content in these deep layers reveals an unexpected and dynamic mixing similar to the <a href="https://www.britannica.com/science/Hadley-cell">Hadley cells</a> in Earth’s atmosphere. This is where masses of hot air rise in equatorial regions and move polewards, before plummeting in the tropics and returning towards the Equator close to Earth’s surface.</p>
<p>One of the goals of the Juno mission was to measure water content in the Jovian atmosphere, which has implications for understanding Solar System formation.</p>
<p>So far Juno has confirmed that the hot spots are indeed very dry regions of descending air with humidity less than 10%.</p>
<h2>The magnetosphere</h2>
<p>Since the <a href="https://radiojove.gsfc.nasa.gov/library/sci_briefs/discovery.html">discovery of strong radio emission from Jupiter</a> in the 1950s – implying the existence of a magnetic field around the planet – every new space mission has slowly added to the ever so complex picture of the Jovian magnetosphere.</p>
<p>The Juno mission is designed to make an unprecedented leap forward in the understanding magnetic field generation processes and also to make a detailed map of the planet’s magnetosphere. </p>
<p>One of the most spectacular consequences of interaction between the magnetosphere and atmosphere of a planet is an auroral display, similar to the northern and southern lights on Earth. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/BWOSGI1WrNA?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Jupiter’s ‘southern lights’ as captured by Juno. (NASA/JPL-Caltech/SWRI)</span></figcaption>
</figure>
<p>The <a href="https://www.missionjuno.swri.edu/spacecraft/juno-spacecraft/">JADE, JEDI and Waves instruments</a> placed on Juno are used to measure the energies of particles that plummet into the polar regions and smash into atmospheric gases, mainly hydrogen that emits radiation, which we see as aurora.</p>
<p>The ultraviolet and infrared maps of this emission allow us to measure how the top layers of the Jovian atmosphere heat up and cool, as well as to understand the dynamics of the magnetosphere. </p>
<p>But why is the <a href="https://www.missionjuno.swri.edu/media-gallery/magnetosphere">magnetosphere</a> worth our attention? Planetary magnetospheres act like protective shields that deflect space radiation harmful to life.</p>
<p>Only planets that can produce magnetic fields have magnetospheres and, lucky for us, Earth has one too. But besides Earth, only the giant planets in our Solar System have appreciable magnetospheres. </p>
<p>Juno measured magnetic field in regions closer to Jupiter than ever before, and the results were very different than the predictions from the previously used models. </p>
<p>The observed magnetic fields are stronger and also more spatially variable than previously assumed. Since it is understood that a magnetic field is formed in the cores of planets via dynamo process, this suggests that magnetic field formation region is actually much larger than expected. </p>
<p>This, in turn, in combination with Juno’s measurements of gravitational field around the planet, tells us that our previous ideas about the core of the planet may have to be revised. </p>
<p>For example, the textbook images of the rather compact core of metallic hydrogen is not consistent with Juno observations. The metallic hydrogen core could be as large as the half of Jupiter’s radius.</p>
<h2>Back on Earth</h2>
<p>During Juno’s closest approach to Jupiter – while the probe makes critical observations of Jovian weather, magnetism and gravity – some of the largest telescopes on Earth support the mission with imaging and spectroscopy of the giant planet.</p>
<p>Although the spatial resolution of such observations is no match for imaging from Juno, ground telescopes have a global view of the planet. </p>
<p>During the sixth approach of Juno to the planet on May 19, some of us were using telescopes on Mauna Kea in Hawaii.</p>
<p>I used a high-resolution infrared spectroscope at the Gemini telescope to map the full extend of auroral hydrogen emission around both planetary poles, while my colleagues were taking infrared images of the same regions at the Subaru telescope. </p>
<p>It is exciting to participate in this critical ground base support of the Juno mission, when the international astronomical community joins for a once in a lifetime opportunity to get a very unique view of our giant neighbour.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/171556/original/file-20170531-23699-1xyn62j.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/171556/original/file-20170531-23699-1xyn62j.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/171556/original/file-20170531-23699-1xyn62j.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/171556/original/file-20170531-23699-1xyn62j.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/171556/original/file-20170531-23699-1xyn62j.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/171556/original/file-20170531-23699-1xyn62j.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/171556/original/file-20170531-23699-1xyn62j.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/171556/original/file-20170531-23699-1xyn62j.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">Here I am in front of the terminals to the Gemini Telescope, in Hawaii.</span>
<span class="attribution"><span class="source">Jen Miller</span></span>
</figcaption>
</figure>
<p>It is also amazing to have a support from many amateur astronomy groups that joined in observations of visible light from the planet.</p>
<p>The raw images from the Juno’s visible light camera are available on the <a href="https://www.nasa.gov/mission_pages/juno/main/index.html">Juno website</a> for public use. People are invited to process such images and submit their work for viewing.</p>
<h2>Future goals for Juno</h2>
<p>Juno’s unique orbit allows for making spectacular images of regions not visited in previous missions. The probe also comes much better equipped than some of its predecessors to visit the planet. </p>
<p>The first results from the mission are already suggesting future revisions, or at least adjustments to models of Jupiter’s atmosphere, interior and magnetism. </p>
<p>This unique study of our largest giant planet will hopefully bear implications on the understanding of the formation and composition of similar but much hotter giant planets discovered around other stars.</p><img src="https://counter.theconversation.com/content/78459/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Lucyna Kedziora-Chudczer 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>We may need to re-think our models of Jupiter’s formation thanks to the first results from Juno probe orbiting the planet, and new observations from Earth.Lucyna Kedziora-Chudczer, Postdoctoral Fellow, Astrophysics Researcher, UNSW SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/554242016-07-05T01:57:30Z2016-07-05T01:57:30ZPlate tectonics: new findings fill out the 50-year-old theory that explains Earth’s landmasses<figure><img src="https://images.theconversation.com/files/129030/original/image-20160701-18317-xgbe00.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Satellite image of California's San Andreas fault, where two continental plates come together.</span> <span class="attribution"><a class="source" href="http://photojournal.jpl.nasa.gov/catalog/PIA14555">NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>Fifty years ago, there was a seismic shift away from the longstanding belief that Earth’s continents were permanently stationary. </p>
<p>In 1966, <a href="https://www.geolsoc.org.uk/Plate-Tectonics/Chap1-Pioneers-of-Plate-Tectonics/John-Tuzo-Wilson">J. Tuzo Wilson</a> published <a href="http://fossilhub.org/wp-content/uploads/2012/10/Wilson1966_did_Atlantic_reopen.pdf">Did the Atlantic Close and then Re-Open?</a> in the journal Nature. The Canadian author introduced to the mainstream the idea that continents and oceans are in continuous motion over our planet’s surface. Known as <a href="https://en.wikipedia.org/wiki/Plate_tectonics">plate tectonics</a>, the theory describes the large-scale motion of the outer layer of the Earth. It explains tectonic activity (things like earthquakes and the building of mountain ranges) at the edges of continental landmasses (for instance, the San Andreas Fault in California and the Andes in South America). </p>
<p>At 50 years old, with a surge of interest in where the surface of our planet has been and where it’s going, scientists are reassessing what plate tectonics does a good job of explaining – and puzzling over where new findings might fit in.</p>
<h2>Evidence for the theory</h2>
