tag:theconversation.com,2011:/id/topics/neptune-18848/articlesNeptune – The Conversation2024-01-05T00:02:45Ztag:theconversation.com,2011:article/2202442024-01-05T00:02:45Z2024-01-05T00:02:45ZHow we discovered that Uranus and Neptune are actually nearly identical in colour<figure><img src="https://images.theconversation.com/files/567647/original/file-20240103-23-otofoy.png?ixlib=rb-1.1.0&rect=29%2C6%2C2153%2C1026&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">This is how we are used to seeing Uranus and Neptune, respectively. But the colours aren't accurate.</span> <span class="attribution"><span class="source">NASA/ JPL/ PlanetS</span></span></figcaption></figure><p>In many images of the two outer gas giants of the Solar System, Neptune typically looks rich blue while Uranus comes across as pale green. But now our new study, published in <a href="https://academic.oup.com/mnras/article-lookup/doi/10.1093/mnras/stad3761">Monthly Notices of the Royal Astronomical Society</a>, has revealed that these two ice giants are actually very similar shades of greenish blue.</p>
<p>The study follows <a href="https://doi.org/10.1029/2022JE007189">our previous work</a> in 2022 that analysed the spectra (light broken down by wavelength) of light reflected off Uranus and Neptune from several sources, including the <a href="https://science.nasa.gov/mission/hubble/observatory/design/space-telescope-imaging-spectrograph/">space telescope imaging spectrograph</a> on the Hubble space telescope. These were recorded in 2002 (Uranus) and 2003 (Neptune). </p>
<p>We found that the colours of Uranus and Neptune were actually remarkably similar, with Neptune appearing only slightly bluer – see the image below. The difference in colour was attributed to the difference in opacity of a layer of haze and methane ice. </p>
<p>Ultimately, Neptune has a thinner layer of haze, allowing more sunlight to reach deeper in the atmosphere. At such depth, it can be absorbed by methane gas, which soaks up red light – making the planet appear ever so slightly more blue.</p>
<h2>Reconstructing the colours</h2>
<p>Our reconstructed colours of Uranus and Neptune look very different from previous images, which come from the <a href="https://science.nasa.gov/mission/voyager/voyager-2/">Voyager 2</a> spacecraft’s encounters with these planets in 1986 and 1989 respectively.</p>
<p>So, did the colours of Uranus and Neptune change between the late 1980s and early 2000s? Or do we need to consider more carefully how observations of planets are converted to the “true” colour that would be observed by an average human observer? The answer, it turns out, is a bit of both.</p>
<p>Colour images of planets <a href="https://theconversation.com/from-neptunes-blue-hue-to-jupiters-red-spot-are-the-colours-of-the-planets-real-62513">are highly processed</a>. The red, green and blue components are usually recorded separately by spacecraft. They are then sent back to Earth as black-and-white images, where they can be combined in colour. However such images may not reveal the true colour the human eye would see. </p>
<p>Even light recorded in channels beyond the visible range, such as in ultraviolet, become red, green or blue when displayed. There are several steps involved in this process and, depending on the choices made, a planetary image can have a wide range of appearances.</p>
<p>To determine the truest colour of Uranus and Neptune up to the present day, we combined our Hubble data with more recent observations at the <a href="https://www.eso.org/public/unitedkingdom/teles-instr/paranal-observatory/vlt/">very large telescope</a> in Chile. Both of these instruments record images where each individual pixel is a complete, continuous spectrum covering all colours that can be seen with the human eye – making them more accurate than spacecraft when it comes to colour.</p>
<p>This allowed us to determine unambiguously the actual colour that the human eye would perceive for Uranus and Neptune. We could then reprocess observations made by imaging cameras on Voyager 2 and Hubble taking this into account.</p>
<p>When the reprocessed Voyager 2 observations of Uranus and Neptune are compared with some of the early-release images, it is clear that the early Uranus images correspond fairly well with what we now believe its colour to be. The early Neptune images, however, are a much darker blue than their true colour. </p>
<p>This difference was actually known at the time to the Voyager imaging team, and the captions released with the images explained this fact. However, since the purpose of these images was to communicate the exciting new discoveries of the mission, it was quite sensibly judged that an enhanced version of the images that accentuated the discoveries was preferable over a “true” colour version, where the features appear washed out. </p>
<p>However, the differences in processing became forgotten over time and so now most people, including planetary researchers, just accept that Neptune is much bluer than Uranus, which is not actually the case.</p>
<h2>Uranus changes colour</h2>
<p>Comparing the true colour of Uranus in 1986 with more recent observations, it became clear that Uranus in 1986 was actually slightly greener than it was in the early 2000s. We tried to determine why this was the case by turning to observations made between 1950 and 2016 at the Lowell Observatory in Arizona. These observations contained the overall brightness of Uranus and Neptune almost annually at two wavelengths: green and blue.</p>
<figure class="align-center ">
<img alt="Images of Uranus changing colours as observed with the Hubble telescope." src="https://images.theconversation.com/files/567656/original/file-20240103-29-sx0fr4.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/567656/original/file-20240103-29-sx0fr4.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=584&fit=crop&dpr=1 600w, https://images.theconversation.com/files/567656/original/file-20240103-29-sx0fr4.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=584&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/567656/original/file-20240103-29-sx0fr4.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=584&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/567656/original/file-20240103-29-sx0fr4.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=734&fit=crop&dpr=1 754w, https://images.theconversation.com/files/567656/original/file-20240103-29-sx0fr4.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=734&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/567656/original/file-20240103-29-sx0fr4.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=734&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Uranus’ changing colours as observed by HST/WFC3.</span>
<span class="attribution"><a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>This revealed that Uranus does change colour, becoming greener at the solstices (when the Sun’s path in the sky is the farthest north or south from the planet’s equator) than it is at the equinoxes (when the Sun’s path crosses the planet’s equator). </p>
<p>Part of the reason for this colour change is that Uranus spins almost on its side during its 84-year orbit about the Sun. This means that, during the planet’s solstices, either its north or south pole points almost directly towards the Sun and Earth. Hence, polar latitudes dominate the overall reflectivity.</p>
<p>This led us to develop a model which compared the spectra of Uranus’ polar regions to its equatorial regions. We found that polar regions are more reflective at green and red wavelengths than blue wavelengths, partly because methane is half as abundant near the poles than the equator. </p>
<p>However, this didn’t fully explain the colour change. To match the Lowell Observatory data, we found that we also need to add a “hood” of icy haze over the summer. This modified model then substantially reproduced the Lowell observations and thus explains how the overall colour of Uranus changes during its orbit about the Sun.</p>
<p>So the next time you see an old image of the two gas giants, keep in mind you’re probably not seeing their “true” colour.</p><img src="https://counter.theconversation.com/content/220244/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Patrick Gerard Joseph Irwin has received funding related to this study from the UK Science and Technology Facilities Council.</span></em></p>It turns out Uranus actually changes colour throughout the year.Patrick Gerard Joseph Irwin, Professor of Planetary Physics, University of OxfordLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2004542023-03-15T13:37:40Z2023-03-15T13:37:40ZCurious Kids: How are planets created?<figure><img src="https://images.theconversation.com/files/511620/original/file-20230222-16-c8nmnb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Eight planets, including Earth, revolve around our Sun.</span> <span class="attribution"><span class="source">Illustration by Tobias Roetsch/Future Publishing via Getty Images</span></span></figcaption></figure><p><em>Curious Kids is a <a href="https://theconversation.com/africa/topics/curious-kids-36782">series</a> for children in which we ask experts to answer questions from kids.</em></p>
<p><strong>How are planets created? - (Saba, 6, Kenya)</strong></p>
<p>Thanks for asking such an interesting question, Saba. When you talk about planets you’re probably thinking of the planets in our solar system – the ones orbiting (circling around) our sun. There are eight of these <a href="https://solarsystem.nasa.gov/planets/overview/#otp_planets_of_our_solar_system">planets</a>. One of them is where you and I live: Earth. The others are Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune.</p>
<p>There are many, many more planets way beyond our solar system and our galaxy, the Milky Way. Scientists like us, known as astronomers, have found <a href="https://www.planetary.org/worlds/exoplanets">over 5,000 planets</a> around other stars. We estimate that there may be trillions across the Universe.</p>
<p>How did they come into being? It all starts with a cloud of gas and dust.</p>
<h2>Gas and dust</h2>
<p>These clouds of gas and dust are called nebulae. They float around in space much like the clouds in our sky. There are some regions with more clouds and some with fewer and astronomers can see these using telescopes.</p>
<p>Nebulae contain gases like hydrogen, helium and carbon. When a nebula becomes dense enough its gravity pulls it together into a very dense core. This is a bit like the water in your bath swirling around the drain before getting sucked down. As the cloud gets dense it heats up. When it gets dense and hot enough the atoms – tiny building blocks for all the matter in the world – in the nebula start to fuse.</p>
<p>This process is called nuclear fusion and produces a lot of energy. And the cloud lights up like a firework. This is how a new star is born, just like our Sun was <a href="https://spaceplace.nasa.gov/sun-age/en/">4.5 billion years ago</a>.</p>
<p>A small amount of gas and dust remains around new stars in a spinning disc. Planets are formed from this disc of material.</p>
<h2>Protoplanets</h2>
<p>As the disc rotates, the material in it, small bits of rock and ice, lump together and get bigger and bigger. That forms what we call planetesimals, which collide with each other like bumper cars, creating even larger bodies known as protoplanets.</p>
<p>The protoplanets keep growing. While this is happening, they can attract gases from the surrounding disc, creating a thick atmosphere. This process is called accretion and it is how gas giant planets like Jupiter and Saturn are formed. If a protoplanet forms from heavier elements in the outer solar system it can create an ice giant. The planets Neptune and Uranus are ice giants.</p>
<p>Even after the planet is formed it can keep changing over time through processes such as volcanic activity, tectonic movement, and erosion. On Earth, mountains like Mount Kilimanjaro in Tanzania – the country next door to Kenya – formed from large volcanoes. And even larger mountains like the Himalayas have formed from tectonic plates colliding. Tectonic plates are big pieces of the Earth’s outer layer; sometimes they crash into each other and that creates things like mountains.</p>
<h2>Millions of years</h2>
<p>The way I’ve described this makes it sound as though planets are formed quickly. But the process which begins with those clouds of gas and dust takes millions of years to transform into the beautiful and diverse worlds we see in our Solar System and beyond.</p><img src="https://counter.theconversation.com/content/200454/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Daniel Cunnama receives funding from the National Research Foundation and the South African Astronomical Observatory. </span></em></p>It all starts with a cloud of gas and dust.Daniel Cunnama, Science Engagement Astronomer, South African Astronomical Observatory, South African Astronomical ObservatoryLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1759182022-02-21T05:00:45Z2022-02-21T05:00:45ZJupiter, Saturn, Uranus, Neptune: why our next visit to the giant planets will be so important (and just as difficult)<figure><img src="https://images.theconversation.com/files/446075/original/file-20220213-17-1s3hlo0.jpg?ixlib=rb-1.1.0&rect=15%2C3%2C2580%2C1191&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.youtube.com/watch?v=sOpMrVnjYeY&t=2225s&ab_channel=SpaceX">SpaceX</a></span></figcaption></figure><p>The giant planets – Jupiter, Saturn, Uranus and Neptune - are some of the most awe-inspiring in our Solar System, and have great importance for space research and our comprehension of the greater universe.</p>
<p>Yet they remain the least explored – especially the “ice giants” Uranus and Neptune – due to their distance from Earth, and the extreme conditions spacecraft must survive to enter their atmospheres. As such, they’re also the least understood planets in the Solar System.</p>
<p>Our <a href="https://arc.aiaa.org/doi/10.2514/1.A34282">ongoing</a> <a href="https://doi.org/10.2514/1.J060560">research</a> looks at how to overcome the harsh entry conditions experienced during giant planet missions. As we look forward to potential future missions, here’s what we might expect.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/446070/original/file-20220213-13-wp9do.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/446070/original/file-20220213-13-wp9do.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/446070/original/file-20220213-13-wp9do.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=227&fit=crop&dpr=1 600w, https://images.theconversation.com/files/446070/original/file-20220213-13-wp9do.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=227&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/446070/original/file-20220213-13-wp9do.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=227&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/446070/original/file-20220213-13-wp9do.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=286&fit=crop&dpr=1 754w, https://images.theconversation.com/files/446070/original/file-20220213-13-wp9do.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=286&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/446070/original/file-20220213-13-wp9do.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=286&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 about ten times as large as Earth – with a 69,911km radius (compared to Earth’s 6,371km radius).</span>
<span class="attribution"><span class="source">Beinahegut</span></span>
</figcaption>
</figure>
<h2>But first, what are giant planets?</h2>
<p>Unlike rocky planets, giant planets don’t have a surface to land on. Even in their lower atmospheres they remain gaseous, reaching extremely high pressures that would crush any spacecraft well before it could land on anything solid.</p>
<p>There are two types of giant planets: gas giants and ice giants. </p>
<p>The larger Jupiter and Saturn are gas giants. These are mainly made of hydrogen and helium, with an outer gaseous layer and a partially liquid “metallic” layer below that. They’re also believed to have a small rocky core. </p>
<p>Uranus and Neptune have similar outer atmospheres and rocky cores, but their inner layer is made up of about 65% water and other so-called “ices” (although these technically remain liquid) such as <a href="https://www.lpi.usra.edu/icegiants/mission_study/Exec-Summary.pdf">methane and ammonia</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/446068/original/file-20220213-17-gke7kv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/446068/original/file-20220213-17-gke7kv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/446068/original/file-20220213-17-gke7kv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/446068/original/file-20220213-17-gke7kv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/446068/original/file-20220213-17-gke7kv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/446068/original/file-20220213-17-gke7kv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/446068/original/file-20220213-17-gke7kv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/446068/original/file-20220213-17-gke7kv.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">Relative size and composition of the giant planets in our solar system (with Earth also shown for comparison).</span>
<span class="attribution"><span class="source">JPL/Caltech (based on material from the Lunar and Planetary Institute)</span></span>
</figcaption>
</figure>
<h2>Slingshots to the edge of the Solar System</h2>
<p>Any giant planet mission is extremely difficult. Still, there have been some past missions sent to the gas giants.</p>
<p>NASA’s 1989 Galileo mission had to slingshot around Venus and Earth to give it enough momentum to <a href="https://www.nasa.gov/feature/30-years-ago-galileo-off-to-orbit-jupiter">get to Jupiter</a>, which it orbited for eight years. The 2011 <a href="https://spaceflight101.com/juno/juno-mission-trajectory-design/">Juno mission</a> spent five years in transit, using a flyby around Earth to reach Jupiter (which it still orbits).</p>
<p>Similarly, the Cassini-Huygens mission run by NASA and the European Space Agency (ESA) <a href="https://sci.esa.int/web/cassini-huygens/-/31240-getting-to-saturn">took seven years</a> to reach Saturn. The spacecraft spent 13 years exploring the planet and its surrounds, and launched a probe to explore Saturn’s moon, <a href="https://solarsystem.nasa.gov/missions/cassini/science/titan/">Titan</a>.</p>
<p>Flight times get even longer for the two ice giants, which are much further from the Sun. Neither has had a dedicated mission so far. </p>
<h2>A complex journey</h2>
<p>The last and only spacecraft to visit the ice giants was <a href="https://solarsystem.nasa.gov/missions/voyager-2/in-depth/">Voyager 2</a>, which flew by Uranus in 1986 and Neptune in 1989. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/446498/original/file-20220215-17-rqmzoy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/446498/original/file-20220215-17-rqmzoy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/446498/original/file-20220215-17-rqmzoy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/446498/original/file-20220215-17-rqmzoy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/446498/original/file-20220215-17-rqmzoy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/446498/original/file-20220215-17-rqmzoy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/446498/original/file-20220215-17-rqmzoy.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">
<figcaption>
<span class="caption">Voyager 2, the only spacecraft ever to have visited Neptune, took a photo of the planet in 1989.</span>
<span class="attribution"><span class="source">NASA/JPL</span></span>
</figcaption>
</figure>
<p>While momentum is building for a return, it won’t be simple. If we launch during the next convenient <a href="https://www.lpi.usra.edu/icegiants/mission_study/Exec-Summary.pdf">launch windows</a> of 2030–34 for Uranus and 2029–30 for Neptune, flight times would vary from 11 to 15 years.</p>
<p>A major issue is power. The Juno spacecraft is the most distant object from the Sun to have <a href="https://www.jpl.nasa.gov/news/nasas-juno-spacecraft-breaks-solar-power-distance-record">used solar panels</a>. It orbits Jupiter, which is <a href="https://solarsystem.nasa.gov/planets/jupiter/in-depth/">five times further away</a> from the Sun than Earth is. Yet, where Juno’s solar cells would generate 14 kilowatts of continuous power on Earth, they only <a href="https://www.jpl.nasa.gov/news/nasas-juno-spacecraft-breaks-solar-power-distance-record">generate 0.5kW at Jupiter</a>. </p>
<p>Meanwhile, Uranus and Neptune are <a href="https://solarsystem.nasa.gov/planets/uranus/in-depth/">20</a> and <a href="https://solarsystem.nasa.gov/planets/neptune/in-depth/">30</a> times further away, respectively, from the Sun than Earth is. Power for these missions would have to be generated from the radioactive <a href="https://solarsystem.nasa.gov/missions/galileo/in-depth/#otp_spacecraft_and_instruments">decay of plutonium</a> (the power source for both the Galileo and Cassini missions). </p>
<p>This radioactive decay can damage and interfere with instruments. It is therefore reserved for spacecraft which really need it, such as missions operating far away from the Sun. </p>
<hr>
<p>
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Read more:
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<hr>
<h2>Fighting the heat</h2>
<p>The massive scale of giant planets means orbit speeds for incoming spacecraft are incredibly fast. And these speeds greatly heat up the spacecraft. </p>
<p>The Galileo probe entered Jupiter’s atmosphere at <a href="https://solarsystem.nasa.gov/missions/galileo-probe/in-depth/">47.5 kilometres per second</a>, surviving the harshest entry conditions ever experienced by an entry probe. The shock layer which formed at the front of the spacecraft during entry reached a temperature of 16,000°C – around three times the temperature of the Sun’s surface.</p>
<p>Even so, the distribution of the <a href="https://arc.aiaa.org/doi/10.2514/2.3293">heat shield’s</a> mass was found to be inefficient – showing we still have a lot to learn about entering giant planets.</p>
<p>Proposed future probe missions to Uranus and Neptune would occur at slower entry speeds of <a href="https://link.springer.com/article/10.1007/s11214-020-0638-2">22km/s and 26km/s</a>, respectively. </p>
