tag:theconversation.com,2011:/ca/topics/earth-like-planets-23346/articlesEarth-like planets – The Conversation2019-11-05T12:16:34Ztag:theconversation.com,2011:article/1221042019-11-05T12:16:34Z2019-11-05T12:16:34ZNASA’s TESS spacecraft is finding hundreds of exoplanets – and is poised to find thousands more<figure><img src="https://images.theconversation.com/files/299057/original/file-20191028-113998-1v3p675.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">This artist's impression shows a view of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the solar system. </span> <span class="attribution"><a class="source" href="https://www.jpl.nasa.gov/news/news.php?feature=6600">ESO/M. Kornmesser</a></span></figcaption></figure><p>Within just 50 light-years from Earth, there are about 1,560 stars, likely orbited by several thousand planets. About a thousand of these extrasolar planets – known as exoplanets – may be rocky and have a composition similar to Earth’s. Some may even harbor life. Over 99% of these alien worlds remain undiscovered — but this is about to change. </p>
<p>With NASA’s new exoplanet-hunter space telescope <a href="https://tess.mit.edu">TESS</a>, the all-sky search is on for possibly habitable planets close to our solar system. TESS — <a href="https://youtu.be/mpViVEO-ymc">orbiting Earth every 13.7 days</a> — and ground-based telescopes are poised to find hundreds of planets over the next few years. This could transform astronomers’ understanding of alien worlds around us and provide targets to scan with next-generation telescopes for <a href="https://ui.adsabs.harvard.edu/abs/2013ApJ...764..182S">signatures of life</a>. In just over a year, TESS has identified <a href="https://tess.mit.edu/publications/">more than 1,200 planetary candidates</a>, 29 of which astronomers have already <a href="https://tess.mit.edu/publications/">confirmed as planets</a>. Given TESS’s unique ability to simultaneously search tens of thousands of stars for planets, the mission is <a href="https://doi.org/10.3847/1538-4365/aae3e9">expected to yield over 10,000 new worlds</a>.</p>
<p>These are exciting times for astronomers and, especially, for those of us exploring exoplanets. <a href="http://apai.space">We</a> <a href="https://brackham.github.io">are</a> members of the planet-hunting <a href="http://project-eden.space">Project EDEN</a>, which also supports TESS’s work. We use telescopes on the ground and in space to find exoplanets to understand their properties and potential for harboring life.</p>
<h2>Undiscovered worlds all around us</h2>
<p>Worlds around us await discovery. Take, for example, Proxima Centauri, an unassuming, faint red star, invisible without a telescope. It is one of over a hundred billion or so such stars within our galaxy, unremarkable except for its status as our next-door neighbor. Orbiting Proxima is a fascinating but mysterious world, called Proxmia b, <a href="https://doi.org/10.1038/nature19106">discovered</a> only in 2016. </p>
<p>Scientists know surprisingly little about <a href="https://exoplanets.nasa.gov/exoplanet-catalog/7167/proxima-centauri-b/">Proxima b</a>. Astronomers name the first planet discovered in a system “b”. This planet has never been seen with human eyes or by a telescope. But we know it exists due to its gravitational pull on its host star, which makes the star wobble ever so slightly. This slight wobble was found in measurements collected by a <a href="https://doi.org/10.1038/nature19106">large, international group of astronomers from data taken with multiple ground-based telescopes</a>. Proxima b <a href="https://doi.org/10.3847/2041-8213/aa5f51">very likely has a rocky composition similar to Earth’s,</a> but higher mass. It receives about the same amount of heat as Earth receives from the Sun.</p>
<p>And that is what makes this planet so exciting: It lies in the “habitable” zone and just might have properties similar to Earth’s, like a surface, liquid water, and — who knows? — maybe even an atmosphere bearing the telltale chemical signs of life.</p>
<p><a href="https://tess.mit.edu">NASA’s TESS mission</a> launched in April 2018 to hunt for other broadly Earth-sized planets, but with a different method. TESS is looking for rare dimming events that happen when planets pass in front of their host stars, blocking some starlight. These transit events indicate not only the presence of the planets, but also their sizes and orbits.</p>
<p>Finding a new transiting exoplanet is a big deal for astronomers like us because, unlike those found through stellar wobbles, worlds seen transiting can be studied further to determine their densities and atmospheric compositions.</p>
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<figcaption>
<span class="caption">By measuring the depth of the dip in brightness and knowing the size of the star, scientists can determine the size or radius of the planet.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/mission_pages/kepler/multimedia/images/transit-light-curve.html">NASA Ames</a></span>
</figcaption>
</figure>
<h2>Red dwarf suns</h2>
<p>For us, the most exciting exoplanets are the smallest ones, which TESS can detect when they orbit small stars called red dwarfs – stars with masses less than half the mass of our Sun.</p>