<p>Although the widespread acceptance of the theory of plate tectonics is younger than Barack Obama, German scientist <a href="https://en.wikipedia.org/wiki/Alfred_Wegener">Alfred Wegener</a> first advanced the hypothesis back in 1912.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/125595/original/image-20160607-15061-lvdpu.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/125595/original/image-20160607-15061-lvdpu.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/125595/original/image-20160607-15061-lvdpu.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=707&fit=crop&dpr=1 600w, https://images.theconversation.com/files/125595/original/image-20160607-15061-lvdpu.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=707&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/125595/original/image-20160607-15061-lvdpu.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=707&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/125595/original/image-20160607-15061-lvdpu.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=888&fit=crop&dpr=1 754w, https://images.theconversation.com/files/125595/original/image-20160607-15061-lvdpu.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=888&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/125595/original/image-20160607-15061-lvdpu.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=888&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 map of the original supercontinent, Pangaea, with modern continent outlines.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Pangaea_continents.svg">Kieff</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>He noted that the Earth’s current landmasses could fit together like a jigsaw puzzle. After analyzing fossil records that showed similar species once lived in now geographically remote locations, meteorologist Wegener proposed that the continents had <a href="https://en.wikipedia.org/wiki/Continental_drift">once been fused</a>. But without a mechanism to explain how the continents could actually “drift,” most geologists dismissed his ideas. His “amateur” status, combined with <a href="http://www.smithsonianmag.com/science-nature/when-continental-drift-was-considered-pseudoscience-90353214/?no-ist">anti-German sentiment</a> in the period after World War I, meant his hypothesis was deemed speculative at best.</p>
<p>In 1966, Tuzo Wilson built on earlier ideas to provide a missing link: the Atlantic ocean had opened and closed at least once before. By studying rock types, he found that parts of New England and Canada were of European origin, and that parts of Norway and Scotland were American. From this evidence, Wilson showed that the Atlantic Ocean had opened, closed and re-opened again, taking parts of its neighboring landmasses with it.</p>
<p>And there it was: proof our planet’s continents were not stationary.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/129045/original/image-20160701-18294-p1ohek.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/129045/original/image-20160701-18294-p1ohek.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/129045/original/image-20160701-18294-p1ohek.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=409&fit=crop&dpr=1 600w, https://images.theconversation.com/files/129045/original/image-20160701-18294-p1ohek.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=409&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/129045/original/image-20160701-18294-p1ohek.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=409&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/129045/original/image-20160701-18294-p1ohek.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=515&fit=crop&dpr=1 754w, https://images.theconversation.com/files/129045/original/image-20160701-18294-p1ohek.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=515&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/129045/original/image-20160701-18294-p1ohek.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=515&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 15 major plates on our planet’s surface.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Plates_tect2_en.svg">USGS</a></span>
</figcaption>
</figure>
<h2>How plate tectonics works</h2>
<p>Earth’s crust and top part of the mantle (the next layer in toward the core of our planet) run about 150 km deep. Together, they’re called the <a href="https://www.geolsoc.org.uk/Plate-Tectonics/Chap2-What-is-a-Plate/Mechanical-properties-lithosphere-and-asthenosphere">lithosphere</a> and make up the “plates” in plate tectonics. We now know there are 15 major plates that cover the planet’s surface, moving at around the speed at which our fingernails grow.</p>
<p>Based on <a href="https://www.youtube.com/watch?v=phZeE7Att_s">radiometric dating</a> of rocks, we know that no ocean is more than 200 million years old, though our continents are much older. The oceans’ opening and closing process – called the <a href="https://www.youtube.com/watch?v=I_q3sAcuzIY">Wilson cycle</a> – explains how the Earth’s surface evolves. </p>
<p>A continent breaks up due to changes in the way molten rock in the Earth’s interior is flowing. That in turn acts on the lithosphere, changing the direction plates move. This is how, for instance, South America broke away from Africa. The next step is continental drift, sea-floor spreading, ocean formation – and hello, Atlantic Ocean. In fact, the Atlantic is still opening, generating new plate material in the middle of the ocean and making the flight from New York to London a few inches longer each year. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/125567/original/image-20160607-15024-19y7pwa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/125567/original/image-20160607-15024-19y7pwa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=464&fit=crop&dpr=1 600w, https://images.theconversation.com/files/125567/original/image-20160607-15024-19y7pwa.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=464&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/125567/original/image-20160607-15024-19y7pwa.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=464&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/125567/original/image-20160607-15024-19y7pwa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=583&fit=crop&dpr=1 754w, https://images.theconversation.com/files/125567/original/image-20160607-15024-19y7pwa.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=583&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/125567/original/image-20160607-15024-19y7pwa.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=583&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A simplified ‘Wilson Cycle’.</span>
<span class="attribution"><span class="source">Philip Heron</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Oceans close when their tectonic plate sinks beneath another, a process geologists call subduction. Off the Pacific Northwest coast of the United States, the ocean is slipping under the continent and into the mantle below the lithosphere, creating in slow motion Mount St Helens and the Cascade mountain range. </p>
<p>In addition to undergoing spreading (construction) and subduction (destruction), plates can simply rub up against each other - usually generating large earthquakes. These interactions, also discovered by Tuzo Wilson back in the 1960s, are termed “conservative.” All three processes occur at the edges of plate boundaries.</p>
<p>But the conventional theory of plate tectonics stumbles when it tries to explain some things. For example, what produces mountain ranges and earthquakes that occur within continental interiors, far from plate boundaries?</p>
<h2>Gone but not forgotten</h2>
<p>The answer may lie in a <a href="http://doi.org/10.1038/ncomms11834">map of ancient continental collisions</a> my colleagues and I assembled. </p>
<p>Over the past 20 years, improved computer power and mathematical techniques have allowed researchers to more clearly look below the Earth’s crust and explore the deeper parts of our plates. Globally, we find many instances of scarring left over from the ancient collisions of continents that formed our present-day continental interiors.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/129046/original/image-20160701-18291-vfhlh0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/129046/original/image-20160701-18291-vfhlh0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/129046/original/image-20160701-18291-vfhlh0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=354&fit=crop&dpr=1 600w, https://images.theconversation.com/files/129046/original/image-20160701-18291-vfhlh0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=354&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/129046/original/image-20160701-18291-vfhlh0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=354&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/129046/original/image-20160701-18291-vfhlh0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=445&fit=crop&dpr=1 754w, https://images.theconversation.com/files/129046/original/image-20160701-18291-vfhlh0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=445&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/129046/original/image-20160701-18291-vfhlh0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=445&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Present day plate boundaries (white) with hidden ancient plate boundaries that may reactivate to control plate tectonics (yellow). Regions where anomalous scarring beneath the crust are marked by yellow crosses.</span>