<p>For this, NASA have developed a tough but relatively lightweight material woven from carbon fibre, called <a href="https://www.nasa.gov/ames/heeet">HEEET</a> (Heatshield for Extreme Entry Environment Technology), designed specifically for surviving giant planet and Venusian entry. </p>
<p>While the material has been tested with a <a href="https://www.nasa.gov/centers/ames/entry-systems-vehicle-development/tps-materials.html">full-scale prototype</a>, it has yet to fly on a mission.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/446526/original/file-20220215-8037-1brq7ct.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/446526/original/file-20220215-8037-1brq7ct.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/446526/original/file-20220215-8037-1brq7ct.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/446526/original/file-20220215-8037-1brq7ct.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/446526/original/file-20220215-8037-1brq7ct.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/446526/original/file-20220215-8037-1brq7ct.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/446526/original/file-20220215-8037-1brq7ct.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">
<figcaption>
<span class="caption">It’s planned NASA’s HEEET material will be used for future ice giant entry missions.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<h2>The next steps</h2>
<p>In 2024, NASA’s Europa Clipper mission <a href="https://europa.nasa.gov/">will launch</a> to investigate Jupiter’s moon Europa, which is believed to house an <a href="https://europa.nasa.gov/why-europa/overview/">ocean of liquid water</a> below its icy surface, where signs of life may be found. The <a href="https://www.nasa.gov/press-release/nasas-dragonfly-will-fly-around-titan-looking-for-origins-signs-of-life/">Dragonfly</a> mission, planned to launch in 2026, will similarly aim to search for signs of life on Saturn’s moon Titan.</p>
<p>There are plans for a joint <a href="https://www.sciencedirect.com/science/article/pii/S0032063318303507">NASA-ESA mission</a> to visit one of the ice giants within the upcoming launch window. But while there has been <a href="https://www.lpi.usra.edu/icegiants/documents_presentations/">extensive</a> <a href="https://sci.esa.int/web/future-missions-department/-/61307-cdf-study-report-ice-giants">preparation</a>, it’s undecided which ice giant will be visited. </p>
<p>A single mission to both planets is being considered. An entry probe is planned, too. But if the mission visits both planets, it’s undecided which planet’s <a href="https://www.sciencedirect.com/science/article/pii/S003206331830350">atmosphere the probe would explore</a>.</p>
<p>If we want to meet the upcoming launch window, it’s expected mission concepts will need to be finalised <a href="https://www.sciencedirect.com/science/article/pii/S0032063320300040">by 2025</a>, at the latest. In other words, crunch time is coming. </p>
<p>Should a mission go forward, the two most important <a href="https://www.lpi.usra.edu/icegiants/mission_study/Full-Report.pdf">goals</a> for NASA’s scientists will be to determine the interior makeup of ice giants (exactly what they are made of) and their composition (how they are formed).</p>
<p>Other objectives will include studying their magnetic fields, which are <a href="https://www.lpi.usra.edu/icegiants/mission_study/Full-Report.pdf">very different</a> to gas giants and all other types of planets. </p>
<p>They’ll also want to study the heat released by both Uranus and Neptune, which both have average temperatures of around -200°C. All giant planets are meant to be very slowly cooling down, as they release energy gained during their formation. </p>
<p>This heat release can be detected for Jupiter, Saturn and Neptune. Uranus, however, doesn’t seem to release heat – and scientists don’t know why.</p><img src="https://counter.theconversation.com/content/175918/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Chris James receives funding from the University of Queensland, the Australian Research Council, and the U.S. Office of Naval Research. </span></em></p><p class="fine-print"><em><span>Yu Liu 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>There has never been a dedicated mission sent to the “ice giants”, Uranus and Neptune. But there may be one on the horizon.Chris James, ARC DECRA Fellow, Centre for Hypersonics, School of Mechanical and Mining Engineering, The University of QueenslandYu Liu, Honorary Fellow, The University of QueenslandLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1710552021-11-08T11:38:56Z2021-11-08T11:38:56ZCurious Kids: what is the coldest planet in the Solar System?<figure><img src="https://images.theconversation.com/files/429980/original/file-20211103-25-titpeg.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C5333%2C2993&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/planet-uranus-elements-this-image-furnished-763214644">NASA images/Shutterstock</a></span></figcaption></figure><p><strong>What is the coldest planet in the Solar System? – Sejal, aged seven, Bangalore, India</strong></p>
<p>The planets in our Solar System are heated by the Sun. Here on Earth, we are about 100 million miles away from the Sun – a distance that provides the perfect temperature for life. </p>
<p>You might think, then, that the coldest planet in the Solar System would be <a href="https://solarsystem.nasa.gov/planets/neptune/overview/">Neptune</a>, as it is the furthest away from the Sun’s warmth. Neptune is an incredible three billion miles away from the Sun. </p>
<p>However, the coldest planet is not Neptune, <a href="https://solarsystem.nasa.gov/planets/uranus/overview/">but Uranus</a> – even though Uranus is a billion miles closer to the Sun than Neptune. Uranus holds the record for the coldest temperature ever measured in the Solar System: a very chilly <a href="https://solarsystem.nasa.gov/planets/uranus/in-depth/">-224°C</a>. The temperature on Neptune is still very cold, of course – usually around <a href="https://www.ucl.ac.uk/culture-online/ask-expert/your-questions-answered/how-cold-neptune-which-planet-would-you-most-visit">-214°C</a> – but Uranus beats that. </p>
<h2>Knocked sideways</h2>
<p>The reason why Uranus is so cold is nothing to do with its distance from the Sun. Billions of years ago, something big <a href="https://www.nasa.gov/feature/ames/planet-shifting-collision-shaped-uranus-rolling-rotation">crashed into Uranus</a> with so much force that it tipped the planet over onto its side. Uranus still rolls around the Sun on its side today. The impact of the crash also let some of the heat that was trapped inside Uranus escape. </p>
<figure class="align-center ">
<img alt="Image showing blue and purple planet with rings" src="https://images.theconversation.com/files/429972/original/file-20211103-13-ik4zvc.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/429972/original/file-20211103-13-ik4zvc.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=554&fit=crop&dpr=1 600w, https://images.theconversation.com/files/429972/original/file-20211103-13-ik4zvc.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=554&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/429972/original/file-20211103-13-ik4zvc.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=554&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/429972/original/file-20211103-13-ik4zvc.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=696&fit=crop&dpr=1 754w, https://images.theconversation.com/files/429972/original/file-20211103-13-ik4zvc.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=696&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/429972/original/file-20211103-13-ik4zvc.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=696&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A Hubble Space Telescope view showing Uranus surrounded by its 4 major rings and 10 of its 17 known satellites.</span>
<span class="attribution"><a class="source" href="https://images.nasa.gov/details-0301099">NASA</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span>
</figcaption>
</figure>
<p>The heat inside planets is left over from when they were formed. Planets are made when smaller chunks of rock smash together, building the full planet piece by piece over many millions of years. Every time these rocks smash together, the planet gains a little more heat. If you clap your hands together for a long time they will start to feel hot – the same thing happens with planets. </p>
<p>Neptune wasn’t hit by a huge asteroid like Uranus was, so it has been able to hold on to more of its heat. </p>
<p>You might also be surprised to learn that the closest planet to the Sun, Mercury, can also be extremely cold. While the side of Mercury facing the Sun is more than 400°C, the side facing away from the Sun is nearly -200°C. </p>
<hr>
<figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=293&fit=crop&dpr=1 600w, https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=293&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=293&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=368&fit=crop&dpr=1 754w, https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=368&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=368&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<p><em><a href="https://theconversation.com/au/topics/curious-kids-36782">Curious Kids</a> is a series by <a href="https://theconversation.com/uk">The Conversation</a> that gives children the chance to have their questions about the world answered by experts. If you have a question you’d like an expert to answer, send it to <a href="mailto:curiouskids@theconversation.com">curiouskids@theconversation.com</a>. We won’t be able to answer every question, but we’ll do our very best.</em></p>
<hr>
<p>The reason for this is that Mercury <a href="https://solarsystem.nasa.gov/planets/mercury/in-depth/#:%7E:text=Temperatures%20on%20Mercury%20are%20extreme,(minus%20180%20degrees%20Celsius).">does not have any atmosphere</a>, unlike Earth. An atmosphere like ours acts like a blanket, holding heat in and spreading it all around. Because it does not have this blanket, the front side and the back side of Mercury can have very different temperatures.</p>
<h2>Measuring temperatures in space</h2>
<p>For some nearby planets like Mars, we can send probes to study the atmosphere directly from the planet’s surface. However, we haven’t been able to do this for distant planets such as Neptune and Uranus. </p>
<p>Instead, we have to work out how cold they are by measuring their temperature from here on Earth. We do this by studying the light from the planet, which can tell us the types of atoms and molecules which make up the planet’s atmosphere. This information lets us know exactly what the temperature of the planet is: the atoms and molecules act as a kind of temperature “fingerprint” for the planet. </p>
<p>While these planets in our Solar System are incredibly cold, there are even chillier places in the universe. The coldest of all is the <a href="https://www.esa.int/Science_Exploration/Space_Science/Extreme_space/Coldest_place_in_the_Universe">Boomerang Nebula</a>, a cloud of dust and gas 30 million billion miles away from us. There, the temperature reaches -272°C. </p>
<p>Nothing in the Universe can be colder than -273°C, because at that temperature the tiny particles and atoms that everything is made of basically stop moving, and once that happens it’s impossible to go colder. This temperature is known as <a href="https://www.jpl.nasa.gov/news/the-coolest-experiment-in-the-universe">absolute zero</a>. This means it is unlikely that we will ever find anywhere in the Universe colder than the Boomerang Nebula.</p>
<hr>
<p><em>When sending in questions to Curious Kids, make sure you include the asker’s first name, age and town or city. You can:</em></p>
<ul>
<li><em>email <a href="mailto:curiouskids@theconversation.com">curiouskids@theconversation.com</a></em></li>
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<hr><img src="https://counter.theconversation.com/content/171055/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Brad Gibson 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>It can get very cold in space.Brad Gibson, Director of the E.A. Milne Centre for Astrophysics and Head of the Department of Physics and Mathematics, University of HullLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1621922021-06-07T09:59:19Z2021-06-07T09:59:19ZNasa has just rejected missions to moons of Jupiter and Neptune – here’s what we would have found out<figure><img src="https://images.theconversation.com/files/404489/original/file-20210604-27-1ybuyah.png?ixlib=rb-1.1.0&rect=114%2C45%2C2378%2C1363&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A volcanic eruption on Jupiter's moon Io.</span> <span class="attribution"><a class="source" href="https://solarsystem.nasa.gov/resources/1039/galileo-sees-io-erupt/?category=moons/jupiter-moons_io">NASA/JPL/DLR</a></span></figcaption></figure><p>It’s been 30 years since Nasa last visited Venus, with <a href="https://theconversation.com/astronomers-spot-strange-bow-like-structure-in-venus-atmosphere-71347">the Magellan orbiter</a> in 1990. Now, <a href="https://www.nasa.gov/press-release/nasa-selects-2-missions-to-study-lost-habitable-world-of-venus/">two new missions</a> have been selected to explore the deadly atmosphere, crushing pressures and volcanic landscape. </p>
<p>The process dates back to February 2020, when Nasa announced that four missions were to undergo a nine-month peer-review process for feasibility. They were all part of the <a href="https://www.nasa.gov/planetarymissions/discovery.html">Discovery program</a>, started by Nasa in 1992 to bring together scientists and engineers to create exciting, groundbreaking missions. Set aside from the flagship missions – such as <a href="https://theconversation.com/curiosity-catches-a-whiff-of-methane-on-mars-and-a-possibility-of-past-life-35595">Curiosity</a> and <a href="https://theconversation.com/uk/search?q=perseverance">Perseverance</a> – the missions operating under Discovery have taken unique and innovative approaches to exploring the solar system.</p>
<p>The two winning Venus missions, <a href="https://theconversation.com/nasa-has-announced-two-missions-to-venus-by-2030-heres-why-thats-exciting-162133">Davinci and Veritas</a>, have been awarded US$500 million (£354 million) and will be launched some time between 2028 and 2030. But the competition was tough from the two losing missions, which would have gone to <a href="https://www.nasa.gov/planetarymissions/io-volcano-observer">Io</a> and <a href="https://www.nasa.gov/feature/jpl/proposed-nasa-mission-would-visit-neptunes-curious-moon-triton">Triton</a>, respectively moons of Jupiter and Neptune. So what are we missing out on as a result?</p>
<h2>Exploring Jupiter’s bizarre moon</h2>
<p>Io is a strange moon – even among moons, which are strange to begin with. As Jupiter’s innermost moon, orbiting a mere 350,000 km above the cloud tops, it gives Io an extreme <a href="https://www.youtube.com/watch?v=9qHrzs6Mbp4">heating mechanism</a> that makes it the <a href="https://ui.adsabs.harvard.edu/abs/2004Icar..169..140L/abstract">most volcanically active</a> object in the solar system, sporting over four hundred volcanoes. </p>
<p>You might think, given we live on a planet with a fair share of volcanoes, that we’d have a good idea of where all this heat is coming from. In fact, according to Alfred McEwen, principal investigator on the proposed Io Volcanic Explorer or IVO mission, we’re still profoundly ignorant of how it actually works.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/nasa-has-announced-two-missions-to-venus-by-2030-heres-why-thats-exciting-162133">Nasa has announced two missions to Venus by 2030 – here's why that's exciting</a>
</strong>
</em>
</p>
<hr>
<p>IVO was designed to perform multiple fly-bys of the moon and use a suite of instruments to map the activity on and below the surface. By collecting information on Io’s magnetic and gravitational fields, taking videos of the enormous lava eruptions and analysing the gas and dust escaping from the moon, IVO would help scientists learn how Io’s heat is generated and lost.</p>
<p>All of this information is crucial – not just for awesome videos of space volcanoes – because this kind of extreme activity is believed to be an <a href="https://www.nasa.gov/planetarymissions/io-volcano-observer">important aspect</a> of planetary formation and evolution. By understanding the processes that drive change on Io, we can ultimately learn more about how planets and moons came to be.</p>
<h2>The ice giants</h2>
<p>The least explored and understood planets are Uranus and Neptune, and they are home to some of the most bizarre things in the solar system. Uranus has an axial tilt – the angle of its axis of rotation compared to the plane it orbits the Sun – so extreme that it spins on its side. This is thought to be the result of a <a href="http://icc.dur.ac.uk/giant_impacts/">giant collision</a> in the solar system’s past. </p>
<p>Meanwhile, Neptune is home to the <a href="https://www.nasa.gov/feature/jpl/proposed-nasa-mission-would-visit-neptunes-curious-moon-triton">only large moon</a> that orbits backwards around its parent planet, the curious Triton. The peculiar orbital arrangement isn’t where the oddities end. The plane in which Triton orbits is offset by an extreme 23 degrees compared to Neptune’s, and it is believed to have moved to <a href="https://ui.adsabs.harvard.edu/abs/2006Natur.441..192A/abstract">Neptune from the Kuiper Belt</a>, the region beyond Neptune’s orbit filled with icy leftovers from the solar system’s formation. </p>
<figure class="align-center ">
<img alt="A diagram showing the surface of Triton and what the Trident mission was aiming to do." src="https://images.theconversation.com/files/404488/original/file-20210604-25-vy73f1.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/404488/original/file-20210604-25-vy73f1.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=628&fit=crop&dpr=1 600w, https://images.theconversation.com/files/404488/original/file-20210604-25-vy73f1.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=628&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/404488/original/file-20210604-25-vy73f1.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=628&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/404488/original/file-20210604-25-vy73f1.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=789&fit=crop&dpr=1 754w, https://images.theconversation.com/files/404488/original/file-20210604-25-vy73f1.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=789&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/404488/original/file-20210604-25-vy73f1.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=789&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">What the Trident mission would’ve done.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/sites/default/files/thumbnails/image/pia23874-1041.jpg">NASA/JPL-Caltech</a></span>
</figcaption>
</figure>
<p>Triton also has an <a href="https://ui.adsabs.harvard.edu/abs/1992AdSpR..12k.113L/abstract">active ionosphere</a> – a layer of charged particles in its atmosphere ten times more active than any other moon, which isn’t powered by the Sun – as well as a constantly changing and dynamic surface, coated in what might be nitrogen snow. When <a href="https://solarsystem.nasa.gov/moons/neptune-moons/triton/in-depth/">Voyager 2 photographed the moon</a>, it discovered cryovolcanoes – geysers erupting ice and gas up to 8km high, which might indicate a subsurface ocean. </p>
<p>The proposed Trident mission would have explored these many strange things about the moon. It proposed a three-pronged approach using instruments to measure the magnetic field of Triton. It would have identified the presence and structure of a subsurface ocean. High resolution infrared cameras would have allowed the spacecraft to image the entire surface, using the sunlight reflected from Neptune, showing scientists what had changed since the last visit in 1989. Finally, the spacecraft would have tried to discover how Triton’s surface remains so dynamic and young.</p>
<p>Ultimately, Trident and IVO lost out to the Venus missions. It would have been fascinating to once again explore the outer reaches of the solar system, or see the colossal volcanoes of Io. But Venus is a fascinating planet, with mysteries and potential all of its own.</p><img src="https://counter.theconversation.com/content/162192/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Ashley Spindler 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>When two missions to Venus were announced, two others missed out.Ashley Spindler, STFC Innovation Fellow, University of HertfordshireLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1241582019-12-16T13:43:04Z2019-12-16T13:43:04ZPlanetary confusion – why astronomers keep changing what it means to be a planet<figure><img src="https://images.theconversation.com/files/300167/original/file-20191104-88409-13vd5ta.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">In 2015, NASA’s New Horizons spacecraft looked back toward the sun and captured this near-sunset view of the rugged, icy mountains and flat ice plains extending to Pluto’s horizon. </span> <span class="attribution"><a class="source" href="https://www.nasa.gov/image-feature/pluto-s-majestic-mountains-frozen-plains-and-foggy-hazes">NASA/JHUAPL/SwRI</a></span></figcaption></figure><p>As an astronomer, the question I hear the most is why isn’t Pluto a planet anymore? More than 10 years ago, astronomers famously voted to change <a href="https://www.nytimes.com/2006/08/24/science/space/25pluto.html">Pluto’s classification</a>. But the question still comes up. </p>
<p>When I am asked directly if I think Pluto is a planet, I tell everyone my answer is no. It all goes back to the origin of the word “planet.” It comes from the Greek phrase for “wandering stars.” Back in ancient times before the telescope was invented, the mathematician and astronomer Claudius Ptolemy called stars “fixed stars” to distinguish them from the seven wanderers that move across the sky in a very specific way. <a href="https://www.loc.gov/collections/finding-our-place-in-the-cosmos-with-carl-sagan/articles-and-essays/modeling-the-cosmos/ancient-greek-astronomy-and-cosmology">These seven objects are the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn</a>.</p>