<p>Each of these systems is unique. For example, <a href="https://arxiv.org/abs/1906.09267">LP 791-18</a> is a red dwarf star 86 light-years from Earth around which TESS found two worlds. The first is a “super-Earth,” a planet larger than Earth but probably still mostly rocky, and the second is a “mini-Neptune,” a planet smaller than Neptune but gas- and ice-rich. Neither of these planets have counterparts in our solar system.</p>
<p>Among astronomers’ current favorites of the new broadly Earth-sized planets is <a href="https://arxiv.org/abs/1809.07242">LHS 3884b</a>, a scorching “hot Earth” that orbits its sun so quickly that on it you could celebrate your birthday every 11 hours.</p>
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<a href="https://images.theconversation.com/files/299546/original/file-20191030-17924-131wnz4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/299546/original/file-20191030-17924-131wnz4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/299546/original/file-20191030-17924-131wnz4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/299546/original/file-20191030-17924-131wnz4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/299546/original/file-20191030-17924-131wnz4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/299546/original/file-20191030-17924-131wnz4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/299546/original/file-20191030-17924-131wnz4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/299546/original/file-20191030-17924-131wnz4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Artist’s impression of an exoplanet transiting a red dwarf star.</span>
<span class="attribution"><span class="source">ESO/L. Calçada</span></span>
</figcaption>
</figure>
<h2>No Earth-like worlds yet</h2>
<p>But how Earth-like are Earth-sized planets? The promise of finding nearby worlds for detailed studies is already paying off. A team of astronomers <a href="https://arxiv.org/abs/1908.06834">observed the hot super-Earth LHS 3884b</a> with the Hubble Space Telescope and found the planet to be a horrible vacation spot, without even an atmosphere. It is just a bare rock with temperatures ranging from over 700 C (1300 Fahrenheit) at noon to near absolute zero (-460 Fahrenheit) at midnight. </p>
<p>The TESS mission was initially funded for two years. But the spacecraft is in excellent shape and <a href="https://tess.mit.edu/news/nasa-extends-the-tess-mission/">NASA recently extended</a> the mission through 2022, doubling the time TESS will have to scan nearby, bright stars for transits.</p>
<p>However, finding exoplanets around the coolest stars — those with temperatures less than about 2700 C (4900 F) — will still be a challenge due to their extreme faintness. Since ultracool dwarfs provide our best opportunity to find and study exoplanets with sizes and temperatures similar to Earth’s, other focused planet searches are picking up where TESS leaves off.</p>
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<figcaption>
<span class="caption">Illustration of TESS, NASA’s Transiting Exoplanet Survey Satellite.</span>
<span class="attribution"><span class="source">NASA's Goddard Space Flight Center</span></span>
</figcaption>
</figure>
<h2>The worlds TESS can’t find</h2>
<p>In May 2016, a Belgian-led group announced the discovery of a <a href="https://doi.org/10.1038/nature17448">planetary system around the ultracool dwarf they christened TRAPPIST-1</a>. The discovery of the <a href="https://doi.org/10.1038/nature21360">seven transiting Earth-sized exoplanets</a> in the TRAPPIST-1 system was groundbreaking.</p>
<p>It also demonstrated how small telescopes — relative to the powerful behemoths of our age — can still make transformational discoveries. With patience and persistence, the TRAPPIST telescope scanned nearby faint, red dwarf stars from its high-mountain perch in the Atacama desert for small, telltale dips in their brightnesses. Eventually, it spotted transits in the data for the red dwarf TRAPPIST-1, which — although just 41 light-years away — is too faint for TESS’s four 10-cm (4-inch) diameter lenses. Its Earth-sized worlds would have remained undiscovered had the TRAPPIST team’s larger telescope not found them. </p>
<p>Two projects have upped up the game in the search for exo-Earth candidates around nearby red dwarfs. The <a href="https://www.speculoos.uliege.be/cms/c_4259452/fr/portail-speculoos">SPECULOOS team</a> installed four robotic telescopes – also in the Atacama desert – and one in the Northern Hemisphere. Our Exoearth Discovery and Exploration Network – <a href="http://project-eden.space">Project EDEN</a> – uses nine telescopes in Arizona, Italy, Spain and Taiwan to observe red dwarf stars continuously.</p>
<p>The SPECULOOS and EDEN telescopes are much larger than TESS’s small lenses and can find planets around stars too faint for TESS to study, including some of the transiting Earth-sized planets closest to us.</p>