<span class="attribution"><a class="source" href="http://www.nature.com/ncomms/2016/160610/ncomms11834/fig_tab/ncomms11834_F1.html">Philip Heron</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>A map of ancient continental collisions may represent regions of hidden tectonic activity. These old impressions below the Earth’s crust may still govern surface processes – despite being so far beneath the surface. If these deep scarred structures (more than 30 km down) were reactivated, they would cause devastating new tectonic activity. </p>
<p>It looks like previous plate boundaries (of which there are many) may never really disappear. These <a href="https://eos.org/articles/tiny-mineral-grains-could-drive-plate-tectonics">inherited structures contribute to geological evolution</a>, and may be why we see geological activity within current continental interiors.</p>
<h2>Mysterious blobs 2,900 km down</h2>
<p>Modern geophysical imaging also shows <a href="https://en.wikipedia.org/wiki/Large_low-shear-velocity_provinces">two chemical “blobs”</a>
at the boundary of Earth’s core and mantle – thought to possibly stem from our planet’s formation. </p>
<p>These hot, dense piles of material lie beneath Africa and the Pacific. Located more than 2,900 km below the Earth’s surface, they’re difficult to study. And nobody knows where they came from or what they do. When these blobs of anomalous substance interact with cold ocean floor that has subducted from the surface down to the deep mantle, they generate hot plumes of mantle and blob material that cause <a href="https://philheron.com/lips/">super-volcanoes at the surface</a>. </p>
<p>Does this mean plate tectonic processes control how these piles behave? Or is it that the deep blobs of the unknown are actually controlling what we see at the surface, by releasing hot material to break apart continents? </p>
<p><a href="http://doi.org/10.1038/ngeo2733">Answers to these questions</a> have the potential to shake the very foundations of plate tectonics.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/125581/original/image-20160607-15041-th83ur.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/125581/original/image-20160607-15041-th83ur.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=159&fit=crop&dpr=1 600w, https://images.theconversation.com/files/125581/original/image-20160607-15041-th83ur.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=159&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/125581/original/image-20160607-15041-th83ur.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=159&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/125581/original/image-20160607-15041-th83ur.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=200&fit=crop&dpr=1 754w, https://images.theconversation.com/files/125581/original/image-20160607-15041-th83ur.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=200&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/125581/original/image-20160607-15041-th83ur.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=200&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Arizona State seismology expert Ed Garnero’s summary of how far we have come in over 100 years of studying the interior of the Earth.</span>
<span class="attribution"><a class="source" href="http://garnero.asu.edu/research_images/images_interp.html">Ed Garnero</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Plate tectonics in other times and places</h2>
<p>And the biggest question of all remains unsolved: How did plate tectonics even begin?</p>
<p>The early Earth’s interior <a href="http://www.livescience.com/42373-early-earth-crust-dripped.html">had significantly hotter temperatures</a> – and therefore different physical properties – than current conditions. Plate tectonics then may not be the same as what our conventional theory dictates today. What we understand of today’s Earth may have little bearing on its earliest beginnings; we might as well be thinking about <a href="https://theconversation.com/keep-a-lid-on-it-the-controversy-over-earths-oldest-rocks-19825">an entirely different world</a>.</p>
<p>In the coming years, we may be able to apply what we discover about how plate tectonics got started here to actual other worlds – the billions of exoplanets found in the <a href="http://www.bbc.co.uk/science/space/universe/sights/habitable_zones">habitable zone</a> of our universe. </p>
<figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/125589/original/image-20160607-15045-7a0xna.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/125589/original/image-20160607-15045-7a0xna.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=480&fit=crop&dpr=1 600w, https://images.theconversation.com/files/125589/original/image-20160607-15045-7a0xna.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=480&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/125589/original/image-20160607-15045-7a0xna.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=480&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/125589/original/image-20160607-15045-7a0xna.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=603&fit=crop&dpr=1 754w, https://images.theconversation.com/files/125589/original/image-20160607-15045-7a0xna.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=603&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/125589/original/image-20160607-15045-7a0xna.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">
<figcaption>
<span class="caption">Venus has some geologic features, but not plate tectonics.</span>
<span class="attribution"><a class="source" href="http://photojournal.jpl.nasa.gov/catalog/PIA00254">NASA/JPL</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>So far, amazingly, Earth is the only planet we know of that has plate tectonics. In our solar system, for example, <a href="https://en.wikipedia.org/wiki/Venus">Venus</a> is often considered Earth’s twin - just with a hellish climate and complete <a href="http://arstechnica.com/science/2014/04/venus-crust-heals-too-fast-for-plate-tectonics/">lack of plate tectonics</a>.</p>
<p>Incredibly, the ability of a planet to generate complex life is <a href="https://theconversation.com/does-a-planet-need-plate-tectonics-to-develop-life-61303">inextricably linked to plate tectonics</a>. A gridlocked planetary surface has helped produce Venus’ inhabitable toxic atmosphere of 96 percent CO₂. On Earth, <a href="http://doi.org/10.1038/nature13072">subduction helps push carbon down into the planet’s interior</a> and out of the atmosphere.</p>
<p>It’s still difficult to explain how <a href="https://en.wikipedia.org/wiki/Cambrian_explosion">complex life exploded all over our world 500 million years ago</a>, but the processes of removing carbon dioxide from the atmosphere is further <a href="http://www.astrobio.net/news-brief/earths-breathable-atmosphere-tied-plate-tectonics/">helped by continental coverage</a>. An exceptionally slow process starts with carbon dioxide mixing with rain water to wear down continental rocks. This combination can form carbon-rich limestone that subsequently washes away to the ocean floor. The long removal processes (even for geologic time) eventually could create a more breathable atmosphere. It just took 3 billion years of plate tectonic processes to get the right carbon balance for life on Earth.</p>
<h2>A theory works now, but what’s in the future?</h2>
<p>Fifty years on from Wilson’s 1966 paper, geophysicists have progressed from believing continents never moved to thinking that every movement may leave a lasting memory on our Earth. </p>
<p>Life here would be vastly different if plate tectonics changed its style – as we know it can. A changing mantle temperature may affect the interaction of our lithosphere with the rest of the interior, <a href="https://www.sciencenews.org/article/plate-tectonics-just-stage-earth%E2%80%99s-life-cycle">stopping plate tectonics</a>. Or those continent-sized chemical blobs could move from their relatively stable state, causing <a href="http://es.ucsc.edu/%7Ethorne/TL.pdfs/GLM_P4.pdf">super-volcanoes as they release material</a> from their deep reservoirs.</p>
<p>It’s hard to understand what our future holds if we don’t understand our beginning. By discovering the secrets of our past, we may be able to predict the motion of our plate tectonic future.</p><img src="https://counter.theconversation.com/content/55424/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Philip Heron receives funding from Natural Sciences and Engineering Research Council of Canada. He works for the University of Toronto. </span></em></p>Fifty years on from a groundbreaking paper, geophysicists have progressed from believing continents never moved to thinking that every movement may leave a lasting memory on our planet.Philip Heron, Postdoctoral Fellow in Geodynamics, University of TorontoLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/613032016-06-28T19:32:33Z2016-06-28T19:32:33ZDoes a planet need plate tectonics to develop life?<figure><img src="https://images.theconversation.com/files/127698/original/image-20160622-19767-15k26ip.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Some of the Earth's fault lines between tectonic plates in the East Asia region.</span> <span class="attribution"><span class="source">Shutterstock /Mopic</span></span></figcaption></figure><p>Plate tectonics may be a phase in the evolution of planets that has implications for the habitability of exoplanets, <a href="http://www.sciencedirect.com/science/article/pii/S0031920116300280">according to new research published this month</a> in the journal Physics of the Earth and Planetary Interiors.</p>