<p>When people started using the word “planet,” they were referring to those seven objects. Even Earth was not originally called a planet – but the Sun and Moon were. </p>
<p>Since people use the word “planet” today to refer to many objects beyond the original seven, it’s no surprise we argue about some of them.</p>
<p>Although I am trained as an astronomer and I studied more distant objects like stars and galaxies, I have an interest in the objects in our Solar System because I teach several classes on planetary science.</p>
<h2>Asteroids, the first demoted planets</h2>
<p>The word “planet” is used to describe <a href="https://www.skyandtelescope.com/observing/ice-giants-neptune-and-uranus/">Uranus and Neptune</a>, which were discovered in 1781 and 1846 respectively, because they move in the same way that the other “wandering stars” move. Like Saturn and Jupiter, if you look at them through a telescope, they appear bigger than stars, so they were recognized to be more like planets than stars. </p>
<p>Not long after the discovery of Uranus, astronomers discovered additional wandering objects – these were named <a href="https://www.esa.int/About_Us/Welcome_to_ESA/ESA_history/Asteroids_The_discovery_of_asteroids">Ceres, Pallas, Juno and Vesta</a>. At the time they were considered planets, too. Through a telescope they look like pinpoints of light and not disks. With a small telescope, even <a href="http://www.astronomy.com/observing/sky-this-month/2018/08/neptune-at-its-best">distant Neptune appears fuzzier than a star</a>. Even though these other, new objects were called planets at first, astronomers thought they needed a different name since they appear more star-like than planet-like. </p>
<p>William Herschel (who discovered Uranus) is often said to have named them “asteroids” which means “star-like,” but <a href="https://www.skyandtelescope.com/astronomy-news/why-do-we-call-them-asteroids/">recently, Clifford Cunningham</a> claimed that the person who coined that name was Charles Burney Jr., a preeminent Greek scholar. </p>
<p>Today, just like the word “planet,” we use the word “asteroid” differently. Now it refers to objects that are rocky in composition, mostly found between Mars and Jupiter, mostly irregularly shaped, smaller than planets, but bigger than meteoroids. Most people assume there is a strict definition for what makes an object an asteroid. But there isn’t, just like there never was for the word “planet.”</p>
<p>In the 1800s the large asteroids were called planets. Students at the time likely learned that the planets were Mercury, Venus, Earth, Mars, Ceres, Vesta, Pallas, Juno, Jupiter, Saturn, Uranus and, eventually, Neptune. Most books today write that asteroids are different than planets, but there is a <a href="https://arxiv.org/abs/1805.04115">debate among astronomers</a> about whether the term “asteroid” was originally used to mean a small type of planet, rather than a different type of object altogether. </p>
<h2>How are moons different than planets?</h2>
<p>These days, scientists consider properties of these celestial objects to figure out whether an object is a planet or not. For example, you might say that shape is important; planets should be mostly spherical, while asteroids can be lumpy. As astronomers try to fix these definitions to make them more precise, we then create new problems. If we use roundness as an important distinction for objects, what should we call moons? Should moons be considered planets if they are round and asteroids if they are not round? Or are they somehow different from planets and asteroids altogether? </p>
<p>I would argue we should again look to how the word “moon” came to refer to objects that orbit planets. </p>
<p>When astronomers talk about the Moon of Earth, we capitalize the word “Moon” to indicate that it’s a proper name. That is, <a href="http://curious.astro.cornell.edu/observational-astronomy/seti-and-extraterrestrial-life/159-our-solar-system/the-sun/the-solar-system/4-what-are-the-names-of-the-earth-moon-sun-and-solar-system-beginner">the Earth’s moon has the name, Moon</a>. For much of human history, it was the only Moon known, so there was no need to have a word that referred to one celestial body orbiting another. This changed when <a href="http://galileo.rice.edu/sci/observations/jupiter_satellites.html">Galileo discovered four large objects orbiting Jupiter</a>. These are now called Io, Europa, Ganymede and Callisto, the moons of Jupiter. </p>
<p>This makes people think the technical definition of moon is a satellite of another object, and so we call lots of objects that orbit Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, Eris, Makemake, Ida and a <a href="https://stardate.org/radio/program/2018-08-28">large number of other asteroids</a> moons. When you start to look at the variety of moons, some, like Ganymede and Titan, are larger than Mercury. Some are similar in size to the object they orbit. Some are small and irregularly shaped, and some have odd orbits. </p>
<p>So they are not all just like Earth’s Moon. If we try to fix the definition for what is a moon and how that differs from a planet and asteroid, we are likely going to have to reconsider the classification of some of these objects, too. You can argue that Titan has more properties in common with the planets than Pluto does, for example. You can also argue that every single particle in Saturn’s rings is an individual moon, which would mean that Saturn has billions upon billions of moons.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/300166/original/file-20191104-88419-3j0wfr.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/300166/original/file-20191104-88419-3j0wfr.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/300166/original/file-20191104-88419-3j0wfr.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/300166/original/file-20191104-88419-3j0wfr.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/300166/original/file-20191104-88419-3j0wfr.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/300166/original/file-20191104-88419-3j0wfr.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/300166/original/file-20191104-88419-3j0wfr.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/300166/original/file-20191104-88419-3j0wfr.png?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">Family portrait of Pluto’s moons: This composite image shows a sliver of Pluto’s large moon, Charon, and all four of Pluto’s small moons.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/image-feature/charon-and-the-small-moons-of-pluto">NASA/JHUAPL/SwRI</a></span>
</figcaption>
</figure>
<h2>Planets around other stars</h2>
<p>The most recent naming challenge astronomers face arose when they discovering planets far from our Solar System orbiting around distant stars. These objects have been called extrasolar planets, exosolar planets or <a href="http://exoplanets.org/">exoplanets</a>. </p>
<p>Astronomers are currently searching for <a href="https://arxiv.org/abs/1904.10618">exomoons</a> orbiting exoplanets. Exoplanets are being discovered that have properties unlike the planets in our Solar System, so astronomers have started putting them in categories like “hot Jupiter,” “warm Jupiter,” “super-Earth” and “mini-Neptune.” </p>
<p>Ideas for how planets form also suggest that there are planetary objects that have been flung out of orbit from their parent star. This means there are <a href="https://exoplanets.nasa.gov/resources/28/free-floating-planets-may-be-more-common-than-stars/">free-floating planets</a> not orbiting any star. Should planetary objects that are flung out of a solar system also get ejected from the elite club of planets? </p>
<p>When I teach, I end this discussion with a recommendation. Rather than arguing over planet, moon, asteroid and exoplanet, I think we need to do what Herschel and Burney did and coin a new word. For now, I use “world” in my class, but I do not offer a rigorous definition of what makes something a world and what does not. Instead, I tell my students that all of these objects are of interest to study. </p>
<h2>The Sun was once a planet</h2>
<p>A lot of people seem to feel that scientists wronged Pluto by changing its classification. I look at it that Pluto was only originally called a planet because of an accident; scientists were looking for planets beyond Neptune, and when they found Pluto they called it a planet, even though its observable properties should have led them to call it an asteroid. </p>
<p>As our understanding of this object has grown, I feel like the evidence now leads me to call Pluto something besides planet. There are other scientists who disagree, feeling Pluto still should be classified as a planet. </p>
<p>But remember: The Greeks started out calling the Sun a planet given how it moved on the sky. We now know that the properties of the Sun show it to belong in a very different category from the planets; it’s a star, not a planet. If we can stop calling the Sun a planet, why can’t we do the same to Pluto?</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/300165/original/file-20191104-88387-brm58q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/300165/original/file-20191104-88387-brm58q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/300165/original/file-20191104-88387-brm58q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/300165/original/file-20191104-88387-brm58q.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/300165/original/file-20191104-88387-brm58q.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/300165/original/file-20191104-88387-brm58q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/300165/original/file-20191104-88387-brm58q.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/300165/original/file-20191104-88387-brm58q.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">The Kepler-90 planets have a similar configuration to our solar system with small planets found orbiting close to their star, and the larger planets found farther away.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/image-feature/ames/kepler-90-system-planet-sizes">NASA/Ames Research Center/Wendy Stenzel</a></span>
</figcaption>
</figure>
<p>[ <em>Like what you’ve read? Want more?</em> <a href="https://theconversation.com/us/newsletters?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=likethis">Sign up for The Conversation’s daily newsletter</a>. ]</p><img src="https://counter.theconversation.com/content/124158/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Christopher Palma does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Many people are still upset that Pluto was demoted from being a planet. But definitions of various celestial objects are fairly fluid. So whether it is an asteroid or moon or planet is up for debate.Christopher Palma, Associate Dean for Undergraduate Students and Teaching Professor of Astronomy & Astrophysics, Penn StateLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1241632019-10-02T20:13:42Z2019-10-02T20:13:42ZWhat moons in other solar systems reveal about planets like Neptune and Jupiter<figure><img src="https://images.theconversation.com/files/295058/original/file-20191001-173407-17sejqu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Exomoons orbiting an exoplanet outside our solar system.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-illustration/beautiful-exoplanet-exomoons-orbiting-alien-binary-744849175?src=2ZJlWkTDSoIH0Tay3OeqBw-1-1">Dotted Yeti/Shutterstock.com</a></span></figcaption></figure><p>What is the difference between a planet-satellite system as we have with the Earth and Moon, versus a binary planet – two planets orbiting each other in a cosmic do-si-do?</p>
<p><a href="http://www.astro.ucla.edu/%7Ehansen">I am an astronomer</a> interested in planets orbiting nearby stars, and gas giants – Jupiter, Saturn, Uranus and Neptune in our solar system – are the largest and easiest planets to detect. The crushing pressure within their gassy atmosphere means they are unlikely to be hospitable to life. But the rocky moons orbiting such planets could have conditions that are more welcoming. Last year, astronomers discovered a planet-sized exomoon orbiting another gas giant planet outside our solar system.</p>
<p><a href="https://advances.sciencemag.org/content/5/10/eaaw8665">In a new paper</a>, I argue that this exomoon is really what is called a captured planet.</p>
<h2>Is the first detected ‘exomoon’ really a moon?</h2>
<p>True Earth analogues, that orbit Sun-like stars, are very hard to detect, even with the large <a href="http://keckobservatory.org">Keck telescopes</a>. The task is easier if the host star is less massive. But then the planet has to be closer to the star to be warm enough, and the star’s gravitational tides may trap the planet in a state with a permanent hot side and a permanent cold side. This makes such planets less attractive as a potential location that could harbor life. When gas giants orbiting Sun-like stars have rocky moons, these may be more likely places to find life.</p>
<p>In 2018, two astronomers from Columbia University reported the first tentative <a href="http://doi.org/10.1126/sciadv.aav1784">observation of an exomoon</a> – a satellite orbiting a planet that itself orbits another star. One curious feature was that this exomoon <a href="http://exoplanet.eu/catalog/kepler-1625_b_i/">Kepler-1625b-i</a> was much more massive than any moon found in our solar system. It has a mass similar to Neptune and orbits a planet similar in size to Jupiter. </p>
<p>Astronomers expect moons of planets like Jupiter and Saturn to have masses only a few percent of Earth. But this new exomoon was almost a thousand times larger than the corresponding bodies of our solar system – moons like Ganymede and Titan which orbit Jupiter and Saturn, respectively. It is very difficult to explain the formation of such a large satellite using current models of moon formation.</p>
<p>In a new model I developed, I discuss how <a href="https://advances.sciencemag.org/content/5/10/eaaw8665">such a massive exomoon</a> forms through a different process, wherein it is really a captured planet.</p>
<p>All planets, large and small, start by gathering together asteroid-sized bodies to make a rocky core. At this early stage in the evolution of a planetary system, the rocky cores are still surrounded by a gaseous disk left over from the formation of the parent star. If a core can grow fast enough to reach a mass equivalent to 10 Earths, then it will have the gravitational strength to pull gas in from the surrounding space and grow to the massive size of Jupiter and Saturn. However, this gaseous accumulation is short-lived, as the star is draining away most of the gas in the disk, the dust and gas surrounding a newly formed star.</p>
<p>If there are two cores growing in close proximity, then they compete to capture rock and gas. If one core gets slightly larger, it gains an advantage and can capture the bulk of the gas in the neighborhood for itself. This leaves the second body without any further gas to capture. The increased gravitational pull of its neighbor drags the smaller body into the role of a satellite, albeit a very large one. The former planet is left as a super-sized moon, orbiting the planet that beat it out in the race to capture gas.</p>
<h2>A remnant core as a look back into history</h2>
<p>Viewed in this context, the captured planet is unlikely to be habitable. Growing planetary cores have gaseous envelopes, which make them more like Uranus and Neptune – a mix of rocks, ice and gas that would have become a Jupiter if it had not been so rudely cut off by its larger neighbor. </p>
<p>However, there are other implications that are almost as interesting. Studying the cores of giant planets is very difficult, because they are buried under several hundred Earth masses of hydrogen and helium. Currently, the <a href="https://www.nasa.gov/mission_pages/juno/main/index.html">JUNO mission</a> is attempting to do this for Jupiter. However, studying the properties of this exomoon may enable astronomers to see the naked core of a giant gaseous planet when it is stripped of its gaseous envelope. This can provide a snapshot of what Jupiter may have looked like before it grew to its current enormous size.</p>
<p>This exomoon system Kepler-1625b-i is right at the edge of what is detectable with current technology. There may be many more objects like this that could be uncovered with future improvements in telescope capabilities. As astronomers’ census of exoplanets continues to grow, systems like the exomoon and its host highlight an issue that will become more important as we go forward. This exomoon reveals that the properties of a planet are not solely a consequence of its mass and position, but can depend on its history and the environment in which it formed. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/295064/original/file-20191001-173358-h4rsv6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/295064/original/file-20191001-173358-h4rsv6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=474&fit=crop&dpr=1 600w, https://images.theconversation.com/files/295064/original/file-20191001-173358-h4rsv6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=474&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/295064/original/file-20191001-173358-h4rsv6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=474&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/295064/original/file-20191001-173358-h4rsv6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=596&fit=crop&dpr=1 754w, https://images.theconversation.com/files/295064/original/file-20191001-173358-h4rsv6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=596&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/295064/original/file-20191001-173358-h4rsv6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=596&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Exomoons may reveal secrets about how gas giants like Jupiter formed and what is in their core.</span>
<span class="attribution"><a class="source" href="https://www.jpl.nasa.gov/news/press_kits/juno/science/">JPL/NASA</a></span>
</figcaption>
</figure><img src="https://counter.theconversation.com/content/124163/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Bradley Hansen does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>A giant exomoon hundreds of times the size of Earth is revealing secrets about how giant planets like Jupiter and Saturn formed. They might also help astronomers find planets where life may thrive.Bradley Hansen, Professor of Physics and Astronomy, University of California, Los AngelesLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1209452019-08-14T12:32:30Z2019-08-14T12:32:30ZA brief astronomical history of Saturn’s amazing rings<figure><img src="https://images.theconversation.com/files/287419/original/file-20190808-144892-1u8fsji.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">With giant Saturn hanging in the blackness and sheltering Cassini from the Sun's blinding glare, the spacecraft viewed the rings as never before.</span> <span class="attribution"><a class="source" href="https://www.nasa.gov/multimedia/imagegallery/image_feature_1202.html">NASA/JPL/Space Science Institute</a></span></figcaption></figure><p>Many dream of what they would do had they a time machine. Some would travel 100 million years back in time, when dinosaurs roamed the Earth. Not many, though, would think of taking a telescope with them, and if, having done so, observe Saturn and its rings.</p>
<p>Whether our time-traveling astronomer would be able to observe Saturn’s rings is debatable. Have the rings, in some shape or form, existed since the beginnings of the solar system, 4.6 billion years ago, or are they a more recent addition? Had the rings even formed when the <a href="https://theconversation.com/more-bad-news-for-dinosaurs-chicxulub-meteorite-impact-triggered-global-volcanic-eruptions-on-the-ocean-floor-91053">Chicxulub asteroid</a> wiped out the dinosaurs?</p>
<p><a href="https://dornsife.usc.edu/cf/faculty-and-staff/faculty.cfm?pid=1063926">I am a space scientist</a> with a passion for teaching physics and astronomy, and Saturn’s rings have always fascinated me as they tell the story of how the eyes of humanity were opened to the wonders of our solar system and the cosmos.</p>
<h2>Our view of Saturn evolves</h2>
<p>When Galileo first observed Saturn through his telescope in 1610, he was still basking in the fame of <a href="https://www.forbes.com/sites/briankoberlein/2016/01/07/galileos-discovery-of-jupiters-moons-and-how-it-changed-the-world/#dd6cc0f46f07">discovering the four moons of Jupiter</a>. But Saturn perplexed him. Peering at the planet through his telescope, it first looked to him as a planet with two very large moons, then as a lone planet, and then again through his newer telescope, in 1616, as a planet with arms or handles.</p>
<p>Four decades later, <a href="https://www.britannica.com/biography/Christiaan-Huygens">Christiaan Huygens</a> first suggested that Saturn was a ringed planet, and what Galileo had seen were different views of Saturn’s rings. Because of the 27 degrees in the tilt of Saturn’s rotation axis relative to the plane of its orbit, the rings appear to tilt toward and away from Earth with the 29-year cycle of Saturn’s revolution about the Sun, giving humanity an ever-changing view of the rings.</p>
<p>But what were the rings made of? Were they solid disks as some suggested? Or were they made up of smaller particles? As more structure became apparent in the rings, as more gaps were found, and as the motion of the rings about Saturn was observed, astronomers realized that the rings were not solid, and were perhaps made up of a large number of moonlets, or small moons. At the same time, estimates for the thickness of the rings went from Sir William Herschel’s 300 miles in 1789, to Audouin Dollfus’ much more precise <a href="http://solarviews.com/eng/saturnbg.htm">estimate of less than two miles</a> in 1966. </p>