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<a href="https://images.theconversation.com/files/299055/original/file-20191028-113991-u2d4bl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/299055/original/file-20191028-113991-u2d4bl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/299055/original/file-20191028-113991-u2d4bl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=300&fit=crop&dpr=1 600w, https://images.theconversation.com/files/299055/original/file-20191028-113991-u2d4bl.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=300&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/299055/original/file-20191028-113991-u2d4bl.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=300&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/299055/original/file-20191028-113991-u2d4bl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=377&fit=crop&dpr=1 754w, https://images.theconversation.com/files/299055/original/file-20191028-113991-u2d4bl.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=377&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/299055/original/file-20191028-113991-u2d4bl.jpg?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"></a>
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<span class="caption">This artist’s concept shows what the TRAPPIST-1 planetary system may look like, based on available data about the planets’ diameters, masses and distances from the host star, as of February 2018.</span>
<span class="attribution"><a class="source" href="https://exoplanets.nasa.gov/news/1481/new-clues-to-compositions-of-trappist-1-planets/">NASA/JPL-Caltech</a></span>
</figcaption>
</figure>
<h2>The decade of new worlds</h2>
<p>The next decade is likely to be remembered as the time when we opened our eyes to the incredible diversity of other worlds. TESS is likely to find between <a href="https://ui.adsabs.harvard.edu/abs/2018ApJS..239....2B">10,000 and 15,000 exoplanet candidates</a> by 2025. By 2030, the European Space Agency’s <a href="https://sci.esa.int/web/gaia">GAIA</a> and <a href="https://sci.esa.int/web/plato">PLATO</a> missions are expected to find <a href="https://sci.esa.int/web/plato/-/59252-plato-definition-study-report-red-book">another 20,000-35,000 planets</a>. GAIA will look for stellar wobbles introduced by planets, while PLATO will search for planetary transits as TESS does.</p>
<p>However, even among the thousands of planets that will soon be found, the exoplanets closest to our solar system will remain special. Many of these worlds can be studied in great detail – including the search for signs of life. Discoveries of the nearest worlds also represent major steps in humanity’s progress in exploring the universe we live in. After mapping our own planet and then the solar system, we now turn to nearby planetary systems. Perhaps one day Proxima b or another nearby world astronomers have yet to find will be the target for interstellar probes, like <a href="https://breakthroughinitiatives.org/initiative/3">Project Starshot</a>, or even crewed starships. But first we’ve got to put these worlds on the map.</p>
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<figcaption><span class="caption">Timeline of discoveries of exoplanetary systems within 50 lightyears of the Sun. Credit: Project EDEN/ Daniel Apai and Benjamin Rackham.</span></figcaption>
</figure><img src="https://counter.theconversation.com/content/122104/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Daniel Apai receives research funding from NASA, NSF and the Gordon & Betty Moore Foundation. He is an Associate Professor of Astronomy and Planetary Sciences at The University of Arizona's Steward Observatory and Lunar and Planetary Laboratory. He pursues research in the field of extrasolar planets and related technology development.</span></em></p><p class="fine-print"><em><span>Ben Rackham receives funding from the Heising-Simons Foundation, NASA, and the NSF. </span></em></p>Beyond the outer edge of the Solar System, mysterious, unknown worlds await by the thousands. Astronomers can now finally find them and explore them - but will we find another Earth?Daniel Apai, Associate Professor of Astronomy and Planetary Sciences, University of ArizonaBenjamin Rackham, Postdoctoral Fellow, Massachusetts Institute of Technology (MIT)Licensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/880442017-12-01T14:37:28Z2017-12-01T14:37:28ZMetal asteroid Psyche is all set for an early visit from NASA<figure><img src="https://images.theconversation.com/files/197280/original/file-20171201-10169-lmrzbl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption"></span> <span class="attribution"><span class="source">NASA/JPL</span></span></figcaption></figure><p>Three times further away from the sun than the Earth lies an enormous lump of metal. Around 252km in diameter, the metallic “M-class” asteroid 16 Psyche is the target of NASA’s next mission to the belt of giant rocks that encircles the inner solar system. And the space agency now plans to visit it <a href="https://www.jpl.nasa.gov/news/news.php?feature=6854">much sooner</a> than originally planned.</p>
<p>Not only has the launch has been brought forward one year to the summer of 2022, but NASA’s scientists have also found a way to get to Psyche (pronounced SYKe-ee) much faster by taking a more efficient trajectory. The new route means the Psyche spacecraft won’t have to swing around the Earth to build up speed and won’t pass as close to the sun, so it needs less heat protection. It is now due to arrive in 2026, four years earlier than the original timeline.</p>
<p>The main aim of the journey to Psyshe is to gather more information about our own solar system. Psyche is one of many wandering members of the asteroid belt. Unlike the rest of its rocky neighbours, Psyche appears to be entirely made of nickel and iron, just like the Earth’s core. This, together with its size, has led to the theory that it might be the remains of the inside of a planet.</p>