<p>Two of the things that make Earth unique in our solar system are that it has plate tectonics – with the surface broken up into a number of tectonic plates that drift around, moving continents and causing earthquakes – and life. </p>
<p>And there is a school of thought that these two are not unrelated.</p>
<p>Complex life on Earth took a long time to evolve; about 3.5 billion years by current estimates. This was possible as the Earth’s surface has been habitable and in the temperature range for liquid water. </p>
<p>This is a remarkable level of stability, especially as the sun has grown <a href="http://www.universetoday.com/16338/the-sun/">30% brighter</a> over that same interval, meaning that Earth’s atmosphere has evolved, becoming less of a greenhouse than it was 3 billion years ago.</p>
<h2>Plate movements</h2>
<p>Plate tectonics provides a mechanism for this global thermostat. Most volcanism on the Earth occurs at plate boundaries in response to plate tectonics. And the most important volcanic products by mass – by a large amount – are two greenhouse gases: carbon dioxide and water.</p>
<p>As they move over the Earth’s surface, some plates get recycled back into the mantle, at places like the Marianas Trench in the Pacific Ocean.</p>
<p>Enormous amounts of water and carbonate (the mineral form of CO<sub>2</sub>) get <a href="http://www.pnas.org/content/112/30/E3997.full.pdf">recycled back</a> into the interior as they do. </p>
<p>Plate tectonics also form mountains, and one of the major sinks of CO<sub>2</sub> over geological time periods is weathering of mountains, where CO<sub>2</sub> dissolved in rainwater reacts with silicate minerals, forming new minerals, and drawing down atmospheric CO<sub>2</sub> levels.</p>
<p>In concert, these mechanisms act as a thermostat. If the Earth gets too hot, high levels of rainfall and erosion start bringing CO<sub>2</sub> levels down. If the Earth gets too cold and freezes over, the erosion mechanism stops.</p>
<p>But volcanism, due to plate tectonics, continues pumping CO<sub>2</sub> into the atmosphere, and levels build up, eventually melting the icecaps. It was this mechanism that allowed Earth to recover from a global ice age in the Neoproterozoic, about 600 million years ago.</p>
<h2>Habitable planets</h2>
<p>This association between habitability, and plate tectonics, has become so entrenched that the search for habitable exosolar planets has focused on super earths. These are rocky planets larger than Earth where the odds for plate tectonics were thought to be higher.</p>
<p>But the case is not so clear cut. Over the past decade, simulations of these super earths suggested that they may not have <a href="http://www.nature.com/news/2007/071019/full/news.2007.176.html">plate tectonics</a>, but rather be in a stagnant-lid state, where a hot interior powers high levels of volcanism, but without moving plates.</p>
<p>Our recent work has looked at the question from an evolutionary viewpoint. How do Earth-like planets evolve from their hot, violent beginnings to their eventual cool, quiescent twilights, radiating their last heat to space?</p>
<p>We found that the evolutionary track a planet takes depends not only on its size, but on how it starts. For example, two planets identical in every other way, but with different starting temperatures, may evolve down very different evolutionary paths.</p>
<p>We also found that plate tectonics may simply be a phase in the evolution of planets, and that planets may begin and end with stagnant lids.</p>
<p>The video (below) shows a simulation of a planet from an initial post-magma ocean state, through more than 700 million years of evolution. The hot interior precludes plate tectonics, and the system evolves in a hot stagnant-lid state.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/7OZSFzufdzQ?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<p>The planetary community has long accepted that as the Earth lost its internal heat, it would eventually settle into a quiescent stagnant state <a href="http://www.abc.net.au/science/articles/2015/07/14/4271527.htm">much like Mars</a> or the Moon today.</p>
<p>The idea that planets may begin in a stagnant lid, though, is more surprising.</p>
<p>Intuitively, this seems an inefficient way for a planet to lose heat. Recycling of plates today is extremely effective at cooling the mantle. Yet one of the main issues in the study of Earth’s thermal evolution is that Earth must have lost its heat less efficiently in the past, to explain its current internal temperatures.</p>
<h2>Look to Jupiter’s moon Io</h2>
<p>An early stagnant lid on Earth provides a mechanism for that. We even have an analogue for this behaviour in Jupiter’s moon Io today.</p>
<p>Io is the most volcanic body in the solar system, a result of Jupiter’s tidal influence, and it operates in a stagnant heat-pipe mode, where it loses its heat primarily through volcanic heat pipes rather than plates.</p>
<p>In a 2013 study, US scientists William Moore and Alexander Webb demonstrated that this regime may have operated under the conditions of the <a href="http://www.nature.com/nature/journal/v501/n7468/full/nature12473.html">early stagnant Earth</a>.</p>
<p>Resolving the issue for Earth is tricky, as the geological record for the first 500 million years – the <a href="http://www.britannica.com/science/Hadean-Eon">Hadean Eon</a> – is missing. </p>
<p>Later geology has been interpreted in the context of stagnant-lid episodes, interspersed by dramatic tectonic events, though this is still contentious. But while geology is lacking, we do have samples from the Hadean in the geochemical signature of the Earth and in <a href="https://theconversation.com/keep-a-lid-on-it-the-controversy-over-earths-oldest-rocks-19825">tiny mineral grains of zircon</a>.</p>
<p>Zircons have provided incredible insights into the makeup of Hadean rocks, and the existence of surface water 4.4 billion years ago, but they are equivocal when it comes to determining tectonic state. </p>
<p>The most recent work suggests they may be crystallising from melt sheets formed by <a href="https://cosmosmagazine.com/space/ancient-asteroid-impacts-yield-evidence-for-the-nature-of-the-early-earth">meteorite impacts</a> on the early Earth. </p>
<p>In contrast, the long-lived isotopic signatures of Hadean processes survived for billions of years in Earth’s mantle, and are recorded in ancient volcanic rocks. The mixing of this material provides an important constraint for the tectonics of the Earth, and supports the idea that the Earth was largely stagnant.</p>
<p>If our conclusions are right, and plate tectonics is an adolescent phase in the evolution of Earth-like planets, then this has big implications for habitability.</p>
<h2>Life on Earth</h2>
<p>Life evolved on the Earth very early. There is evidence in carbon isotopes from <a href="http://www.sciencemag.org/news/2015/10/scientists-may-have-found-earliest-evidence-life-earth">Hadean zircons</a>, and solid fossil evidence from <a href="http://www.smh.com.au/technology/sci-tech/planets-oldest-fossils-found-in-pilbara-experts-say-20130101-2c3qs.html">3.5 billion years ago</a>. It probably evolved on a planet with a stagnant lid, not plate tectonics.</p>
<p>Volcanic degassing evidently provided enough of a greenhouse effect to keep the planet from freezing, despite <a href="http://www.sciencemag.org/news/2016/05/earth-s-ancient-atmosphere-was-half-thick-it-today">lower atmospheric pressures</a>. This was probably helped by low levels of mountain building and subduction of carbonate material, both of which need tectonics.</p>
<p>The evidence we have suggests most of the Earth’s continents were below sea-level before 3 billion years ago, and so Earth’s atmospheric CO<sub>2</sub> had nowhere to go.</p>
<p>These conclusions also impact the search for habitable exoplanets. For a long time there has been an <a href="http://discovermagazine.com/2015/sept/14-super-earths">ingrained assumption</a> that habitable exoplanets must possess plate tectonics like the Earth.</p>
<p>The simplistic view of Venus supports this, as it does not have plate tectonics, and is extremely inhospitable to surface life. Yet Venus and Earth may have diverged for very different reasons early in their history.</p>
<p>It is entirely possible that the best analogue for early Earth, on which life evolved, is a warm, stagnant-lid planet in a distant star system. This increases the exploration space for habitable planets, and in doing so, the chances of life elsewhere in the universe.</p><img src="https://counter.theconversation.com/content/61303/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Craig O'Neill receives funding from the ARC. </span></em></p>Earth is the only planet in our solar system with both plate tectonics and life. Is there a connection?Craig O'Neill, Director of the Macquarie Planetary Research Centre/Associate Professor in Geodynamics, Macquarie UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/565112016-05-26T20:08:43Z2016-05-26T20:08:43ZStars with planets on strange orbits: what’s going on?<figure><img src="https://images.theconversation.com/files/123924/original/image-20160525-25239-1md452s.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">An artist's impression of a transiting Jupiter-mass exoplanet around a star slightly more massive than the sun.</span> <span class="attribution"><a class="source" href="http://www.eso.org/public/images/eso0638a/">ESO</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>All the planets in our solar system orbit close to the sun’s equatorial plane. Of the <a href="https://theconversation.com/somewhere-out-there-could-be-a-giant-new-planet-in-our-solar-system-so-where-is-it-53501">eight confirmed planets</a>, the Earth’s orbit is the most tilted, but even that tilt is still small, at just seven degrees.</p>