<p>Astronomers understanding of the rings changed dramatically with the <a href="https://solarsystem.nasa.gov/missions/pioneer-11/in-depth/">Pioneer 11</a> and twin Voyager missions to Saturn. <a href="https://voyager.jpl.nasa.gov/assets/images/galleries/images-voyager-took/saturn/6bg.jpg">Voyager’s now famous photograph of the rings</a>, backlit by the Sun, showed for the first time that what appeared as the vast A, B and C rings in fact comprised millions of smaller ringlets. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/287555/original/file-20190809-144868-1oi82mi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/287555/original/file-20190809-144868-1oi82mi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/287555/original/file-20190809-144868-1oi82mi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=469&fit=crop&dpr=1 600w, https://images.theconversation.com/files/287555/original/file-20190809-144868-1oi82mi.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=469&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/287555/original/file-20190809-144868-1oi82mi.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=469&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/287555/original/file-20190809-144868-1oi82mi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=589&fit=crop&dpr=1 754w, https://images.theconversation.com/files/287555/original/file-20190809-144868-1oi82mi.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=589&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/287555/original/file-20190809-144868-1oi82mi.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=589&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Voyager 2 false color image of Saturn’s B and C rings showing many ringlets.</span>
<span class="attribution"><a class="source" href="https://voyager.jpl.nasa.gov/assets/images/galleries/images-voyager-took/saturn/6bg.jpg">NASA</a></span>
</figcaption>
</figure>
<p>The Cassini mission to Saturn, having spent over a decade orbiting the ringed giant, gave planetary scientists even more spectacular and surprising views. The magnificent ring system of Saturn is between 10 meters and one kilometer thick. The combined mass of its particles, which are 99.8% ice and most of which are less than one meter in size, is about 16 quadrillion tons, less than 0.02% the mass of Earth’s Moon, and less than half the mass of Saturn’s moon <a href="https://doi.org/10.1126/science.aat2965">Mimas</a>. This has led some scientists to speculate whether the rings are a result of the breakup of one of Saturn’s moons or the capture and breakup of a stray comet. </p>
<h2>The dynamic rings</h2>
<p>In the four centuries since the invention of the telescope, rings have also been discovered around <a href="https://doi.org/10.1007/978-0-387-73981-6_4">Jupiter</a>, <a href="https://www.universetoday.com/19288/uranus-rings/">Uranus</a> and <a href="https://www.universetoday.com/21635/rings-of-neptune/">Neptune</a>, the giant planets of our solar system. The reason why the giant planets are adorned with rings and Earth and the other rocky planets are not was first proposed by Eduard Roche, a French astronomer in 1849. </p>
<p>A moon and its planet are always in a gravitational dance. Earth’s moon, by pulling on opposite sides of the Earth, causes the ocean tides. Tidal forces also affect planetary moons. If a moon ventures too close to a planet, these forces can overcome the gravitational “glue” holding the moon together and tear it apart.
This causes the moon to break up and spread along its original orbit, forming a ring. </p>
<p>The <a href="https://www.britannica.com/science/Roche-limit">Roche limit</a>, the minimum safe distance for a moon’s orbit, is approximately 2.5 times the planet’s radius from the planet’s center. For enormous Saturn, this is a distance of 87,000 kilometers above its cloud tops and matches the location of Saturn’s outer F ring. For Earth, this distance is less than 10,000 kilometers above its surface. An asteroid or comet would have to venture very close to the Earth to be torn apart by tidal forces and form a ring around the Earth. Our own Moon is a very safe 380,000 kilometers away.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/287560/original/file-20190809-144873-yojp3b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/287560/original/file-20190809-144873-yojp3b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=253&fit=crop&dpr=1 600w, https://images.theconversation.com/files/287560/original/file-20190809-144873-yojp3b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=253&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/287560/original/file-20190809-144873-yojp3b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=253&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/287560/original/file-20190809-144873-yojp3b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=318&fit=crop&dpr=1 754w, https://images.theconversation.com/files/287560/original/file-20190809-144873-yojp3b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=318&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/287560/original/file-20190809-144873-yojp3b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=318&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">NASA’s Cassini spacecraft about to make one of its dives between Saturn and its innermost rings as part of the mission’s grand finale.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/image-feature/illustration-of-cassini-spacecrafts-grand-finale-dive">NASA/JPL-Caltech</a></span>
</figcaption>
</figure>
<p>The thinness of planetary rings is caused by their ever-changing nature. A ring particle whose orbit is tilted with respect to the rest of the ring will eventually collide with other ring particles. In doing so, it will lose energy and settle into the plane of the ring. Over millions of years, all such errant particles either fall away or get in line, leaving only the very thin ring system people observe today.</p>
<p>During the last year of its mission, the Cassini spacecraft dived repeatedly through the 7,000 kilometer gap between the clouds of Saturn and its inner rings. These unprecedented observations made one fact very clear: <a href="https://doi.org/10.1126/science.aat3760">The rings are constantly changing</a>. Individual particles in the rings are continually jostled by each other. Ring particles are steadily raining down onto Saturn.</p>
<p>The shepherd moons Pan, Daphnis, Atlas, Pandora and Prometheus, measuring between eight and 130 kilometers across, quite literally shepherd the ring particles, <a href="https://doi.org/10.1126/science.aat2349">keeping them in their present orbits</a>. Density waves, caused by the motion of shepherd moons within the rings, jostle and reshape the rings. Small moonlets are forming from ring particles that coalesce together. All this indicates that the rings are ephemeral. Every second <a href="https://doi.org/10.1126/science.aat2382">up to 40 tons of ice from the rings</a> rain down on Saturn’s atmosphere. That means the rings may last only several tens to hundreds of millions of years. </p>
<p>Could a time-traveling astronomer have seen the rings 100 million years ago? One indicator for the age of the rings is their dustiness. Objects exposed to the dust permeating our solar system for long periods of time grow dustier and darker. </p>
<p>Saturn’s rings are extremely bright and dust-free, seeming to indicate that they <a href="https://www.scientificamerican.com/article/how-old-are-saturns-rings-the-debate-rages-on/">formed anywhere from 10 to 100 million years ago</a>, if astronomers’ understanding of how icy particles gather dust is correct. One thing is for certain. The rings our time-traveling astronaut would have seen would have looked very different from the way they do today. </p>
<p><em>This story has been corrected to reflect that it was Christiaan Huygens, not Giovanni Cassini, who first suggested that Saturn had rings.</em></p>
<p>[ <em>Like what you’ve read? Want more?</em> <a href="https://theconversation.com/us/newsletters?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=likethis">Sign up for The Conversation’s daily newsletter</a>. ]</p><img src="https://counter.theconversation.com/content/120945/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Vahe Peroomian does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Although the rings of Saturn may look like a permanent fixture of the planet, they are ever-changing. New analyses of the rings reveal how and when they were made, from what and whether they’ll last.Vahe Peroomian, Associate Professor of Physics and Astronomy, USC Dornsife College of Letters, Arts and SciencesLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1124362019-02-28T19:42:54Z2019-02-28T19:42:54ZThere are missing objects at the fringe of the solar system – new study puzzles astronomers<figure><img src="https://images.theconversation.com/files/261286/original/file-20190227-150698-64ge6x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Where are the smallest of the icy worlds we thought resided in the Kuiper belt?</span> <span class="attribution"><span class="source">ESO/Flickr</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>In the dimly lit spaces of our solar system beyond Neptune’s orbit lies the <a href="https://theconversation.com/after-pluto-theres-still-plenty-of-the-solar-system-left-to-explore-45002">Kuiper Belt</a>. This a region between about 35 and 50 times further from the sun than the Earth, populated by icy bodies so sparsely distributed that they never had the chance to collide and merge into planet sized objects. </p>
<p>Pluto is the largest that we know of, but only just. And over the past two decades, telescope surveys <a href="https://theconversation.com/how-we-discovered-840-minor-planets-beyond-neptune-and-what-they-can-tell-us-96431">have found</a> a couple of thousand more ranging down in size to only a few tens of kilometres across. The trouble is that most of the objects of that size or smaller are too faint to be spotted by telescopes. So it will be difficult to ever work out how many small but unseen bodies there actually are in the Kuiper belt. Now a new paper, <a href="http://science.sciencemag.org/cgi/doi/10.1126/science.aap8628">published in Science</a>, has used an ingenious method to help us find out.</p>
<p>This is important, because scientists believe Kuiper Belt objects are survivors from the solar system’s birth, developing from a primordial cloud of dust and gas. That means that their size distribution could have a lot to tell us about how the material from which the planets grew was initially assembled.</p>
<h2>Counting craters</h2>
<p>Instead of counting the small Kuiper belt objects directly, the researchers behind the new study counted the craters made by the random sample of objects that have impacted the surfaces of Pluto and its largest moon, <a href="https://theconversation.com/images-of-plutos-moon-charon-show-huge-fractures-and-hints-of-icy-lava-flows-48533">Charon</a>. There, craters 13km across would have been made by objects only 1km-2km in size. That is already way below the telescopic detection limit for Kuiper belt objects themselves, but images from <a href="https://theconversation.com/new-horizons-finally-gets-up-close-with-pluto-for-15-minutes-44603">the flyby of NASA’s New Horizons mission in 2015</a> allow craters as small as 1.4km to be mapped. Those must have been made by impacts of Kuiper Belt objects not much bigger than 100 metres in size.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/260674/original/file-20190225-26171-1u9e0w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/260674/original/file-20190225-26171-1u9e0w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=360&fit=crop&dpr=1 600w, https://images.theconversation.com/files/260674/original/file-20190225-26171-1u9e0w.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=360&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/260674/original/file-20190225-26171-1u9e0w.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=360&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/260674/original/file-20190225-26171-1u9e0w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=453&fit=crop&dpr=1 754w, https://images.theconversation.com/files/260674/original/file-20190225-26171-1u9e0w.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=453&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/260674/original/file-20190225-26171-1u9e0w.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=453&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Details of the ancient cratered surface of Charon’s Vulcan Planitia.</span>
<span class="attribution"><span class="source">: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/K. Singer</span></span>
</figcaption>
</figure>
<p>The researchers’ analysis shows that for craters of 13km or larger, on both Pluto and Charon, the frequency of impacts of various sizes seems to match with what would be expected from the known size distribution for Kuiper belt objects. However, for smaller craters the abundance falls off dramatically, and so by implication must the abundance of the Kuiper Belt objects capable of making those craters. The same does not happen for the well-documented asteroids that collide with the bodies in the region of Jupiter, Mars and Earth, nor is it consistent with theoretical models.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/260748/original/file-20190225-26177-o5p85w.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/260748/original/file-20190225-26177-o5p85w.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/260748/original/file-20190225-26177-o5p85w.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=363&fit=crop&dpr=1 600w, https://images.theconversation.com/files/260748/original/file-20190225-26177-o5p85w.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=363&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/260748/original/file-20190225-26177-o5p85w.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=363&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/260748/original/file-20190225-26177-o5p85w.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=457&fit=crop&dpr=1 754w, https://images.theconversation.com/files/260748/original/file-20190225-26177-o5p85w.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=457&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/260748/original/file-20190225-26177-o5p85w.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=457&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">800km wide view of part of Cthulhu Regio, extracted from the most detailed colour map of Pluto.</span>
<span class="attribution"><span class="source">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</span></span>
</figcaption>
</figure>
<p>Interpretation of the most heavily cratered terrains led the researchers to rule out that the small craters have been erased by geological resurfacing such as cryovolcanic activity (eruptions of icy fluids) during the past four billion years. This reinforces the conclusion that smaller craters were never made in the expected numbers, so there must be a mysterious corresponding deficit of Kuiper belt objects less than about 1-2km in size.</p>
<h2>Blorping and flomping</h2>
<p>When the researchers, led by Kelsi Singer of the Southwest Research Institute (Boulder, Colorado), wrote their paper no one had yet seen a small Kuiper Belt object in detail. However, New Horizons recently flew past a 30km long object known as 2014 MU₆₉ <a href="https://gizmodo.com/new-horizons-scientists-double-down-on-ultima-thule-nic-1831439791">(more controversially nicknamed “Ultima Thule”</a>) on January 1, and has now transmitted probably the best images we are going to get. </p>
<p>Sometimes described as “snowman-shaped”, it is a two-lobed “contact binary”, almost certainly formed by a merger of two round objects that happened so slowly and gently that neither component was deformed in the process. But what happened before that? If you look at the larger of the two lobes, in particular, you can make out what looks like traces of component parts that merged vigorously enough to squish together into an approximate sphere, but with insufficient violence to smash each other apart.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/260705/original/file-20190225-26181-17snz3t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/260705/original/file-20190225-26181-17snz3t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/260705/original/file-20190225-26181-17snz3t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=490&fit=crop&dpr=1 600w, https://images.theconversation.com/files/260705/original/file-20190225-26181-17snz3t.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=490&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/260705/original/file-20190225-26181-17snz3t.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=490&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/260705/original/file-20190225-26181-17snz3t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=616&fit=crop&dpr=1 754w, https://images.theconversation.com/files/260705/original/file-20190225-26181-17snz3t.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=616&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/260705/original/file-20190225-26181-17snz3t.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=616&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Kuiper belt object 2014 MU₆₉. The two-lobed object is about 30km from end to end.</span>
<span class="attribution"><span class="source">NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute, National Optical Astronomy Observatory</span></span>
</figcaption>
</figure>
<p>These ideas have inspired <a href="https://www.hou.usra.edu/meetings/lpsc2019/pdf/1248.pdf">quirky new terms</a>. “Blorping” refers to the collisional merging of material to assemble each of the lobes, and “flomping” describes the coming together when two lobes meet without causing any deformation. More importantly, this could offer an insight into the processes that robbed the Kuiper belt of the smaller objects that would otherwise have impacted to make small craters on Pluto and Charon.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/260770/original/file-20190225-26174-1qfnbjm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/260770/original/file-20190225-26174-1qfnbjm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=549&fit=crop&dpr=1 600w, https://images.theconversation.com/files/260770/original/file-20190225-26174-1qfnbjm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=549&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/260770/original/file-20190225-26174-1qfnbjm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=549&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/260770/original/file-20190225-26174-1qfnbjm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=690&fit=crop&dpr=1 754w, https://images.theconversation.com/files/260770/original/file-20190225-26174-1qfnbjm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=690&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/260770/original/file-20190225-26174-1qfnbjm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=690&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Detail of Charon’s Vulcan Planitia, where small craters are deficient in numbers.</span>
<span class="attribution"><a class="source" href="http://pluto.jhuapl.edu/Galleries/Featured-Images/image.php?page=12&gallery_id=2&image_id=324">NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute</a></span>
</figcaption>
</figure>
<p>The relative lack of small Kuiper Belt objects may be because, instead of breaking each other apart in collisions, they tended to merge by blorping – eventually growing into objects like 2014 MU₆₉. If this is correct, then when we try to count them, we see a record of growth rather than collisional fragmentation. </p>
<p>Orbital speeds are slower the further you get from the sun, so we would expect collisions to be less violent in the Kuiper Belt than in the inner solar system. But even so, a “blorp” event to fuse two lumps together rather than break them apart probably requires the ices that make up the bulk of their substance to be a lot less brittle and more squishy than we might have expected. That is crucial information, as these lumps are made from the raw material that the solar system formed from, shedding important light on its evolution. </p>
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<strong>
Read more:
<a href="https://theconversation.com/images-of-plutos-moon-charon-show-huge-fractures-and-hints-of-icy-lava-flows-48533">Images of Pluto's moon Charon show huge fractures and hints of icy 'lava flows'</a>
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Read more:
<a href="https://theconversation.com/stunning-crystal-clear-images-of-pluto-but-what-do-they-mean-47517">Stunning, crystal-clear images of Pluto – but what do they mean?</a>
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Read more:
<a href="https://theconversation.com/nasa-mission-brings-pluto-into-sharp-focus-but-its-still-not-a-planet-40495">NASA mission brings Pluto into sharp focus – but it's still not a planet</a>
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<hr>
<img src="https://counter.theconversation.com/content/112436/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Rothery is co-leader of the European Space Agency's Mercury Surface and Composition Working Group, and a Co-Investigator on MIXS (Mercury Imaging X-ray Spectrometer) that is now on its way to Mercury on board the European Space Agency's Mercury orbiter BepiColombo. He has received funding from the UK Space Agency and the Science & Technology Facilities Council for work related to Mercury BepiColombo, and is currently funded by the European Commission under its Horizon 2020 programme for work on planetary geological mapping (776276 Planmap). He is author of Planet Mercury - from Pale Pink Dot to Dynamic World (Springer, 2015), Moons: A Very Short Introduction (Oxford University Press, 2015) and Planets: A Very Short Introduction (Oxford University Press, 2010). He is Educator on the Open University's free learning Badged Open Course (BOC) on Moons and its equivalent FutureLearn Moons MOOC, and chair of the Open University's level 2 course on Planetary Science and the Search for Life.</span></em></p>There’s a mysterious lack of small bodies beyond Neptune, but a ‘snowman-shaped’ object may help explain why.David Rothery, Professor of Planetary Geosciences, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1120302019-02-22T09:44:16Z2019-02-22T09:44:16ZWhat’s the weather like on Uranus and Neptune? New images give important clues<figure><img src="https://images.theconversation.com/files/260425/original/file-20190222-195867-tbkvkc.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Uranus (left) and Neptune (right) seen by Hubble.