<p>Asteroids are made up of <a href="https://www.astrobio.net/meteoritescomets-and-asteroids/primitive-particles/">primitive materials</a>, leftovers from the dust cloud from which our solar system originated. Different types of asteroids resemble the various steps it took to form planets from this dust cloud. This means they reveal a lot about the <a href="https://academic.oup.com/astrogeo/article/41/1/1.12/182262">origin and evolution of our solar system</a>. <a href="http://www.abc.net.au/news/science/2017-03-06/16-psyche-asteroid-like-no-other-metal-world-nasa-mission/8316054">Scientists think</a> Psyche could be what’s left of an exposed metal core of a planet very similar to Earth.</p>
<p>We actually derive much of our knowledge about asteroids and the evolution of planets from the <a href="https://www.meteorite.com/study-of-meteorites/">study of meteorites</a>. Many asteroids and comets are primitive <a href="https://socratic.org/questions/what-are-protoplanetary-bodies-and-what-do-they-do">protoplanetary bodies</a> accumulated from the same dust cloud our solar system originates from. As these protoplanetary bodies collide, gravity pulls them together into ever-larger bodies. Eventually these bodies become big and hot enough to partially melt, allowing heavy materials such as iron to sink to the core – and lighter material such as silicon to rise to the surface. </p>
<p>This process, known as <a href="https://www.windows2universe.org/glossary/differentiation.html">differentiation</a>, explains why Earth and other planets such as Mercury, Venus or Mars have an iron core and silicon-rich mantle and crust. The 16 Psyche asteroid is thought to be the leftover iron core of a planet stripped of its mantle in a giant collision.</p>
<p>But many questions regarding the formation of Psyche remain. How do you strip a planet of its mantle only leaving the core? Perhaps there is an alternative formation mechanism of an iron-rich body that does not involve differentiation? Psyche may once have been molten and, if so, did it cool from the inside out or from its surface to the core?</p>
<p>Also, <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/MagEarth.html">Earth’s magnetic field</a> comes from a liquid outer core circling around a solid inner core. Did these processes occur on Psyche and create a magnetic field? What elements other than iron accumulate in a core? And how does the surface geology of an iron body look compared to a rocky or icy body?</p>
<h2>Avoiding collisions</h2>
<p>There are other reasons for visiting asteroids. For one thing, possible collisions with Earth can have devastating effects. The impact of an 15km-wide <a href="http://large.stanford.edu/courses/2015/ph240/xu2/">asteroid approximately</a> 65m years ago is linked to the <a href="https://news.nationalgeographic.com/2016/06/what-happened-day-dinosaurs-died-chicxulub-drilling-asteroid-science/">extinction of the dinosaurs</a>. And the explosion of the 30m-diameter <a href="https://www.space.com/33623-chelyabinsk-meteor-wake-up-call-for-earth.html">Chelyabinsk asteroid</a> over Russia in 2013 led to injuries and damage on the ground. We need to know as much as possible about the composition and physical make-up of asteroids to devise the best ways to defend our planet.</p>
<p>Asteroids also provide resources. Those containing water or other valuable materials may act as stepping stones for human exploration of the solar system. And asteroids crossing Earth’s orbit may become convenient targets for mining operations, providing materials that are running out on Earth and potentially taking environmentally detrimental extraction methods off Earth. Companies including <a href="https://www.planetaryresources.com/">Planetary Resources</a> and countries like <a href="http://www.spaceresources.public.lu/en.html">Luxembourg</a> have already started to pursue these ideas in earnest.</p>
<p>The Psyche spacecraft will carry four instruments to gather as much information about the asteroid as it can: a camera, a <a href="https://mars.nasa.gov/odyssey/mission/instruments/grs/">gamma-ray spectrometer</a> to record what chemical elements are there, a magnetometer, and a radio gravity experiment. The data these devices collect should help us work out if Psyche is the frozen core of a former planet or simply a lump of unmelted metal. If it is a core, then it might help us determine exactly what’s at the centre of our own planet.</p>
<p>Lindy Elkins-Tanton, the lead scientist of the mission, probably summarised it best: <a href="https://www.nasa.gov/press-release/nasa-selects-two-missions-to-explore-the-early-solar-system">“We learn about inner space by visiting outer space”</a>.</p><img src="https://counter.theconversation.com/content/88044/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Christian Schroeder receives funding from the UK Space Agency. </span></em></p>A new trajectory means the mission to uncover core facts about the asteroid belt will happen sooner than planned.Christian Schroeder, Senior Lecturer in Environmental Science and Planetary Exploration, University of StirlingLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/769182017-05-09T14:16:34Z2017-05-09T14:16:34ZCurious Kids: What plants could grow in the Goldilocks zone of space?<figure><img src="https://images.theconversation.com/files/167267/original/file-20170430-13003-h72pjp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Almost every star has planets – so there are more planets in our galaxy than there are stars. </span> <span class="attribution"><span class="source">NASA Ames/JPL-Caltech/T. Pyle</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p><em>This is an article from <a href="https://theconversation.com/au/topics/curious-kids-36782">Curious Kids</a>, a new series for children. The Conversation is asking kids to send in questions they’d like an expert to answer. All questions are welcome – serious, weird or wacky!</em> </p>