<p>It was natural, then, for astronomers to expect that planets orbiting other stars would behave the same way – forming and evolving on orbits aligned with their host star’s equators. </p>
<p>But in recent years, <a href="http://theconversation.com/explainer-how-to-find-the-orbit-of-an-exoplanet-56682">new observations</a> have revealed that the story is somewhat more complicated, at least for the oddest planets known, the Hot Jupiters.</p>
<h2>An explosion of exoplanets</h2>
<p>In just two decades, we have gone from knowing one planetary system (our own) to thousands, with <a href="http://exoplanetarchive.ipac.caltech.edu/">3,268 exoplanets</a> now known. This has driven a massive rethink of our models of planetary formation. </p>
<p>Based on a sample of one system, astronomers once expected most planetary systems to have small, rocky planets (like Earth) orbiting close to their host star, and massive, Jupiter-like planets orbiting farther out.</p>
<p>With the discovery of the first exoplanets, this simple model was shattered. Those planets, the Hot Jupiters, were different from anything we had expected. </p>
<p>Comparable in mass to Jupiter, they move on incredibly short period orbits, almost skimming the surfaces of their host star. Instead of Jupiter’s sedate 12-year orbit, they whizz around with periods of days, or even hours. Finding planets on such extreme orbits meant a major rethink.</p>
<p>As a result, a new suite of theories were born. Rather than planets forming sedately at a fixed distance from a star, we picture migratory planets, drifting huge distances as they grow. </p>
<p>The evidence for such migration abounds, even <a href="https://theconversation.com/by-jupiter-the-gas-giants-trojans-were-captured-not-pre-formed-6496">within the solar system</a>.</p>
<p>Then came another set of shocking discoveries. Rather than moving in the same plane as their host star’s equator, some Hot Jupiters turned out to have <a href="http://adsabs.harvard.edu/abs/2013ApJ...774L...9A">highly tilted orbits</a>. Some even move on retrograde orbits, in the opposite direction to their star’s rotation. </p>
<p>How did those planets get onto such crazy orbits?</p>
<h2>Rethinking planet formation</h2>
<p>The most widely accepted model of planet formation is “core accretion”, where planets form slowly, in a circumstellar disk of material. We’ve even caught systems in the act, partway through formation.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/115974/original/image-20160322-32285-76tclf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/115974/original/image-20160322-32285-76tclf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/115974/original/image-20160322-32285-76tclf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=394&fit=crop&dpr=1 600w, https://images.theconversation.com/files/115974/original/image-20160322-32285-76tclf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=394&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/115974/original/image-20160322-32285-76tclf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=394&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/115974/original/image-20160322-32285-76tclf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=495&fit=crop&dpr=1 754w, https://images.theconversation.com/files/115974/original/image-20160322-32285-76tclf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=495&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/115974/original/image-20160322-32285-76tclf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=495&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Within each of these circumstellar disks, seen against the backdrop of the Orion nebula, planets are being born.</span>
<span class="attribution"><span class="source">NASA, ESA, M. Robberto (STSI/ESA), the HST Orion Treasury Project Team and L. Ricci (ESO)</span></span>
</figcaption>
</figure>
<p>Within those disks, dust and ice particles gradually grow by devouring their neighbours. In the hot inner reaches, the amount of solid material is limited as it is too warm for gaseous water to condense to form ice, so planets grow slowly. </p>
<p>Farther out, vast amounts of ice contribute to the more rapid growth of planetary cores. Eventually, those cores gain enough mass (around ten times the mass of Earth) to capture gasses from their surroundings.</p>
<p>When a planet reaches this critical mass, it begins to accrete gas from the disk, and undergoes rapid growth, becoming a fully fledged gas giant. </p>
<p>In the process, the interaction between the planet and the disk causes it to migrate inwards. Depending on the properties of the disk, the planet can move vast distances, even ending up devoured by its host. </p>
<p>This rapid growth and migration comes to an end when the host star clears any remaining gas and dust from the system.</p>
<p>The planets continue to drift as they scatter and devour the larger debris that remains. That process continues even today in the solar system, albeit at a snail’s pace.</p>
<p>But this simple model fails to explain the latest discoveries of planets on highly inclined orbits. The migration described above typically happens within the disk, keeping the planet close to the star’s equatorial plane. </p>
<p>To excite it to a highly inclined orbit requires something more.</p>
<h2>Highly inclined planets</h2>
<p>To date, astronomers have measured the orbital <a href="http://www2.mps.mpg.de/homes/heller/">inclinations of 91 exoplanets</a> and more than a third (36) move on orbits that are significantly misaligned, tilted by more than 20 degrees. Nine of them move on retrograde orbits.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/115778/original/image-20160321-30935-5aqssi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/115778/original/image-20160321-30935-5aqssi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/115778/original/image-20160321-30935-5aqssi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=424&fit=crop&dpr=1 600w, https://images.theconversation.com/files/115778/original/image-20160321-30935-5aqssi.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=424&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/115778/original/image-20160321-30935-5aqssi.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=424&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/115778/original/image-20160321-30935-5aqssi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=533&fit=crop&dpr=1 754w, https://images.theconversation.com/files/115778/original/image-20160321-30935-5aqssi.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=533&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/115778/original/image-20160321-30935-5aqssi.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=533&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 polar orbit of WASP-79b.</span>
<span class="attribution"><span class="source">ESO/B Addison</span></span>
</figcaption>
</figure>
<p>Were there one or two misaligned planets, we could write them off as a fluke of nature. But the number found is far too large to be coincidence.</p>
<p>Astronomers have developed new models, featuring evolution that allows migrating planets to become misaligned. The most promising share a common theme, a period of high eccentricity migration.</p>
<h2>A problem solved?</h2>
<p>High eccentricity migration models run as follows. Giant planets form, as expected, on initially circular orbits, well aligned with their host’s equator. As the systems evolve, the planet’s orbit is perturbed by other massive objects in the same system (most likely, a companion star). </p>
<p>As a result, the planet’s orbit becomes significantly less circular (more eccentric). At the same time, its inclination can be pumped up, becoming misaligned. If a planet’s orbit is sufficiently tilted, compared to that of its perturber, an additional effect can kick in, known as the <a href="http://large.stanford.edu/courses/2007/ph210/raman1/">Kozai-Lidov mechanism</a>.</p>