</span> <span class="attribution"><span class="source">NASA, ESA, A. Simon (NASA Goddard Space Flight Center), and M.H. Wong and A. Hsu (University of California, Berkeley)</span></span></figcaption></figure><p>The outer region of the solar system may be the least explored, but scientists have managed to unravel several of its mysteries in recent weeks. On New Year’s Day, the NASA spacecraft New Horizons <a href="https://solarsystem.nasa.gov/news/807/new-horizons-successfully-explores-ultima-thule/">encountered the icy object Ultima Thule</a> for the first time, shedding light on how it formed. Astronomers <a href="https://www.nature.com/articles/s41586-019-0909-9">have also just discovered</a> a previously unknown moon orbiting Neptune, which has been dubbed “Hippocamp”.</p>
<p>Another discovery, thanks to <a href="http://hubblesite.org/image/4320/news_release/2019-06">new images</a> from the <a href="https://theconversation.com/telescopes-on-the-ground-may-be-cheaper-but-hubble-shows-why-they-are-not-enough-40724">Hubble Space Telescope</a>, is that there’s a variety of intriguing weather patterns in the atmospheres of both Neptune and Uranus. So what would it be like to go there?</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/260211/original/file-20190221-195892-1cgrhm9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/260211/original/file-20190221-195892-1cgrhm9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=360&fit=crop&dpr=1 600w, https://images.theconversation.com/files/260211/original/file-20190221-195892-1cgrhm9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=360&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/260211/original/file-20190221-195892-1cgrhm9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=360&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/260211/original/file-20190221-195892-1cgrhm9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=452&fit=crop&dpr=1 754w, https://images.theconversation.com/files/260211/original/file-20190221-195892-1cgrhm9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=452&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/260211/original/file-20190221-195892-1cgrhm9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=452&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Artist’s impression of Neptune and Hippocamp.</span>
<span class="attribution"><span class="source">ESA/Hubble</span></span>
</figcaption>
</figure>
<p>Having four times the diameter of the Earth, we typically refer to Uranus and Neptune as “ice giants”. Unlike the gas giants, Saturn and Jupiter, Neptune and Uranus are lower in hydrogen and helium and higher in concentrations of heavier material such as methane, water and ammonia.</p>
<p>Uranus is especially interesting as it is also the only planet in the solar system that <a href="https://theconversation.com/how-did-uranus-end-up-on-its-side-weve-been-finding-out-109894">rotates on its side</a>. A northern summer on Uranus lasts 21 years with the north pole receiving constant sunlight, while the south pole sees continual darkness.</p>
<p>This tilt to the Uranian axis is believed to be the result of an early solar system collision with an <a href="https://www.space.com/41076-uranus-weird-til-icy-rock-crash.html">object at least as large as the Earth</a>. Such a collision would either have released the internal heat reserves of the planet or created a layer of particles which effectively insulate the interior of the planet – <a href="http://iopscience.iop.org/article/10.3847/1538-4357/aac725/meta">preventing heat flow</a> to space. Neptune, having avoided such an encounter, still has an <a href="https://theconversation.com/curious-kids-how-does-heat-travel-through-space-if-space-is-a-vacuum-111889">outward heat flow</a>. As such, both planets are almost the same temperature (within a few degrees) despite Uranus being 33% closer to the sun.</p>
<h2>Weather on Uranus</h2>
<p>The <a href="https://planetaryweather.blogspot.com/2013/05/why-does-uranus-emit-very-little-heat.html">absence of any significant internal heat flow</a> on Uranus means that this planet’s atmosphere is distinctly less active than Neptune’s. In fact the Uranian atmosphere in winter is the coldest planetary atmosphere in the solar system. When Voyager 2 flew past Uranus in 1986, the planet appeared as a largely featureless green-blue disc. In the years since, however, scientists have realised that even this apparently cold, dead world has a surprisingly dynamic atmosphere. </p>
<p>But the new images from the Hubble Space Telescope show a previously unseen huge white cloud likely composed of ammonia or methane ice enveloping the north pole (see top image). Clearly visible at the edge of this huge cloud system is a smaller cloud of methane ice which rotates around the larger cloud edge. These cloud structures may be seasonal, resulting from the current constant sunlight at the north pole. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/260201/original/file-20190221-195873-21rioa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/260201/original/file-20190221-195873-21rioa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=480&fit=crop&dpr=1 600w, https://images.theconversation.com/files/260201/original/file-20190221-195873-21rioa.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=480&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/260201/original/file-20190221-195873-21rioa.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=480&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/260201/original/file-20190221-195873-21rioa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=603&fit=crop&dpr=1 754w, https://images.theconversation.com/files/260201/original/file-20190221-195873-21rioa.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=603&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/260201/original/file-20190221-195873-21rioa.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">Uranus.</span>
</figcaption>
</figure>
<p>Around the equator of Uranus we can also see a thin band of cloud (top image), though how this cloud band remains so narrow is not currently understood. Wind speeds on Uranus are so high that they can blow clouds along at up to 560mph, which would spread clouds outwards over a large area. All planetary atmospheres possess a <a href="https://theconversation.com/beast-from-the-east-the-science-behind-europes-siberian-chill-92385">latitudinal circulation system</a> which should, in theory, also distribute this cloud band over wider latitudes. It could be that these methane clouds are somehow constrained by these circulation patterns, due to altitude or chemical instability. </p>
<p>If we could visit Uranus, the winds at a depth equivalent to the atmospheric pressure of Earth’s surface could reach up to 250 metres per second, or roughly three times as fast as a category five hurricane. Be sure to bring your coat, too, as temperatures at this depth are a frigid -200C.</p>
<h2>Weather on Neptune</h2>
<p>As strong as the winds of Uranus are, they are nothing compared to those found on the other ice giant. Neptune boasts supersonic wind speeds of over <a href="https://media.giphy.com/media/d2W6sksZ9o3qopUc/giphy.gif">1,300mph</a>, and numerous storm systems. The most famous of these features was the <a href="https://www.nasa.gov/content/25-years-ago-voyager-2-captures-images-of-neptune">Great Dark Spot</a> which was observed in close up by Voyager 2 in 1989. This huge storm system covered an area roughly equivalent to one sixth of the surface area of Earth. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/260204/original/file-20190221-195876-q3t31n.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/260204/original/file-20190221-195876-q3t31n.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=599&fit=crop&dpr=1 600w, https://images.theconversation.com/files/260204/original/file-20190221-195876-q3t31n.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=599&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/260204/original/file-20190221-195876-q3t31n.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=599&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/260204/original/file-20190221-195876-q3t31n.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=753&fit=crop&dpr=1 754w, https://images.theconversation.com/files/260204/original/file-20190221-195876-q3t31n.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=753&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/260204/original/file-20190221-195876-q3t31n.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=753&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Neptune with Great Dark Spot.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>In the latest Hubble images, a different storm system is visible near the North pole, accompanied by bright clouds of methane ice crystals. The reason these features appear darker than their surroundings is because they are holes offering a view into deeper layers of the Neptunian atmosphere, much like the eye of a hurricane on Earth allows you to see the surface from space. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/260199/original/file-20190221-195861-l5t3qp.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/260199/original/file-20190221-195861-l5t3qp.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=300&fit=crop&dpr=1 600w, https://images.theconversation.com/files/260199/original/file-20190221-195861-l5t3qp.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=300&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/260199/original/file-20190221-195861-l5t3qp.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=300&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/260199/original/file-20190221-195861-l5t3qp.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=377&fit=crop&dpr=1 754w, https://images.theconversation.com/files/260199/original/file-20190221-195861-l5t3qp.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=377&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/260199/original/file-20190221-195861-l5t3qp.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=377&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Neptune seen by the Hubble Space Telescope.</span>
<span class="attribution"><span class="source">NASA/ESA/M.H.Wong/A.I. H</span></span>
</figcaption>
</figure>
<p>Like on Jupiter and Saturn, these gigantic storm systems are believed to be powered by heat flowing out of the planet, left over from the planet’s birth some 4.5 billion years ago. Once again, a visit here would be problematic, with similar temperatures to Uranus but double the wind speed. In fact, Neptune is the windiest planet in the solar system.</p>
<p>The ice giants are the <a href="https://www.sciencedirect.com/science/article/pii/S0032063314002955">most commonly observed type of “exoplanet”</a> – planets orbiting stars other than our sun. If we know more about Uranus and Neptune, we can therefore understand more about planets throughout the universe. </p>
<p>Of course, the ideal plan would be to travel to these worlds. Sadly, apart from the great distance involved, the exceptionally cold temperatures, massive storms and strong winds make them particularly unsuitable for a human visit. So for now, we shall just have to rely on telescopes like Hubble to tell us about our local ice giants.</p><img src="https://counter.theconversation.com/content/112030/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 Hubble Space Telescope has spotted clouds and storms on the solar system’s ice giants.Gareth Dorrian, Post Doctoral Research Associate in Space Science, Nottingham Trent UniversityIan Whittaker, Lecturer, Nottingham Trent UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/964312018-05-25T09:21:55Z2018-05-25T09:21:55ZHow we discovered 840 minor planets beyond Neptune – and what they can tell us<figure><img src="https://images.theconversation.com/files/218467/original/file-20180510-5968-7lw5y4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The Canada-France-Hawaii Telescope (CFHT) at sunset, which observed the OSSOS survey.</span> <span class="attribution"><span class="source">wikipedia</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Our solar system is a tiny but wonderfully familiar corner of the vast, dark universe – we have even been able to land spacecraft on our celestial neighbours. Yet its outer reaches are still remarkably unmapped. Now we <a href="https://ui.adsabs.harvard.edu/#abs/2018ApJS..236...18B/abstract">have discovered</a> 840 small worlds in the distant and hard-to-explore region beyond Neptune. This is the largest set of discoveries ever made, increasing the number of distant objects with well known paths around the sun by 50%.</p>
<p>These little icy worlds are important as they help us tell the solar system’s history. They can also help us <a href="https://theconversation.com/our-discovery-of-a-minor-planet-beyond-neptune-shows-there-might-not-be-a-planet-nine-after-all-75656">test the idea</a> that there’s a yet unseen planet lurking in the outer solar system. </p>
<p>Our planetary system as we see it today is not as it formed. When the sun was newborn, it was surrounded by a massive disk of material. Encounters with tiny, growing planets – including some of the worlds we’ve just discovered – moved the giant planets outward from the sun until they settled into their present locations. The growing planets, on the other hand, went everywhere, scattering both inward and outward. </p>
<p>Planetary migration also happened in far away systems around many other stars. Fortunately, the celestial bodies in our own planetary system are comparatively close by, making it the only place where we can see the intricate details of how migration happened. Mapping the minor planet populations that are left over from the disk lets us reconstruct the history of how the big planets were pushed into place.</p>
<h2>Mapping the sky</h2>
<p>The new discoveries were made as part of a five year project called the <a href="http://www.ossos-survey.org/">Outer Solar System Origins Survey</a> (OSSOS). The observations, conducted in 2013-2017, used the imaging camera of one of the world’s major telescopes – the <a href="http://www.cfht.hawaii.edu/">Canada-France-Hawaii Telescope</a> on Maunakea in Hawaii. The survey looked for faint, slow-moving points of light within eight big patches of sky near the plane of the planets and away from the dense star fields of the Milky Way. </p>
<p>With 840 discoveries made at distances between six and 83 astronomical units (au) – one such unit is the distance between the sun and the Earth – the survey gives us a very good overview of the many sorts of orbits these “trans-Neptunian objects” have.</p>
<p>Earlier surveys have suffered from losing some of their distant discoveries – when too few observations occur, the predicted path of a minor planet in the sky will be so uncertain that a telescope can’t spot it again, and it is considered “lost”. This happens more to objects with highly tilted and elongated orbits, producing a bias in what’s currently known about these populations.</p>
<p>Our new survey successfully tracked all its distant discoveries. The frequent snapshots we made of the 840 objects over several years meant that each little world’s orbit could be determined very precisely. In total, more than 37,000 hand-checked measurements of the hundreds of discoveries precisely pinned down their arcs across the sky.</p>
<p>We also created an accompanying software “simulator” (a computer model), which provides a powerful tool for testing the inventory and history of our solar system. This lets theorists <a href="https://arxiv.org/abs/1802.00460">test out their models</a> of how the solar system came to be in the shape we see it today, comparing them with our real discoveries.</p>
<h2>Strange new worlds</h2>
<p>The new icy and rocky objects fall into two main groups. One includes those that reside on roundish orbits in the Kuiper belt, which extends from 37au to approximately 50au from the sun. The other consists of worlds that orbit in a careful dance of avoidance with Neptune as it travels around the sun. These “resonant” trans-Neptunian objects, which include Pluto, were pushed into their current elongated orbits during Neptune’s migration outwards. </p>
<p>In the Kuiper belt, we found 436 small worlds. Their orbits confirm that a concentrated “kernel” of the population nestles on almost perfectly round, flat orbits at 43 to 45au. These quiet orbits may have been undisturbed since the dawn of the solar system, a leftover fraction of the original disk. Soon, we will see a member of this group up close: the <a href="https://theconversation.com/new-horizons-is-an-old-spacecraft-but-it-will-transform-our-knowledge-of-pluto-44524">New Horizons spacecraft</a>, which <a href="https://theconversation.com/new-horizons-finally-gets-up-close-with-pluto-for-15-minutes-44603">visited Pluto in 2015</a>, will be flying by a world that’s about the size of London on New Year’s Day 2019.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/218465/original/file-20180510-185500-4w73js.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/218465/original/file-20180510-185500-4w73js.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=379&fit=crop&dpr=1 600w, https://images.theconversation.com/files/218465/original/file-20180510-185500-4w73js.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=379&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/218465/original/file-20180510-185500-4w73js.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=379&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/218465/original/file-20180510-185500-4w73js.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=477&fit=crop&dpr=1 754w, https://images.theconversation.com/files/218465/original/file-20180510-185500-4w73js.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=477&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/218465/original/file-20180510-185500-4w73js.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=477&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The dwarf planet candidate 2015 RR245 is on an exceptionally distant orbit, but is one of the few dwarf planets that could one day be reached by a spacecraft mission.</span>
<span class="attribution"><span class="source">Alex Parker/OSSOS</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>We found 313 resonant trans-Neptunian objects, with the survey showing that they exist <a href="https://arxiv.org/abs/1802.05805">as far out as an incredible 130au</a> – and are <a href="https://arxiv.org/abs/1604.08177">far more abundant</a> than previously thought. Among these discoveries is the dwarf planet 2015 RR245, which is about half the size of Britain. It may have hopped onto its current orbit at 82au <a href="https://arxiv.org/abs/1607.06970">after an encounter with Neptune</a> hundreds of millions of years ago. It was once among the <a href="https://arxiv.org/abs/1803.07521">90,000 scattered objects</a> of smaller size that we estimate currently exist. </p>
<h2>Are there more planets?</h2>
<p>Among the most unusual of the discoveries are nine little worlds on incredibly distant orbits, never coming closer to the sun than Neptune’s orbit, and taking as long as 20,000 years to travel around our star. Their existence implies an unseen population of hundreds of thousands of trans-Neptunian objects on similar orbits.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/220427/original/file-20180525-51141-1jeqew.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/220427/original/file-20180525-51141-1jeqew.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=357&fit=crop&dpr=1 600w, https://images.theconversation.com/files/220427/original/file-20180525-51141-1jeqew.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=357&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/220427/original/file-20180525-51141-1jeqew.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=357&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/220427/original/file-20180525-51141-1jeqew.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=449&fit=crop&dpr=1 754w, https://images.theconversation.com/files/220427/original/file-20180525-51141-1jeqew.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=449&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/220427/original/file-20180525-51141-1jeqew.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=449&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Artist’s concept of Planet Nine.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech/Robert Hurt</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>How these objects got on their present paths is unclear — some orbit so far out that, even at their closest approach, they are barely tugged by Neptune’s gravity. One explanation that has been put forward is that a yet unseen large planet, sometimes called “Planet Nine”, could be causing them to cluster in space. However, our nine minor planets all seem to be <a href="https://arxiv.org/abs/1706.05348">spread out smoothly</a>, rather than clustering. Perhaps the shepherding of such a large planet is more subtle – or these orbits instead <a href="https://theconversation.com/our-discovery-of-a-minor-planet-beyond-neptune-shows-there-might-not-be-a-planet-nine-after-all-75656">formed in a different way</a>.</p>
<p>The history of our solar system is just beginning to be told. We hope this new set of discoveries will help piece together the story.</p><img src="https://counter.theconversation.com/content/96431/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Michele Bannister receives funding from the STFC, and has previously been funded by Canada's NRC and NSERC. She serves on the committee of the AAS's Division of Planetary Sciences.</span></em></p>Discovery of many icy worlds helps unravel the solar system’s history.Michele Bannister, Research Fellow, planetary astronomy, Queen's University BelfastLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/877972017-11-27T14:26:32Z2017-11-27T14:26:32ZHow insights into ‘supercritical fluids’ could help us understand the interior of the giant gas planets<figure><img src="https://images.theconversation.com/files/196498/original/file-20171127-2046-q1nylf.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Jupiter seen by Juno.</span> <span class="attribution"><span class="source">Justin Cowart/Flickr</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>The temperature and pressure inside Jupiter range from about -100°c near the edge to about 15,000°c and 50m times the Earth’s atmospheric pressure in the middle. Saturn, Uranus and Neptune are similar pressure cookers. As we descend into Jupiter, we may see matter in the gas state, in the liquid state and in another, less well-known state, called the <a href="https://theconversation.com/explainer-what-is-a-supercritical-fluid-26537">“supercritical fluid” state</a>. </p>