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<p>What plants could grow in the Goldilocks zone of space? <strong>– Jesse, 9, Miranda.</strong></p>
</blockquote>
<p>Imagine a planet like the Earth, orbiting a distant star. Could that planet have life? Well, life on Earth (the only life we know) needs liquid water. So to find life on another planet, we think that that planet would have to be “just right”.</p>
<p>If the planet is too close to its star, it will be too hot, and any oceans would boil. Too distant, and any oceans would freeze. Somewhere in between lies the “Goldilocks Zone” - not too hot, not too cold, but just right. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/167297/original/file-20170430-13007-1m0bzvf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/167297/original/file-20170430-13007-1m0bzvf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/167297/original/file-20170430-13007-1m0bzvf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=337&fit=crop&dpr=1 600w, https://images.theconversation.com/files/167297/original/file-20170430-13007-1m0bzvf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=337&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/167297/original/file-20170430-13007-1m0bzvf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=337&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/167297/original/file-20170430-13007-1m0bzvf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/167297/original/file-20170430-13007-1m0bzvf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/167297/original/file-20170430-13007-1m0bzvf.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 ‘Goldilocks Zone’ for the Solar system and the TRAPPIST-1 system. Too close to the star, you’d be too hot (red). Too distant, and you’d be too cold (blue). In between, things might be just right for liquid water…</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech</span></span>
</figcaption>
</figure>
<p>We still haven’t discovered life on any other planet, so we can’t say for sure what such life would be like. One thing is certain, though - alien life will be very, very different to anything on Earth. </p>
<p>We can try to imagine plants that could live on other planets based on the facts we <em>do</em> have. Using your imagination like this is a very important part of being a scientist, trying to explore the vast ocean of the unknown.</p>
<h2>So many planets</h2>
<p>One thing we’ve learned over the last 20 years is that planets are everywhere. Almost every star has planets – so there are more planets in our galaxy than there are stars. </p>
<p>“Goldilocks planets” could be bigger or smaller than the Earth. Smaller ones have weaker gravity, so you would weigh far less there than on the Earth. Plants (and animals!) growing there could easily be much taller than on Earth, since it would be easier for them to grow up! </p>
<p>On a bigger planet, more massive than Earth, plants would probably be much shorter – thanks to the stronger gravity on such a world. We can work out how strong gravity would be on different planets. This <a href="https://www.exploratorium.edu/ronh/weight/">cool website</a> lets you work out how much you would weigh on other objects in the Solar system, for example.</p>
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<a href="https://images.theconversation.com/files/167295/original/file-20170430-13003-j98tjr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/167295/original/file-20170430-13003-j98tjr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/167295/original/file-20170430-13003-j98tjr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=337&fit=crop&dpr=1 600w, https://images.theconversation.com/files/167295/original/file-20170430-13003-j98tjr.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=337&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/167295/original/file-20170430-13003-j98tjr.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=337&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/167295/original/file-20170430-13003-j98tjr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/167295/original/file-20170430-13003-j98tjr.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/167295/original/file-20170430-13003-j98tjr.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">On the left, the Earth. To the right, Earth’s bigger cousin. On a more massive planet, gravity would be stronger - which would have a big effect on any life that evolved!</span>
<span class="attribution"><span class="source">NASA/JPL</span></span>
</figcaption>
</figure>
<h2>Water, wind and light</h2>