<p>Under the Kozai-Lidov mechanism, a planet’s orbit can yaw wildly in space. As its orbit becomes more inclined (compared to the perturber), it also becomes more circular. Then the oscillation changes direction, and the orbit swings back towards that of the perturber, while becoming more eccentric. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/123931/original/image-20160525-25247-lhod2y.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/123931/original/image-20160525-25247-lhod2y.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/123931/original/image-20160525-25247-lhod2y.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/123931/original/image-20160525-25247-lhod2y.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/123931/original/image-20160525-25247-lhod2y.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/123931/original/image-20160525-25247-lhod2y.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/123931/original/image-20160525-25247-lhod2y.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/123931/original/image-20160525-25247-lhod2y.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">Artists impression of HD 189733 b, a Hot Jupiter so close to its host that its atmosphere is being boiled off into space.</span>
<span class="attribution"><span class="source">NASA's Goddard Space Flight Center</span></span>
</figcaption>
</figure>
<p>These oscillations can be so extreme that they cause a planet to become star-grazing, skimming its host’s surface with every pass. During these close encounters, the star and planet interact tidally with the planet raising tides on the star, and the star raising tides on the planet. </p>
<p>These tides exert a strong damping force, causing the planet’s orbit to decay rapidly. The point of closest approach remains roughly the same, but the apocentre (the greatest separation distance) shrinks. The planet’s orbit is rapidly circularised as it decouples from the distant perturber, but remains highly tilted.</p>
<h2>But what comes next?</h2>
<p>The theory makes testable predictions. To make misaligned planets this way requires a perturber.</p>
<p>In some cases, the companion will be long gone, the binary star system torn asunder by passing stars, for example. But for most, the smoking gun should still be there. Binary companions, waiting to be discovered. </p>
<p>Astronomers are using new instruments on the world’s largest telescopes to attempt to detect the perturbers, if they’re there. </p>
<p>Some stars, by chance or association, appear to be very close together. To see whether a star has a true companion isn’t just a case of seeing if there’s another star in the same bit of sky, though chances are, there is. </p>
<p>Instead, we have to watch those neighbours for months, or years. If they’re truly are a couple, they’ll move together, drifting in lockstep against the background stars. </p>
<p>One of us (Brett Addison) is currently actively involved in this search, using the <a href="https://obs.carnegiescience.edu/Magellan">Magellan Clay Telescope</a> in Chile. The preliminary results are already in – with no strong correlations observed between systems with stellar companions and those with inclined planets.</p>
<p>Still, the search goes on.</p><img src="https://counter.theconversation.com/content/56511/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Many of the new planets found in other star systems have some extraordinary orbital behavior. So what’s going on?Brett Addison, Postdoc astrophysicist, Mississippi State UniversityJonti Horner, Vice Chancellor's Senior Research Fellow, University of Southern QueenslandLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/521162015-12-11T09:25:19Z2015-12-11T09:25:19ZRarity of Jupiter-like planets means planetary systems exactly like ours may be scarce<figure><img src="https://images.theconversation.com/files/105138/original/image-20151209-15588-1cyskbt.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Artist's depiction of the newly discovered Jupiter-like planet orbiting the star HD 32963. </span> <span class="attribution"><span class="source">Stefano Meschiari</span></span></figcaption></figure><p>Is our little corner of the galaxy a special place? As of this date, we’ve <a href="http://exoplanets.org">discovered more than 1,500 exoplanets</a>: planets orbiting stars other than our sun. Thousands more will be added to the list in the coming years as we confirm planetary candidates by alternative, independent methods.</p>
<p>In the hunt for other planets, we’re especially interested in those that might potentially host life. So we focus our modern exoplanet surveys on planets that might be similar to Earth: low-mass, rocky and with just the right temperature to allow for liquid water. But what about the other planets in the solar system? The <a href="https://en.wikipedia.org/wiki/Copernican_principle">Copernican principle</a> – the idea that the Earth and the solar system are not unique or special in the universe – suggests the architecture of our planetary system should be common. But it doesn’t seem to be.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/105306/original/image-20151210-7425-1kd373r.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/105306/original/image-20151210-7425-1kd373r.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/105306/original/image-20151210-7425-1kd373r.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=429&fit=crop&dpr=1 600w, https://images.theconversation.com/files/105306/original/image-20151210-7425-1kd373r.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=429&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/105306/original/image-20151210-7425-1kd373r.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=429&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/105306/original/image-20151210-7425-1kd373r.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=539&fit=crop&dpr=1 754w, https://images.theconversation.com/files/105306/original/image-20151210-7425-1kd373r.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=539&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/105306/original/image-20151210-7425-1kd373r.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=539&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 mass-period diagram. Each dot marks the mass and orbital period of a confirmed exoplanet.</span>
<span class="attribution"><span class="source">Stefano Meschiari</span></span>
</figcaption>
</figure>
<p>The figure above, called a <em>mass-period diagram</em>, provides a visual way to compare the planets of our solar system with those we’ve spotted farther away. It charts the orbital periods (the time it takes for a planet to make one trip around its central star) and the masses of the planets discovered so far, compared with the properties of solar system planets.</p>
<p>Planets like Earth, Jupiter, Saturn and Uranus occupy “empty” parts of the diagram – we haven’t found other planets with similar masses and orbits so far. At face value, this would indicate that the majority of planetary systems do not resemble our own solar system.</p>
<p>The solar system lacks close-in planets (planets with orbital periods between a few and a few tens of days) and super-Earths (a class of planets with masses a few times the mass of the Earth often detected in other planetary systems). On the other hand, it does feature several long-period gaseous planets with very nearly circular orbits (Jupiter, Saturn, Uranus and Neptune). </p>
<p>Part of this difference is due to selection effects: close-in, massive planets are easier to discover than far-out, low-mass planets. In light of this discovery bias, astronomers <a href="http://aasnova.org/2015/09/25/how-normal-is-our-solar-system/">Rebecca Martin and Mario Livio</a> convincingly argue that our solar system is actually <a href="http://dx.doi.org/10.1088/0004-637X/810/2/105">more typical than it seems at first glance</a>.</p>
<p>There is a sticking point, however: Jupiter still stands out. It’s an outlier based both on its orbital location (with a corresponding period of about 12 years) and its very-close-to-circular orbit. Understanding whether Jupiter’s relative uniqueness is a real feature, or another product of selection effects, has real implications for our understanding of exoplanets.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/3afEX8a2jPg?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Jupiter as seen by the Hubble Space Telescope.</span></figcaption>
</figure>
<h2>Throwing its weight around</h2>
<p>According to our understanding of how our solar system formed, Jupiter shaped much of the other planets’ early history. Due to its gravity, it influenced the <a href="http://www.sciencedaily.com/releases/2011/06/110605132437.htm">formation of Mars</a> and Saturn. It potentially facilitated the development of life by shielding Earth from cosmic collisions that would have delayed or extinguished it, and by funneling water-rich bodies towards it. And its gravity <a href="http://doi.org/10.1073/pnas.1423252112">likely swept the inner solar system of solid debris</a>. Thanks to this clearing action, Jupiter might have prevented the formation of super-Earth planets with massive atmospheres, thereby ensuring that the inner solar system is populated with small, rocky planets with thin atmospheres. </p>
<p>Without Jupiter, it looks unlikely that we’d be here. As a consequence, figuring out if Jupiter is a relatively common type of planet might be crucial to understanding whether terrestrial planets with a similar formation environment as Earth are abundant in the galaxy.</p>