<p>Understanding supercritical fluids is not only important for planetary scientists, it is also used in industrial processes such as power generation and food processing.</p>
<p>When we boil water on Earth, it changes “phase” going from a liquid to a gas state. This is due to a sudden dramatic change in the density and other properties called a “phase transition”. However, if you squeezed water to 1,000 times atmospheric pressure and then heated it while keeping the pressure on, you would no longer observe boiling as such. The water molecules would whizz around with more energy, and the density would gradually go down, but there would be no sudden boiling (phase transition). This is what constitutes the supercritical fluid state – it’s neither a liquid nor a gas.</p>
<p>Exactly how liquids and supercritical fluids behave has caused scientists to scratch their heads for decades. But new research has shed light on this problem, raising hopes that we can soon gain a much better understanding of what goes on deep inside the giant gas planets.</p>
<p>Scientists have long assumed that liquids and supercritical fluids behave like dense gases, with molecules constantly moving around freely. But in the 1930s, the Russian physicist Yakov Ilyich Frenkel questioned this assumption, proposing that under certain conditions <a href="https://books.google.co.uk/books/about/Kinetic_Theory_of_Liquids.html?id=ORdSQwAACAAJ&redir_esc=y">they would instead behave like solids</a> (where atoms are stuck), except that the atoms occasionally hop from place to place. We can call liquids and supercritical fluids under these conditions “dense liquids”.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/196499/original/file-20171127-2004-11li5v8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/196499/original/file-20171127-2004-11li5v8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=364&fit=crop&dpr=1 600w, https://images.theconversation.com/files/196499/original/file-20171127-2004-11li5v8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=364&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/196499/original/file-20171127-2004-11li5v8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=364&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/196499/original/file-20171127-2004-11li5v8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=457&fit=crop&dpr=1 754w, https://images.theconversation.com/files/196499/original/file-20171127-2004-11li5v8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=457&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/196499/original/file-20171127-2004-11li5v8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=457&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Ethane in three forms: subcritical, critical and supercritical.</span>
<span class="attribution"><a class="source" href="http://en.wikipedia.org/wiki/File:CriticalPointMeasurementEthane.jpg">Dr. Sven Horstmann</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Ignored for decades, this approach has got a second life in the last decade as it has been successfully <a href="https://physicsworld.com/cws/article/news/2012/jun/13/phonon-theory-sheds-light-on-liquid-thermodynamics">used to predict</a> the heat capacity of liquids. Heat capacity is a crucial property of liquids, determining the way heat is stored and flows around planets, power stations and everything in between. </p>
<p>A dividing line (the “Frenkel line”) should therefore be drawn, up to arbitrarily high pressures and temperatures, between conditions where dense liquids behave similarly to gases, and conditions where Frenkel’s approach – assuming similar behaviour to solids – is valid. But how should the line be defined? How sudden is it? These questions need to be addressed by experiments. </p>
<h2>Powerful experiments</h2>
<p>This year, two groundbreaking studies have been published in which this line has been charted from observations. In the <a href="https://en.wikipedia.org/wiki/Structure_of_liquids_and_glasses#Diffraction">first study</a>, one of the most powerful <a href="https://en.wikipedia.org/wiki/Synchrotron">synchrotron light sources</a> in the world (the Advanced Photon Source near Chicago) was used to <a href="https://journals.aps.org/prb/abstract/10.1103/PhysRevB.95.134114">pin down the pressure</a> – 6,500 times the Earth’s atmosphere – at which one of the most fundamental model fluids, supercritical neon, starts to behave as a dense liquid as modelled by Frenkel. </p>
<p>In the second study, data from another powerful X-ray source (the European Synchrotron Radiation Facility in Grenoble) was combined with measurements in my laboratory in Manchester <a href="https://journals.aps.org/pre/accepted/ff07bYc4Of81815d43ae6e7606da3acaa5c3d9c33">to determine</a> the way in which the atoms in methane molecules vibrate to make a similar observation. We found that the methane starts to behave as a dense liquid at about 2,000 atmospheres pressure.</p>
<p>We found that one key piece of evidence in the puzzle was already there in the literature, <a href="https://www.researchgate.net/publication/280016864_Etude_par_spectroscopie_Raman_du_methane_comprime_jusqu%27a_3_kbar_Application_a_la_mesure_de_pression_dans_les_inclusions_fluides_contenues_dans_les_mineraux">dating back to 1986</a>; a demonstration that the vibrations in gaseous methane behave in completely the opposite way to vibrations that we are used to seeing in dense liquids and solids. Its importance had simply not been recognised.</p>
<p>Our study had an added bonus compared to the neon study – methane is everywhere in our solar system. The gas giants Uranus and Neptune are full of it, and perhaps understanding methane will answer a lot of the mysteries these planets pose. Planetary scientists have lost sleep for decades over questions such as how the composition changes as you delve into Uranus and Neptune and whether Uranus’ surface really is the coldest place in the solar system. </p>
<p>The hope is now to apply these new results on the liquid and supercritical fluid states of matter to answer these and other long-standing-mysteries.</p><img src="https://counter.theconversation.com/content/87797/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>John Proctor 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>New experimental results on methane may help us to uncover whether Uranus really is the coldest planet.John Proctor, Senior Lecturer in Physics, University of SalfordLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/825122017-08-18T05:03:33Z2017-08-18T05:03:33ZFrom the edge of the Solar System, Voyager probes are still talking to Australia after 40 years<figure><img src="https://images.theconversation.com/files/182362/original/file-20170817-16233-1tfxoku.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Both Voyager spacecraft are only in communication with Earth via a Canberra tracking station.</span> <span class="attribution"><span class="source">NASA/JPL</span></span></figcaption></figure><p>This month marks 40 years since NASA launched the two Voyager space probes on their mission to explore the outer planets of our Solar System, and Australia has been helping the US space agency keep track of the probes at every step of their epic journey.</p>
<p>CSIRO operates NASA’s tracking station in Canberra, a set of four radio telescopes, or dishes, known as the Canberra Deep Space Communication Complex (<a href="https://www.cdscc.nasa.gov/">CDSCC</a>).</p>
<p>It’s one of three tracking stations spaced around the globe, which form the <a href="https://deepspace.jpl.nasa.gov/">Deep Space Network</a>. The other two are at <a href="https://www.gdscc.nasa.gov/">Goldstone</a>, in California, and <a href="https://www.mdscc.nasa.gov/index.php?ChangeLang=en">Madrid</a>, in Spain. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/182364/original/file-20170817-16209-lr9vy4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/182364/original/file-20170817-16209-lr9vy4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/182364/original/file-20170817-16209-lr9vy4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=718&fit=crop&dpr=1 600w, https://images.theconversation.com/files/182364/original/file-20170817-16209-lr9vy4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=718&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/182364/original/file-20170817-16209-lr9vy4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=718&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/182364/original/file-20170817-16209-lr9vy4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=902&fit=crop&dpr=1 754w, https://images.theconversation.com/files/182364/original/file-20170817-16209-lr9vy4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=902&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/182364/original/file-20170817-16209-lr9vy4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=902&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 Canberra Deep Space Communication Complex (CDSCC).</span>
<span class="attribution"><span class="source">CSIRO</span></span>
</figcaption>
</figure>
<p>Between them they provide NASA, and other space exploration agencies, with continuous, two-way radio communication coverage to every part of the Solar System. </p>
<hr>
<p><em><strong>Read more:</strong>
<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></em></p>
<hr>
<p>Four decades on and the Australian tracking station is now the only one with the right equipment and position to be able to communicate with both of the probes as they continue to push back the boundaries of deep space exploration.</p>
<h2>The launch of Voyagers</h2>
<p>The <a href="https://www.jpl.nasa.gov/voyager/mission/">Voyagers’ primary purpose</a> was to fly by Jupiter and Saturn. If all the scientific objectives were met at Saturn, then Voyager 2 would be targeted to continue on to Uranus and Neptune.</p>
<p>At each planetary encounter – running on power equivalent to the light bulb in your refrigerator – the Voyagers would transmit photographs and scientific data back to Earth before being accelerated towards their next target by the planet’s gravity, like a slingshot.</p>
<p>Timed to take advantage of a favourable alignment of the outer planets not expected to recur for another 175 years, Voyager 2 launched first on August 20, 1977, followed by Voyager 1 on September 5. Although launched second, Voyager 1 was sent on a faster trajectory and was timed to arrive at Jupiter ahead of Voyager 2. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/182365/original/file-20170817-16222-2j23u0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/182365/original/file-20170817-16222-2j23u0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/182365/original/file-20170817-16222-2j23u0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=718&fit=crop&dpr=1 600w, https://images.theconversation.com/files/182365/original/file-20170817-16222-2j23u0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=718&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/182365/original/file-20170817-16222-2j23u0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=718&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/182365/original/file-20170817-16222-2j23u0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=902&fit=crop&dpr=1 754w, https://images.theconversation.com/files/182365/original/file-20170817-16222-2j23u0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=902&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/182365/original/file-20170817-16222-2j23u0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=902&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Voyager 2 launches aboard Titan-Centaur rocket.</span>
<span class="attribution"><span class="source">NASA/JPL</span></span>
</figcaption>
</figure>
<p>When Voyager 1 arrived at Jupiter in 1979 the mission’s scientific discoveries began.</p>
<h2>Jupiter revealed close up</h2>
<p>The world watched as the Voyagers’ cameras sent back – via the tracking stations – close up images of Jupiter and its moons, letting us see these worlds in detail for the very first time. </p>
<p>From the turbulence surrounding huge storms on Jupiter, to a volcano erupting on Jupiter’s moon Io, to hints that the icy surface of Europa probably conceals an ocean underneath, the Voyager mission started to reveal the outer Solar System to us in inspiring detail.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/182371/original/file-20170817-13444-1301l19.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/182371/original/file-20170817-13444-1301l19.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/182371/original/file-20170817-13444-1301l19.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=718&fit=crop&dpr=1 600w, https://images.theconversation.com/files/182371/original/file-20170817-13444-1301l19.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=718&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/182371/original/file-20170817-13444-1301l19.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=718&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/182371/original/file-20170817-13444-1301l19.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=902&fit=crop&dpr=1 754w, https://images.theconversation.com/files/182371/original/file-20170817-13444-1301l19.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=902&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/182371/original/file-20170817-13444-1301l19.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=902&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Getting close to the Jupiter.</span>
<span class="attribution"><span class="source">NASA/JPL</span></span>
</figcaption>
</figure>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/182372/original/file-20170817-13501-1jy19nt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/182372/original/file-20170817-13501-1jy19nt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/182372/original/file-20170817-13501-1jy19nt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=502&fit=crop&dpr=1 600w, https://images.theconversation.com/files/182372/original/file-20170817-13501-1jy19nt.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=502&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/182372/original/file-20170817-13501-1jy19nt.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=502&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/182372/original/file-20170817-13501-1jy19nt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=630&fit=crop&dpr=1 754w, https://images.theconversation.com/files/182372/original/file-20170817-13501-1jy19nt.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=630&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/182372/original/file-20170817-13501-1jy19nt.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=630&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Peering into Jupiter’s famous red spot.</span>
<span class="attribution"><span class="source">NASA/JPL</span></span>
</figcaption>
</figure>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/182373/original/file-20170817-13480-lq7fqz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/182373/original/file-20170817-13480-lq7fqz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/182373/original/file-20170817-13480-lq7fqz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/182373/original/file-20170817-13480-lq7fqz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/182373/original/file-20170817-13480-lq7fqz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/182373/original/file-20170817-13480-lq7fqz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/182373/original/file-20170817-13480-lq7fqz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/182373/original/file-20170817-13480-lq7fqz.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">Voyager 1 captures a volcanic eruption on Jupiter’s moon Io.</span>
<span class="attribution"><span class="source">NASA/JPL</span></span>
</figcaption>
</figure>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/182366/original/file-20170817-15619-60go96.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/182366/original/file-20170817-15619-60go96.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/182366/original/file-20170817-15619-60go96.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=718&fit=crop&dpr=1 600w, https://images.theconversation.com/files/182366/original/file-20170817-15619-60go96.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=718&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/182366/original/file-20170817-15619-60go96.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=718&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/182366/original/file-20170817-15619-60go96.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=902&fit=crop&dpr=1 754w, https://images.theconversation.com/files/182366/original/file-20170817-15619-60go96.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=902&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/182366/original/file-20170817-15619-60go96.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=902&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Voyager 1 image of Ganymede, Jupiter’s largest moon and the largest moon in the Solar System at 5,262km in diameter (compared to Earth’s Moon at 3,475km diameter).</span>
<span class="attribution"><span class="source">NASA/JPL/Image processed by Bjӧrn Jόnsson</span></span>
</figcaption>
</figure>
<p>Indeed, during the course of their 12-year mission, the Voyagers discovered 24 new moons orbiting the outer planets and refined NASA’s use of the Deep Space Network to listen to signals from distant spacecraft.</p>
<h2>To Saturn and beyond</h2>
<p>After Jupiter, both Voyagers went on to encounter Saturn. Voyager 1 achieved the major goal of closely approaching Saturn’s giant moon, Titan. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/182375/original/file-20170817-13469-vt878q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/182375/original/file-20170817-13469-vt878q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/182375/original/file-20170817-13469-vt878q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=718&fit=crop&dpr=1 600w, https://images.theconversation.com/files/182375/original/file-20170817-13469-vt878q.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=718&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/182375/original/file-20170817-13469-vt878q.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=718&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/182375/original/file-20170817-13469-vt878q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=902&fit=crop&dpr=1 754w, https://images.theconversation.com/files/182375/original/file-20170817-13469-vt878q.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=902&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/182375/original/file-20170817-13469-vt878q.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=902&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Both Voyagers passed by the ringed planet Saturn.</span>
<span class="attribution"><span class="source">NASA/JPL</span></span>
</figcaption>
</figure>
<p>Following this encounter, with its primary mission ended, Voyager 1 was flung on a northward trajectory above the plain of the orbits of the planets. Voyager 2 was subsequently targeted to travel outward on an extended mission to visit the next two gas giant worlds.</p>
<p>When Voyager 2 flew past Uranus in January 1986, the signals being received were much weaker than when it flew by Saturn, five years earlier. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/182382/original/file-20170817-13444-wbh9xl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/182382/original/file-20170817-13444-wbh9xl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/182382/original/file-20170817-13444-wbh9xl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=567&fit=crop&dpr=1 600w, https://images.theconversation.com/files/182382/original/file-20170817-13444-wbh9xl.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=567&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/182382/original/file-20170817-13444-wbh9xl.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=567&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/182382/original/file-20170817-13444-wbh9xl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=713&fit=crop&dpr=1 754w, https://images.theconversation.com/files/182382/original/file-20170817-13444-wbh9xl.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=713&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/182382/original/file-20170817-13444-wbh9xl.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=713&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Voyager 2 captures Uranus.</span>
<span class="attribution"><span class="source">NASA/JPL</span></span>
</figcaption>
</figure>
<p>Consequently, CSIRO’s radio telescope at Parkes was linked, or arrayed, with NASA’s dishes in Canberra to boost Voyager 2’s weak radio signal.</p>
<p>This was the first time an array of telescopes had been used to track a spacecraft. Yet this array would be insufficient to receive the even fainter signals expected when Voyager 2 reached Neptune in 1989.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/182537/original/file-20170818-28120-3yf8fv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/182537/original/file-20170818-28120-3yf8fv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/182537/original/file-20170818-28120-3yf8fv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=428&fit=crop&dpr=1 600w, https://images.theconversation.com/files/182537/original/file-20170818-28120-3yf8fv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=428&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/182537/original/file-20170818-28120-3yf8fv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=428&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/182537/original/file-20170818-28120-3yf8fv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=538&fit=crop&dpr=1 754w, https://images.theconversation.com/files/182537/original/file-20170818-28120-3yf8fv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=538&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/182537/original/file-20170818-28120-3yf8fv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=538&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">CDSCC staff at Parkes monitoring the encounter with Uranus’ moon, Miranda, in 1986.</span>
<span class="attribution"><span class="source">CSIRO</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>So in the time between the encounters, NASA expanded Canberra’s largest dish from 64 metres to 70 metres in diameter to increase its sensitivity, and then linked it again with the Parkes 64 metre dish, to maximise the data capture at Neptune. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/182378/original/file-20170817-13430-t2hfq7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/182378/original/file-20170817-13430-t2hfq7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/182378/original/file-20170817-13430-t2hfq7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/182378/original/file-20170817-13430-t2hfq7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/182378/original/file-20170817-13430-t2hfq7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/182378/original/file-20170817-13430-t2hfq7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/182378/original/file-20170817-13430-t2hfq7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/182378/original/file-20170817-13430-t2hfq7.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">Neptune’s bright wispy cirrus-type clouds can been seen against the blue atmosphere.</span>
<span class="attribution"><span class="source">NASA/JPL/ Image processed by Bjӧrn Jόnsson</span></span>
</figcaption>
</figure>
<p>The increased size and sensitivity of the Canberra dish also meant that it was able to support Voyager’s ongoing journey beyond the outer planets.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/182539/original/file-20170818-30283-fivs8d.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/182539/original/file-20170818-30283-fivs8d.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/182539/original/file-20170818-30283-fivs8d.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=461&fit=crop&dpr=1 600w, https://images.theconversation.com/files/182539/original/file-20170818-30283-fivs8d.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=461&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/182539/original/file-20170818-30283-fivs8d.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=461&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/182539/original/file-20170818-30283-fivs8d.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=580&fit=crop&dpr=1 754w, https://images.theconversation.com/files/182539/original/file-20170818-30283-fivs8d.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=580&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/182539/original/file-20170818-30283-fivs8d.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=580&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Robina Otrupcek tracking Voyager 2 at Neptune from the CSIRO Parkes telescope on the day before the close approach in 1989.</span>
<span class="attribution"><span class="source">CSIRO</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<h2>The Pale Blue Dot</h2>
<p>In 1990 Voyager 1 turned its cameras towards home. The resulting photograph, known as the Pale Blue Dot, is our most distant view of Earth, a fraction of a pixel floating in a deep black sea. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/182376/original/file-20170817-13501-t0lmj2.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/182376/original/file-20170817-13501-t0lmj2.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/182376/original/file-20170817-13501-t0lmj2.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=381&fit=crop&dpr=1 600w, https://images.theconversation.com/files/182376/original/file-20170817-13501-t0lmj2.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=381&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/182376/original/file-20170817-13501-t0lmj2.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=381&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/182376/original/file-20170817-13501-t0lmj2.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=479&fit=crop&dpr=1 754w, https://images.theconversation.com/files/182376/original/file-20170817-13501-t0lmj2.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=479&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/182376/original/file-20170817-13501-t0lmj2.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=479&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">This pale blue dot, less than a pixel in size, is Voyager 1’s view of Earth.</span>
<span class="attribution"><span class="source">NASA/JPL</span></span>
</figcaption>
</figure>
<p>The legendary astrophysicist Carl Sagan, involved with Voyager since its inception, reflected that this distant view of the tiny stage on which we play out our lives should inspire us “to preserve and cherish that pale blue dot, the only home we’ve ever known”.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/wupToqz1e2g?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The Pale Blue Dot.</span></figcaption>
</figure>
<p>Both Voyagers have long since left the outer planets behind, two explorers heading into the galaxy in different directions, still sending data back to Earth and answering questions we didn’t even know to ask when they were launched 40 years ago.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/the-pale-blue-dot-and-other-selfies-of-earth-39118">The pale blue dot and other 'selfies' of Earth</a>
</strong>
</em>
</p>
<hr>
<h2>Voyagers only talk to Australia</h2>
<p>The Canberra tracking station continues to receive signals from both Voyager spacecraft every day, and is currently the only tracking station capable of exchanging signals with Voyager 2, owing to the spacecraft’s position as it heads on its southward path out of the Solar System.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/182383/original/file-20170817-13501-5sxt70.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/182383/original/file-20170817-13501-5sxt70.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/182383/original/file-20170817-13501-5sxt70.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=411&fit=crop&dpr=1 600w, https://images.theconversation.com/files/182383/original/file-20170817-13501-5sxt70.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=411&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/182383/original/file-20170817-13501-5sxt70.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=411&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/182383/original/file-20170817-13501-5sxt70.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=516&fit=crop&dpr=1 754w, https://images.theconversation.com/files/182383/original/file-20170817-13501-5sxt70.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=516&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/182383/original/file-20170817-13501-5sxt70.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=516&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 Parkes telescope tracking Voyager 2 at Neptune on the day of the close approach.</span>
<span class="attribution"><span class="source">CSIRO</span></span>
</figcaption>
</figure>
<p>Due to their respective distances, tens of billions of kilometres from home, the signal strength from both spacecraft is very weak, only one-tenth of a billion-trillionth of a watt.</p>
<p>In 2012, Voyager 1 became the first spacecraft to have entered interstellar space, the region between the stars. Lying beyond the influence of the magnetic bubble generated by our Sun, Voyager 1 is able to directly study the composition of the interstellar medium, for the first time.</p>
<p>Voyager 1 is still receiving commands that can only be sent from Canberra’s dishes. It is the only station with the high-power transmitter that can transmit a signal strong enough to be received by the spacecraft.</p>
<p>It has been an epic voyage for two spacecraft no bigger than small buses, two brilliant robots with an eight track tape deck to record data and 256kB of memory.</p>
<h2>A golden message</h2>
<p>The scientists and engineers at NASA’s Jet Propulsion Laboratory in California, who built the Voyagers and continue to operate them, planned ahead for Voyager’s legacy and its journey beyond our Solar System. </p>
<p>On board both spacecraft they placed a golden record, similar in concept to a vinyl record, featuring one and a half hours of world music and greetings to the universe in 55 different languages.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/182386/original/file-20170817-13469-1geh4xq.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/182386/original/file-20170817-13469-1geh4xq.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/182386/original/file-20170817-13469-1geh4xq.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=311&fit=crop&dpr=1 600w, https://images.theconversation.com/files/182386/original/file-20170817-13469-1geh4xq.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=311&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/182386/original/file-20170817-13469-1geh4xq.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=311&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/182386/original/file-20170817-13469-1geh4xq.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=391&fit=crop&dpr=1 754w, https://images.theconversation.com/files/182386/original/file-20170817-13469-1geh4xq.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=391&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/182386/original/file-20170817-13469-1geh4xq.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=391&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 golden record and instructions on how to play it.</span>
<span class="attribution"><span class="source">NASA/JPL</span></span>
</figcaption>
</figure>
<p>The cover art features a pictorial representation of how to play the record and a map reference to Earth’s location in our galaxy based on the positions of surrounding pulsars.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/Bhuq9rNO_FQ?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The first of the 31 recordings. Click on the video to hear the rest.</span></figcaption>
</figure>
<p>By 2030, both Voyagers will be out of power, their scientific instruments deactivated, no longer able to exchange signals with Earth. They will continue on at their current speeds of more than 17 kilometres per second, carrying their golden records like messages in bottles across the vast ocean of interstellar space. </p>
<p>Heading in opposite directions, southward and northward out of the Solar System, it will be 40,000 years before Voyager 2 passes within a handful of light years of the closest star system along its flight path, and 296,000 years before Voyager 1 passes by the bright star Sirius.</p>
<p>Beyond that, we may imagine them surviving for billions of years as the only traces of a civilisation of human explorers in the far reaches of our galaxy.</p><img src="https://counter.theconversation.com/content/82512/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 Voyager space probes sent back some amazing images of the planets in the outer Solar System, and they’re still talking to Earth every day via Australia’s tracking station.John Sarkissian, Operations Scientist, CSIROEd Kruzins, Facilities Program Director Nasa Operations Canberra Deep Space Communication Complex , CSIROLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/604072016-06-03T10:12:59Z2016-06-03T10:12:59ZA stolen exoplanet that will kill us all? Here’s what we do know about ‘Planet Nine’<figure><img src="https://images.theconversation.com/files/125097/original/image-20160603-11585-8vnwyc.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Artist's impression of Planet Nine.</span> <span class="attribution"><span class="source">Tomruen, nagualdesign; background taken from File:ESO </span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Ever since <a href="https://theconversation.com/claims-about-new-planets-that-turned-out-to-be-wrong-and-why-planet-nine-may-be-different-53573">a study suggested</a> that a “Planet Nine” could be lurking in the outskirts of our solar system, astronomers have been busy trying to pin it down. </p>
<p>As nobody has actually observed the planet yet, this research has been largely computational. The existence of the planet was only suggested after scientists noticed that objects in its vicinity were moving strangely.</p>
<p>Since it was proposed in January, astronomers have modelled Planet Nine’s structure, orbit, estimated threat to Earth and possible origin. But with all this data at hand, are we any closer to actually finding it? Let’s take a look at some recent results and what they really mean.</p>
<h2>It may be an exoplanet</h2>
<p>The <a href="http://mnrasl.oxfordjournals.org/content/460/1/L109">latest such study</a> has come up with two different possible scenarios for Planet Nine’s origin. One is that it may have started as a forming outer planet core from our own early solar system which was expelled to the edge of the solar system by some process, perhaps a collision. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/gVSEK9yvr3s?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Simulation of the sun stealing an exoplanet.</span></figcaption>
</figure>
<p>However, another possibility is that our sun may have stolen Planet Nine from a nearby star in the Milky Way 4.5 billion years ago, which would make it our nearest extrasolar planet. As star formation regions are relatively dense with stars – the sun was born in a cluster with perhaps 1,000 other stars – these can indeed interact. </p>
<p>While this is entirely possible, the research assumes that Neptune-sized objects were relatively common in this region – something we simply don’t know. The study also suggest that further observations and modelling of the positions and orbits of minor objects in the solar system beyond Neptune now may provide further clues as to the origin of the proposed Planet Nine – whether this is core expulsion as originally proposed or exoplanet capture.</p>
<p>At the moment, the lack of direct observations of Planet Nine and the whole range of objects which may be affected by it mean that the explanations are poorly constrained. In the meantime, this kind of work provides interesting ideas – but ultimately we need proof. Excitingly, if it does exist and turns out to be a captured exoplanet, it is likely to be our best bet for visiting an exoplanet in the near future.</p>
<h2>It could be made up of iron and ice</h2>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/125100/original/image-20160603-11600-oett2w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/125100/original/image-20160603-11600-oett2w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=884&fit=crop&dpr=1 600w, https://images.theconversation.com/files/125100/original/image-20160603-11600-oett2w.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=884&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/125100/original/image-20160603-11600-oett2w.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=884&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/125100/original/image-20160603-11600-oett2w.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1111&fit=crop&dpr=1 754w, https://images.theconversation.com/files/125100/original/image-20160603-11600-oett2w.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1111&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/125100/original/image-20160603-11600-oett2w.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1111&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Planet Nine could be similar to Uranus.</span>
<span class="attribution"><span class="source">NASA, ESA, and M. Showalter</span></span>
</figcaption>
</figure>
<p>Other <a href="http://www.sciencealert.com/astrophysicists-have-predicted-what-planet-nine-would-be-made-of">computer simulations</a> assume that Planet Nine was a distant ice giant similar to Neptune and Uranus. They calculate the evolution of the size, temperature, luminosity and colour of such a body, having moved from its possible formation point nearer the sun to its distant position at about 700 AU.</p>
<p>This research suggests Planet Nine is like a “mini-Uranus”, with an iron core, silicate mantle, water ice shell and hydrogen/helium outer layers. Its temperature would be about -226°C (or 47 Kelvin) – and most of this would be internal heat rather than absorbed sunlight, which means it would be difficult to see with optical telescopes. In fact, the planet would reflect very little sunlight, which means it would be visible in infrared wavelengths (heat) instead of visible light. </p>
<p>The research is useful for scientists modelling and searching for the planet, as it helps them to know what they should look for.</p>
<h2>It may be harder to detect than we thought</h2>
<p>The proposers of Planet Nine, and other astronomers, are busy using an array of telescopes to search for their target. These includes further work with data from the <a href="http://wise2.ipac.caltech.edu/docs/release/allsky/">WISE survey</a>, as well as <a href="http://www.lpl.arizona.edu/css/">Catalina Sky Survey</a> and <a href="http://pan-starrs.ifa.hawaii.edu/public/">Pan-STARRS</a>. So far, these searches have been unsuccessful. Current and planned searches are underway and proposed using telescopes including<a href="http://subarutelescope.org/"> Subaru at Mauna Kea observatory</a>, <a href="http://www.almaobservatory.org/">ALMA</a>, the <a href="http://www.darkenergysurvey.org/">Dark Energy Survey </a> and the <a href="http://www.jwst.nasa.gov/">James Webb Space Telescope</a>.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/125104/original/image-20160603-11616-1hmzqfd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/125104/original/image-20160603-11616-1hmzqfd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=444&fit=crop&dpr=1 600w, https://images.theconversation.com/files/125104/original/image-20160603-11616-1hmzqfd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=444&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/125104/original/image-20160603-11616-1hmzqfd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=444&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/125104/original/image-20160603-11616-1hmzqfd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=558&fit=crop&dpr=1 754w, https://images.theconversation.com/files/125104/original/image-20160603-11616-1hmzqfd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=558&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/125104/original/image-20160603-11616-1hmzqfd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=558&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Mauna Kea’s Subaru.</span>
<span class="attribution"><span class="source">Denys/wikimedia</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>But there may be other ways to spot it. For example, there was <a href="http://arxiv.org/abs/1602.06116">a prediction</a> that the orbit of <a href="https://www.nasa.gov/mission_pages/cassini/main/">the Cassini spacecraft</a> at Saturn may be affected by Planet Nine’s gravitational pull, based on its possible location in Cetus at about 630 AU (1 AU = the distance between the Earth and the sun). However, many scientists are sceptical of this. </p>
<p>So why haven’t we seen it? The study that modelled its interior also postulated how easy it would be to detect the object using surveys such as WISE, and estimated that Planet Nine’s current size is just 3.7 times Earth. This is considerably less than the 10 times our planet which was initially suggested. They therefore argue that it would be very hard to spot the planet with current instruments, but suggest future telescopes may be able to. </p>
<p>What’s more, some suggest that the planet may currently be at aphelion (its farthest point from the sun), which would also make it even more difficult to see. However, one study has managed to make the search area smaller by <a href="https://www.theguardian.com/science/2016/feb/24/search-narrows-for-planet-nine-along-sprawling-orbit-of-thousands-of-years">modelling the orbit</a> and its inclination. The search is narrowing, but slowly.</p>
<h2>It probably won’t wipe us out</h2>
<p>With its suggested orbit between 200 and 1,200-2,000 AU – much further away from us than the sun– it seems we should be safe from Planet Nine. But conspiracy theorists <a href="http://metro.co.uk/2016/04/06/mysterious-ninth-planet-in-our-solar-system-could-wipe-out-life-on-earth-5799703/">were quick to suggest that</a>, as this may be the first of several such objects, at least one may have our name on it. However, there is no evidence that any known or postulated object poses any threat to Earth. But observers are keeping a careful eye on near-Earth objects for potential problems.</p>