<p>Despite their relative heft, it’s a challenge to discover Jupiter analogs – those planets with periods and masses similar to Jupiter’s. Astronomers typically discover them using an indirect detection technique called the <a href="https://en.wikipedia.org/wiki/Doppler_spectroscopy">Doppler radial velocity method</a>. The gravitational pull of the planet causes tiny shifts in the wavelength of features in the spectrum of the star, in a distinctive, periodic pattern. We can detect these shifts by periodically capturing the star’s light with a telescope and turning it into a spectrum with <a href="https://www2.keck.hawaii.edu/inst/hires/">a spectrograph</a>. This periodic signal, based on a planet’s long orbital period, can require monitoring a star over many years, even decades.</p>
<h1>Are Jupiter-like planets rare?</h1>
<p><a href="http://arxiv.org/abs/1512.00417">In a recent paper</a>, Dominick Rowan, a high school senior from New York, and his coauthors (including astronomers from the University of Texas, the University of California at Santa Cruz and me) analyzed the Doppler data for more than 1,100 stars. Each star was observed with the <a href="http://www.keckobservatory.org/">Keck Observatory telescope</a> in Hawaii; many of them had been monitored for a decade or more. To analyze the data, he used the <a href="https://www.r-project.org">open-source statistical environment R</a> together with a freely available application that I developed, called <a href="http://www.stefanom.org/systemic">Systemic</a>. Many universities use an <a href="http://www.stefanom.org/systemic-live">online version</a> to teach how to analyze astronomical data.</p>
<p>Our team studied the available data for each star and calculated the probability that a Jupiter-like planet could have been missed – either because not enough data are available, or because the data are not of high enough quality. To do this, we simulated hundreds of millions of possible scenarios. Each was created with a computer algorithm and represents a set of alternative possible observations. This procedure makes it possible to infer how many Jupiter analogs (both discovered and undiscovered) orbited the sample of 1,100 stars.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/105134/original/image-20151209-15552-1nqxyfv.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/105134/original/image-20151209-15552-1nqxyfv.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/105134/original/image-20151209-15552-1nqxyfv.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/105134/original/image-20151209-15552-1nqxyfv.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/105134/original/image-20151209-15552-1nqxyfv.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/105134/original/image-20151209-15552-1nqxyfv.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/105134/original/image-20151209-15552-1nqxyfv.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/105134/original/image-20151209-15552-1nqxyfv.png?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">Orbit of the newly discovered Jupiter-mass planet orbiting the star HD 32963, compared to the orbits of Earth and Jupiter around the sun.</span>
<span class="attribution"><span class="source">Stefano Meschiari</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>While carrying out this analysis, we discovered a <a href="http://exoplanet.eu/catalog/hd_32963_b/">new Jupiter-like planet</a> orbiting HD 32963, which is a star very similar to the sun in terms of age and physical properties. To make this discovery, we analyzed each star with an automated algorithm that tried to uncover periodic signals potentially associated with the presence of a planet.</p>
<p>We pinpointed the frequency of Jupiter analogs across the survey at approximately 3%. This result is broadly consistent with previous estimates, which were based on a smaller set of stars or a different discovery technique. It greatly strengthens earlier predictions because we took <em>decades</em> of observations into account in the simulations. </p>
<p>This result has several consequences. First, the relative rarity of Jupiter-like planets indicates that true solar system analogs should themselves be rare. By extension, given the important role that Jupiter played at all stages of the formation of the solar system, Earth-like habitable planets with similar formation history to our solar system will be rare.</p>
<p>Finally, it also underscores that Jupiter-like planets do not form as readily around stars as other types of planets do. It could be because not enough solid material is available, or because these gas giants migrate closer to the central stars very efficiently. <a href="http://astrobites.org/2015/08/18/giant-planets-from-far-out-there/">Recent planet-formation simulations</a> tentatively bear out the latter explanation.</p>
<p>Long-running, ongoing surveys will continue to help us understand the architecture of the outer regions of planetary systems. Programs including the Keck planet search and the <a href="http://arxiv.org/abs/1512.02965">McDonald Planet Search</a> have been accumulating data for decades. Discovering ice giants similar to Uranus and Neptune will be even tougher than tracking down these Jupiter analogs. Because of their long orbital periods (84 and 164 years) and the very small Doppler shifts they induce on their central stars (tens of times smaller than a Jupiter-like planet), the detection of Uranus and Neptune analogs lies far in the future.</p><img src="https://counter.theconversation.com/content/52116/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Stefano Meschiari works for the University of Texas at Austin.</span></em></p>Jupiter had a big influence on how our solar system’s planets formed. New research – led by a high school student – tried to nail down how rare Jupiter analogs really are in other planetary systems.Stefano Meschiari, W J McDonald Postdoctoral Fellow, The University of Texas at AustinLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/434042015-07-13T04:35:31Z2015-07-13T04:35:31ZPluto and its collision-course place in our solar system<figure><img src="https://images.theconversation.com/files/88159/original/image-20150713-9492-1f189hg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">New Horizons' look at Pluto's Charon-facing hemisphere reveals intriguing geologic details that are of keen interest to mission scientists. This image was taken on July 11, 2015, when the spacecraft was 4 million km from Pluto.</span> <span class="attribution"><a class="source" href="https://www.nasa.gov/image-feature/new-horizons-last-portrait-of-pluto-s-puzzling-spots">NASA/JHUAPL/SWRI</a></span></figcaption></figure><p>There are just hours to go now before the <a href="https://www.nasa.gov/mission_pages/newhorizons/main/index.html">New Horizons Spacecraft</a> will <a href="https://theconversation.com/new-horizons-close-encounter-with-pluto-will-reveal-its-icy-secrets-44361">tear past Pluto</a> on Tuesday July 14 (about 10pm AEST), giving us our first closeup view of the enigmatic dwarf planet.</p>
<p>As it flies past, <a href="http://pluto.jhuapl.edu/Mission/Spacecraft/Payload.php">the seven instruments on board</a> will capture every moment of their fleeting encounter.</p>
<p>Over the months that follow, that data will <a href="https://theconversation.com/rise-and-shine-new-horizons-awakes-ahead-of-a-date-with-pluto-35332">trickle back to Earth</a>, providing vital new clues to help piece together the story of our solar system’s formation and evolution.</p>
<p>But what do we already know about Pluto and its place in our solar system?</p>
<p>Most science is generally experimental in nature. If you want to find out how something works, you can hit it with a hammer, boil it in a test tube or make it run through a complicated maze - you get the idea.</p>
<p>Astronomy, by contrast, is an observational science. We can’t really experiment (except through <a href="https://theconversation.com/by-jupiter-the-gas-giants-trojans-were-captured-not-pre-formed-6496">clever use of computers</a>). Instead, we gather observations and use them to piece together the story of how, when, why, and where something happened. </p>
<p>So the universe is a crime scene, and astronomers are the detectives examining the clues left behind. Pluto, and its brethren in the space beyond the planets, are particularly important clues for astronomers studying our solar system’s past.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/88138/original/image-20150712-17462-1igwm2d.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/88138/original/image-20150712-17462-1igwm2d.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/88138/original/image-20150712-17462-1igwm2d.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=204&fit=crop&dpr=1 600w, https://images.theconversation.com/files/88138/original/image-20150712-17462-1igwm2d.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=204&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/88138/original/image-20150712-17462-1igwm2d.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=204&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/88138/original/image-20150712-17462-1igwm2d.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=257&fit=crop&dpr=1 754w, https://images.theconversation.com/files/88138/original/image-20150712-17462-1igwm2d.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=257&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/88138/original/image-20150712-17462-1igwm2d.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=257&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Astronomy, an observational science, places astronomers in the role of detectives trying to disentangle the universe around us.</span>