<p>Planet Nine, if it exists, would certainly be a difficult object to detect. We know that the effects on six nearby objects look consistent with its existence, although even this is not universally accepted by astronomers. However, it is certainly strong enough to prompt a detailed search. Thanks to the computational modelling, the search is narrowing. At the same time, technology is developing and observations in this region are improving. If Planet Nine exists, it should be found in the next few years.</p><img src="https://counter.theconversation.com/content/60407/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andrew Coates receives funding from STFC and UKSA. </span></em></p>Why Planet Nine should be found in the next few years … if it exists.Andrew Coates, Professor of Physics, Deputy Director (Solar System) at the Mullard Space Science Laboratory, UCLLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/462942015-08-19T17:09:18Z2015-08-19T17:09:18ZHow did Jupiter and Saturn form? The answer may lie with the humble pebble<figure><img src="https://images.theconversation.com/files/92417/original/image-20150819-10832-6b7mub.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">We know exactly what it looks like but have been unable to explain how it came into being - until now.</span> <span class="attribution"><a class="source" href="http://www.esa.int/spaceinimages/Images/2014/05/Jupiter_and_its_shrunken_great_red_spot">NASA, ESA, and A. Simon (Goddard Space Flight Center)</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>After many decades of exploring the solar system, we still have much to learn about our closest celestial neighbours. One of the biggest remaining puzzles is how the giant gas planets managed to form in the early days of the solar system, with the leading theories suggesting that they could have formed by <a href="http://www.gemini.edu/index.php?q=node/117">repeated collisions between objects</a> about ten times smaller than our own moon. </p>
<p>Now a <a href="http://nature.com/articles/doi:10.1038/nature14675">new model</a> suggests they could have developed with the help of slowly forming, relatively small pebbles – a discovery that may help us answer a number of other questions about the planets.</p>
<p>There is a lot we actually <em>do</em> know about our solar system. The sun and its planets <a href="http://www.universetoday.com/72589/solar-nebula-theory/">formed out of a cloud of gas and dust</a> about <a href="http://lasp.colorado.edu/%7Ebagenal/1010/SESSIONS/11.Formation.html">4.5 billion years ago</a>. We can even <a href="http://www.eso.org/public/news/eso1436/">watch this process in motion</a> in other places in our galaxy. We also know that, once the pieces which eventually form the planets get to a certain size, they can grow by their gravitational pull on smaller objects – trapping them in unstable orbits before they eventually crash into the surface. Violent growth, but growth nonetheless. </p>
<p>But there is a big gap in the understanding of this process, and it’s how a disk of rotating gas and dust can possibly lead to the number of large and small planets that we see today. This is exactly the question that the new paper is trying to answer.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/92414/original/image-20150819-10832-1ptp42v.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/92414/original/image-20150819-10832-1ptp42v.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=358&fit=crop&dpr=1 600w, https://images.theconversation.com/files/92414/original/image-20150819-10832-1ptp42v.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=358&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/92414/original/image-20150819-10832-1ptp42v.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=358&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/92414/original/image-20150819-10832-1ptp42v.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=449&fit=crop&dpr=1 754w, https://images.theconversation.com/files/92414/original/image-20150819-10832-1ptp42v.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=449&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/92414/original/image-20150819-10832-1ptp42v.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=449&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The interior of the gas giants.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Giant_planet#/media/File:Gas_Giant_Interiors.jpg">RHorning/wikimedia</a></span>
</figcaption>
</figure>
<p>Over the past decade, scientists have proposed a number of models <a href="http://www.universetoday.com/72589/solar-nebula-theory/">starting with the assumption</a> that the gas and dust disk forms a bunch of small objects very quickly. If you run a simulation of this scenario (termed the “standard model” in the new study), you find that the dust is able to roll up into clumps, and from there into denser and denser objects until you wind up with something a few centimetres to a metre across. These objects, dubbed “pebbles” (yes, that’s the technical term) can all grow quite quickly and rapidly reach the mass of the Earth. </p>
<p>The problem is that if you assume that this is constantly happening, you wind up with way too many of these Earth-sized proto-planets. And having too many objects would result in them scattering off each other – ending up in crazy orbits that are no longer aligned with the disk surrounding the star.</p>
<p>The disk, however, is the only place where there is a lot of material for these proto-planets to continue growing, which they would need to do in order to form planets that look like Jupiter and Saturn. But in this model, their crazy orbits no longer take them through that material, meaning they stop growing. In this way our model solar system stalls out with a bunch of Earth-sized objects, and no gas giants.</p>
<p>This is a problem for our understanding of how the gas giants formed, because if you always end up forming 100 proto-Earths and no gas giant planets, then the model is clearly incomplete. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/92415/original/image-20150819-10852-dw66qb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/92415/original/image-20150819-10852-dw66qb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/92415/original/image-20150819-10852-dw66qb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/92415/original/image-20150819-10852-dw66qb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/92415/original/image-20150819-10852-dw66qb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/92415/original/image-20150819-10852-dw66qb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/92415/original/image-20150819-10852-dw66qb.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">
<figcaption>
<span class="caption">Artist’s impression of a giant gas exoplanet near our solar system.</span>
<span class="attribution"><span class="source">NASA, ESA, and G. Bacon (STScI)</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>We also know that our solar system isn’t particularly unique; especially with the <a href="http://www.nasa.gov/mission_pages/kepler/main/index.html">treasure trove of information</a> that the planet-hunting Kepler satellite is feeding us, there are an increasing number of solar systems which <a href="https://theconversation.com/teenage-jupiter-may-hold-the-secret-of-how-planets-form-46090">appear to have a few giant planets further out</a>, and smaller, (potentially) rocky planets closer in, just like ours. Cracking how gas giants form is therefore increasingly relevant.</p>
<h2>Mystery solved?</h2>
<p>So what is the solution to these problems? The new model suggests that instead of the disk of gas and dust forming into pebbles rapidly, the proto-planets might build up more slowly. And indeed, if the pebbles form less quickly, then the objects which grow from them are both less numerous and take much longer to grow. </p>
<p>The authors ran another simulation, this time with a slow addition of pebbles into the mix over time. This means that instead of the disk going granular all at once, the pebbles form gradually, here and there. This change allows the largest proto-planets to grow to much larger sizes. With this new model, the smallest objects are either captured by the larger ones, or scattered far away from the disk, which – as in the standard model – stops them from growing. Meanwhile, the largest few objects are able to grow faster and faster (limited largely by how slowly the pebbles are being formed), until they reach a point where they could plausibly go on to develop into a gas giant.</p>
<p>The details of the simulation will be refined as time goes on and computers are able to do more intensive calculations – but it is a promising start that this work has been able to reproduce a number of the features of our solar system, while avoiding some of the problems of the previous model. If these results hold, planetary scientists will be able to push forward, answering more detailed questions about how the planets moved to their current positions, or how and when the gas giants gained all their gas.</p><img src="https://counter.theconversation.com/content/46294/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jillian Scudder 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>How a bunch of pebbles could have created Jupiter, Saturn, Neptune and Uranus.Jillian Scudder, Postdoctoral Research Fellow in Astrophysics, University of SussexLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/450022015-07-23T05:24:09Z2015-07-23T05:24:09ZAfter Pluto there’s still plenty of the solar system left to explore<figure><img src="https://images.theconversation.com/files/89324/original/image-20150722-1460-12z4553.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/nasamarshall/5099713438/">NASA/JPL-Caltech</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc/4.0/">CC BY-NC</a></span></figcaption></figure><p>The past couple of years have been very exciting for space exploration. We’ve watched as spacecraft made visits to <a href="https://theconversation.com/curiosity-catches-a-whiff-of-methane-on-mars-and-a-possibility-of-past-life-35595">Mars</a>, <a href="https://theconversation.com/explainer-what-philae-did-in-its-60-hours-on-comet-67p-34289">comet 67P</a> and, just last week, <a href="https://theconversation.com/new-horizons-finally-gets-up-close-with-pluto-for-15-minutes-44603">Pluto</a>, which for decades marked the edge of our solar system.</p>
<p>Given the fervour that surrounded last week’s New Horizons mission, it’s fair to wonder whether anything could be as exciting as flying past Pluto (with perhaps the exception of <a href="https://theconversation.com/if-we-are-to-find-life-beyond-earth-we-need-to-be-explorers-not-hunters-45001">discovering alien life</a>). We have a basic understanding of our solar system – such as how moons, rings and planets interact in planetary systems, and what their atmospheres are made of. We also have theories about how the solar system was formed and has evolved. But we’re far from finishing exploring our solar system and testing these theories. Several missions over the next decade and beyond will reveal new insights into our patch of the universe.</p>
<h2>What’s next?</h2>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/89323/original/image-20150722-1460-nozv2r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/89323/original/image-20150722-1460-nozv2r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/89323/original/image-20150722-1460-nozv2r.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/89323/original/image-20150722-1460-nozv2r.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/89323/original/image-20150722-1460-nozv2r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/89323/original/image-20150722-1460-nozv2r.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/89323/original/image-20150722-1460-nozv2r.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">
<figcaption>
<span class="caption">Brave new worlds.</span>
<span class="attribution"><span class="source">NASA, ESA, and G. Bacon</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>New Horizons will continue to produce new discoveries as it <a href="https://theconversation.com/new-horizons-is-an-old-spacecraft-but-it-will-transform-our-knowledge-of-pluto-44524">transmits its measurements over the next year</a>, but the <a href="https://theconversation.com/beyond-pluto-new-horizons-mission-is-not-over-yet-44520">next step</a> is a fly-by of another <a href="http://solarsystem.nasa.gov/planets/profile.cfm?Object=KBOs">Kuiper Belt</a> object beyond the orbit of Pluto. The preferred candidate is a body designated <a href="http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2014+MU69&orb=1">2014 MU69</a> that was discovered just <a href="http://hubblesite.org/newscenter/archive/releases/2014/35/image/a/">last year</a>. If approval is granted later this year, a possible fly-by in 2019 could allow us to discovering more about this mysterious object and help us understand what happens at the very edge of our solar system and how it was formed.</p>
<p>In July 2016, <a href="http://www.nasa.gov/mission_pages/juno/images/index.html">NASA’s Juno mission</a> will enter orbit around Jupiter, the first spacecraft to do so since the end of the Galileo mission in 2003. Juno will study the interior of Jupiter, looking at its composition for information that could teach us about the formation of the solar system. It will also study <a href="http://hubblesite.org/newscenter/archive/releases/2000/38/image/a/">Jupiter’s aurora</a> and how the planet connects with its <a href="http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=1589">enormous magnetosphere</a>, the largest physical structure in the solar system.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/89317/original/image-20150722-1426-1ugpmj0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/89317/original/image-20150722-1426-1ugpmj0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/89317/original/image-20150722-1426-1ugpmj0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/89317/original/image-20150722-1426-1ugpmj0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/89317/original/image-20150722-1426-1ugpmj0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/89317/original/image-20150722-1426-1ugpmj0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/89317/original/image-20150722-1426-1ugpmj0.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">
<figcaption>
<span class="caption">Move over Curiosity, here comes ExoMars.</span>
<span class="attribution"><span class="source">ESA</span></span>
</figcaption>
</figure>
<p>Europe’s <a href="http://exploration.esa.int/mars/">ExoMars mission</a> aims to search for signatures of life on Mars using two spacecraft. The Trace Gas Orbiter, due to launch in 2016, will study the distribution of volatile gases such as water, methane and ozone in Mars’ atmosphere, all of which could provide evidence for life. It will also act as a telecommunications relay for <a href="http://exploration.esa.int/mars/51499-exomars-rover/">a rover</a> that will be launched in 2018, which will drill two metres under the surface of the planet in search of similar biosignatures.</p>
<p>There’s more. The joint European-Japanese <a href="http://sci.esa.int/bepicolombo/">BepiColombo mission</a> to Mercury will launch in 2017, with two spacecraft undertaking a detailed study of the planet’s interior, surface and magnetosphere. And in 2018, Japan’s <a href="http://global.jaxa.jp/projects/sat/hayabusa2/">Hayabusa 2</a> spacecraft will arrive at <a href="http://arxiv.org/abs/1302.1199">one of</a> the <a href="http://scienceworld.wolfram.com/astronomy/ApolloAsteroid.html">Apollo asteroids</a> that cross the Earth’s orbit and, after surveying it for a year, will return samples to Earth in 2020.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/89319/original/image-20150722-1432-1d4u20j.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/89319/original/image-20150722-1432-1d4u20j.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=393&fit=crop&dpr=1 600w, https://images.theconversation.com/files/89319/original/image-20150722-1432-1d4u20j.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=393&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/89319/original/image-20150722-1432-1d4u20j.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=393&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/89319/original/image-20150722-1432-1d4u20j.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=493&fit=crop&dpr=1 754w, https://images.theconversation.com/files/89319/original/image-20150722-1432-1d4u20j.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=493&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/89319/original/image-20150722-1432-1d4u20j.jpeg?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">
<figcaption>
<span class="caption">A long way to go for ice.</span>
<span class="attribution"><span class="source">ESA/AOES</span></span>
</figcaption>
</figure>
<p>Further into the future, we have a big first to look forward to. In 2022, the European Space Agency (ESA) will send the <a href="http://sci.esa.int/juice/">Jupiter Icy Moon Explorer (JUICE)</a> mission on a 10-year journey <a href="http://solarsystem.nasa.gov/planets/profile.cfm?Object=Ganymede">to Ganymede</a>, the largest moon in the Solar System. This will be the first time we have put a <a href="https://www.youtube.com/watch?v=j8ZiNNIFzzQ&feature=youtu.be">spacecraft in orbit around the moon</a> of a giant planet. JUICE’s primary aim is to study whether the moons of giant planets can be viable locations for life. A NASA mission called <a href="http://solarsystem.nasa.gov/missions/profile.cfm?MCode=EuropaFlyby">Europa Clipper</a> will also explore another of Jupiter’s moons, Europa, in the late 2020s/early 2030s.</p>
<p>Even with all of these planned missions, there are plenty of other corners of the solar system worth visiting again. Many scientists are not satisfied with simply making measurements from afar but want to get samples back from our moon, Mars and its moon Phobos. These so-called sample return missions still require a huge amount of technology development that will push our capabilities much further. But they are also a stepping stone to <a href="http://www.esa.int/Our_Activities/Human_Spaceflight/Calling_new_partners_for_exploring_the_Moon_and_Mars">human exploration</a> as robotic exploration allows us to test technology and reconnoitre distant hostile environments before we send humans.</p>
<p>Comets also continue to be a focus of attention for space scientists because there is no typical comet. “<a href="http://www.scientificamerican.com/article/main-belt-comet-asteroid/">Main-belt comets</a>”, for example, are a recently discovered class of comet which reside in the asteroid belt and may hold the keys to understanding the source of Earth’s water. The recent <a href="http://news.nationalgeographic.com/2015/06/150623-venus-volcanoes-active-space/">discovery of volcanic activity on Venus</a> is also tempting atmospheric scientists and geologists to look again at Earth’s “evil twin”.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/89326/original/image-20150722-1473-kqouyi.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/89326/original/image-20150722-1473-kqouyi.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=377&fit=crop&dpr=1 600w, https://images.theconversation.com/files/89326/original/image-20150722-1473-kqouyi.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=377&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/89326/original/image-20150722-1473-kqouyi.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=377&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/89326/original/image-20150722-1473-kqouyi.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=474&fit=crop&dpr=1 754w, https://images.theconversation.com/files/89326/original/image-20150722-1473-kqouyi.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=474&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/89326/original/image-20150722-1473-kqouyi.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=474&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The outer reaches.</span>
<span class="attribution"><span class="source">NASA/JPL</span></span>
</figcaption>
</figure>
<p>The outer solar system beyond Saturn is still very poorly explored. Uranus and Neptune have only received single fly-bys, similar to New Horizons at Pluto, with visits in 1986 and 1989 respectively. These ice giant planets form a unique class of planet and are quite different to the gas giants, Jupiter and Saturn. Scientists have been arguing for a <a href="http://www.space.com/13248-nasa-uranus-missions-solar-system.html">return to the ice giants</a> for the last decade. Triton, the largest moon of Neptune is of particular interest because it is suspected to be a Kuiper Belt object that has been pulled into orbit in a similar way to the origins of Pluto. Without radically new technology, Triton is the only opportunity we have to encounter a Pluto-type object multiple times.</p>
<h2>What’s the point?</h2>
<p>It is part of the human condition to explore and ask questions about <a href="http://time.com/3957126/pluto-new-horizons-meaning/">where we came from</a>. Many of science’s big questions, such as: “how did our Solar System evolve?” and “is there life beyond Earth?” aren’t easy to answer without exploring the universe. One of Philae’s main science questions was to try to unravel <a href="https://theconversation.com/why-is-life-left-handed-the-answer-is-in-the-stars-44862">why certain biological molecules</a> are shaped the way they are.</p>
<p>These answers also come with a cost. New Horizons cost around US$700m (£450m), although this only works out at about US$2 (£1.30) for each US citizen. But this cash wasn’t just launched into space. The money for space exploration goes to the same industries that support other sectors we rely on, such as global communications, weather observations and navigation. The same scientists also educate the next generation of scientists and engineers who in turn will ask those same big questions and seek answers amongst the planets.</p><img src="https://counter.theconversation.com/content/45002/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Chris Arridge receives funding from the the Royal Society and Science and Technology Facilities Council. He also chairs the Solar System Advisory Panel which provides advice to the Science and Technology Facilities Council on Solar System research carried out in the UK. He is involved in ESA's JUICE mission to Jupiter and NASA's Cassini mission to Saturn and is active in developing new missions to Uranus.</span></em></p>Space scientists have a busy decade ahead with plans to visit Jupiter, Mars, Mercury and other interplanetary bodies all on the cards.Chris Arridge, Research Fellow/Lecturer, Lancaster UniversityLicensed as Creative Commons – attribution, no derivatives.