<span class="attribution"><a class="source" href="https://xkcd.com/1522/">xkcd</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-sa/4.0/">CC BY-NC-SA</a></span>
</figcaption>
</figure>
<h2>Pluto - a celestial oddball</h2>
<p>In the <a href="http://www.space.com/19824-clyde-tombaugh.html">years since it was discovered</a> in 1930, astronomers have learned a great deal about <a href="http://solarsystem.nasa.gov/planets/profile.cfm?Object=Pluto">Pluto</a>. It’s turned out to be a very unusual object. </p>
<p>It is <a href="https://books.google.com.au/books?id=8nKZnNnWjE0C&pg=PA104&lpg=PA104&dq=pluto+high+albedo&source=bl&ots=5foFPQAzb3&sig=suxMi4j-INx1YfvNb8jdNl2a3iI&hl=en&sa=X&ei=6FGfVfOhHYfJmAWrm4nACg&ved=0CCQQ6AEwBDgK#v=onepage&q=pluto%20high%20albedo&f=false">highly reflective</a>, exuding a tenuous atmosphere when closest to the sun. In addition, it has a family of satellites, including the behemoth <a href="http://solarsystem.nasa.gov/planets/profile.cfm?Object=Plu_Charon">Charon</a>, a little over 1,200km in diameter it is just over half Pluto’s size. </p>
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<span class="caption">Pluto and Charon, as imaged by New Horizons on July 8, 2015.</span>
<span class="attribution"><a class="source" href="http://www.nasa.gov/feature/pluto-and-charon-new-horizons-dynamic-duo">NASA-JHUAPL-SWRI</a></span>
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<p><a href="http://www.universetoday.com/13865/orbit-of-pluto/">Pluto’s orbit</a> is distinctly non-circular, or eccentric. At its closest to the sun (a distance of 4.44 billion km), Pluto passes within the orbit of <a href="http://solarsystem.nasa.gov/planets/profile.cfm?Object=Neptune">Neptune</a>, while at its most distant it lies almost three billion kilometres further away.</p>
<p>Pluto’s orbit is also tilted, or inclined, by about 17 degrees to the plane of the solar system. Pluto wanders both far above and far below the other planets during each 248-year orbit.</p>
<p>The oddities don’t end there. Crossing paths with Neptune, you might expect Pluto to eventually come close to that planet, potentially even crashing into it. But it avoids such a fate due to something called a <a href="https://en.wikipedia.org/wiki/Orbital_resonance">mean-motion resonance</a>. </p>
<p>Pluto’s orbit takes around 50% longer than that of Neptune’s (164 years). Pluto therefore completes two full laps of the sun in around the time it takes Neptune to complete three. This prevents close encounters between Pluto and Neptune. Every time Pluto crosses Neptune’s orbit, Neptune is elsewhere. </p>
<p>It works like this: on the first orbit, Pluto beats Neptune to the point their orbits cross, and the two avoid a collision by a huge distance. By the time Pluto completes another orbit, Neptune has completed one and a half, meaning that it now precedes Pluto, and a collision is again avoided. After another Plutonian year, the two return to where they started, and the dance begins again.</p>
<p>Because Neptune completes three orbits in the time Pluto completes two, we say that they are trapped in 3:2 mean-motion resonance. And it is this resonance that is key to our understanding the solar system’s formation.</p>
<h2>Pluto and planet formation</h2>
<p>Our current best theory is that the solar system formed from a gas and dust-rich protoplanetary disk - much like those observed around young stars in the Orion nebula.</p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/K3GaCIlphrc?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Hubblecast 32: The Proplyds in the Orion Nebula.</span></figcaption>
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<p>For planets, dwarf planets and other assorted debris to form in such an environment, the disk has to be dynamically cold – in other words, as flat as a pancake. </p>
<p>In that scenario, the tiny fragments of dust and ice in the disk collide at such slow speeds that they can stick together, rather than smashing one another apart. </p>
<p>Fast forward uncounted collisions over a few tens of millions of years and a planetary system is born. </p>
<p>This is a surprisingly successful model and matches the clues we observe better than any of its rivals. But, at first glance, Pluto’s peculiar orbit seems to contradict the story. If Pluto formed that way, why does it now move on such an eccentric and inclined orbit? </p>
<p>And Pluto isn’t alone. We now know of <a href="http://www.johnstonsarchive.net/astro/tnos.html">a large population of objects</a> beyond Neptune’s orbit, many of which are also <a href="http://arxiv.org/abs/1205.7065">trapped in resonance with Neptune</a>, and move on inclined and/or eccentric orbits. They’re certainly not what you might expect of a population born from a thin, cold disk of material. </p>
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<span class="caption">The inclination of orbits of the solar system’s small bodies, outward from Saturn’s orbit.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/File:TheTransneptunians_73AU.svg">Wikimedia</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
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<p>And so we have a clue, in the form of the eccentricities and inclinations of Pluto and the other <a href="http://astronomy.swin.edu.au/cosmos/P/Plutinos">Plutinos</a>. But what does it portend?</p>
<h2>Pluto as the yardstick of migration</h2>
<p>As our models of planet formation have become more sophisticated, the simple picture that our planets formed on their current orbits has been overturned. </p>
<p>Based on the <a href="https://theconversation.com/by-jupiter-the-gas-giants-trojans-were-captured-not-pre-formed-6496">evidence</a> frozen in to the solar system’s small body populations, we now think that Jupiter, Saturn, Uranus and Neptune migrated as they grew, spreading out to reach their current dispersed architecture. </p>
<p>Neptune, in particular, was a great wanderer, with <a href="http://www.jontihorner.com/papers/TrojansI.pdf">some models suggesting</a> it formed between one and two billion kilometres closer to the sun than we currently observe it. But how can we tell? </p>
<p>The answer? Pluto’s peculiar orbit and those of the Plutinos.</p>
<h2>The evidence for Neptune’s great journey</h2>
<p>As the planets formed, with Neptune much closer to the sun than it is today, there was a wealth of debris (planetesimals) further out. As Neptune fed, devouring the material closest to it, it scattered material inward from this trans-Neptunian region and, in the process, began to drift outwards. </p>
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<span class="caption">Neptune’s Great Dark Spot and its companion bright smudge as captured by Voyager 2.</span>
<span class="attribution"><a class="source" href="http://solarsystem.nasa.gov/multimedia/display.cfm?Category=Planets&IM_ID=2424">NASA</a></span>
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<p>As Neptune moved, so did the location of its resonances. Objects were captured as the planet swept outwards, forced to move in lockstep with the giant. </p>
<p>As it travelled further, Neptune ensnared more objects. Once caught, few escaped, and the rest were carried inexorably outwards, swept ahead of the giant planet. As they were pushed, the force driving them acted to excite their orbits, increasing their eccentricities and their inclinations. </p>
<p>Eventually, Neptune’s migration all but ceased, and the population of Plutinos was frozen to that we observe today - the clue that reveals the magnitude of Neptune’s rapid outward march. </p>
<h2>A well travelled enigma</h2>
<p>This brings us back to Pluto. From its orbit, and its link to Neptune, we can tell that Neptune must have formed closer to the sun and then moved outwards. </p>
<p>That also means that Pluto must have formed closer to the sun than its current orbit. We can estimate where it formed, to some degree, based on its current excitement. </p>
<p>And this is where we come to the hero of the hour - the New Horizons spacecraft. The measurements the probe makes in the coming hours as it passes Pluto should give us an independent measure of where it formed, adding a vital new clue to the mix. </p>
<p>Will it support our theories, or will we have to start again from scratch? We will have to see what the data reveals, and that’s part of the beauty and thrill of the observational detective missions such as these.</p><img src="https://counter.theconversation.com/content/43404/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The New Horizons spacecraft is only hours away from its closest approach to Pluto. It’s hoped the brief encounter will help answer many questions about the oddball member of our solar system.Jonti Horner, Vice Chancellor's Senior Research Fellow, University of Southern QueenslandJonathan P. Marshall, Vice Chancellor's Post-doctoral Research Fellow, UNSW SydneyLicensed as Creative Commons – attribution, no derivatives.