<p>Also, not all planets are equally wet. Some are likely dry, desert worlds, while other “Earth-like” worlds might <a href="https://theconversation.com/giant-impacts-planet-formation-and-the-search-for-life-elsewhere-33478">have oceans tens, or hundreds of kilometres deep</a>. What kind of plants could grow on those desert or water worlds?</p>
<p>If a “Goldilocks planet” had a thin atmosphere (like Mars), even the strongest winds would push more gently than a soft breeze here on Earth. Any plants probably wouldn’t need to be very strong to protect against bad weather. With a really thick atmosphere, though, winds push harder – and any plants in those conditions would have to evolve to be really tough.</p>
<p>And then we get to light. Plants on Earth have evolved to use the light from the Sun to get their energy, using a chemical called chlorophyll. It absorbs blue and red light, but reflects green light. </p>
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<a href="https://images.theconversation.com/files/167296/original/file-20170430-12970-q19l5i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/167296/original/file-20170430-12970-q19l5i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/167296/original/file-20170430-12970-q19l5i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=337&fit=crop&dpr=1 600w, https://images.theconversation.com/files/167296/original/file-20170430-12970-q19l5i.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=337&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/167296/original/file-20170430-12970-q19l5i.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=337&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/167296/original/file-20170430-12970-q19l5i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/167296/original/file-20170430-12970-q19l5i.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/167296/original/file-20170430-12970-q19l5i.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">Sunset on a planet around a red dwarf star. What kind of life would thrive under a red Sun?</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech</span></span>
</figcaption>
</figure>
<p>That chemical is really useful for Earth plants, because the Sun gives off lots of energy in the blue and the red. But imagine a dull, cool, red star. That star would be red because it doesn’t give off much yellow or blue light – and so plants using chlorophyll would starve! </p>
<p>But there are probably lots of other chemicals that plants on those worlds could use to live under their own suns. There may even be life on planets out there with more than one sun - with each star a different colour in the daylight sky!</p>
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<a href="https://images.theconversation.com/files/167294/original/file-20170430-12979-19lcj4m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/167294/original/file-20170430-12979-19lcj4m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/167294/original/file-20170430-12979-19lcj4m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=375&fit=crop&dpr=1 600w, https://images.theconversation.com/files/167294/original/file-20170430-12979-19lcj4m.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=375&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/167294/original/file-20170430-12979-19lcj4m.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=375&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/167294/original/file-20170430-12979-19lcj4m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=471&fit=crop&dpr=1 754w, https://images.theconversation.com/files/167294/original/file-20170430-12979-19lcj4m.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=471&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/167294/original/file-20170430-12979-19lcj4m.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=471&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A world with two suns, orbiting a giant planet (like Jupiter). What kind of life could thrive on such a world?</span>
<span class="attribution"><span class="source">IAU/L. Calçada</span></span>
</figcaption>
</figure>
<p>Put all that together and you have lots of fuel for your imagination! Plants that are different colours to those on Earth, using different coloured starlight. Tall, wispy plants, living on worlds with low gravity and thin atmospheres. Squat, low, strong plants on worlds that are massive, or have thick atmospheres. </p>
<p>Plants on other planets are bound to be even weirder than the strangest ones we find on Earth – and probably stranger than we can even imagine!</p>
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<span class="attribution"><a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
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<p class="fine-print"><em><span>Jonti Horner does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Plants on other planets are bound to be even weirder than the strangest ones we find on Earth – if they even exist.Jonti Horner, Vice Chancellor's Senior Research Fellow, University of Southern QueenslandLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/733522017-02-22T19:18:43Z2017-02-22T19:18:43ZSeven Earth-sized planets discovered orbiting a nearby star<figure><img src="https://images.theconversation.com/files/157806/original/image-20170222-20297-13i0lwk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">An artist's concept of what it could look like on the surface of one of the exoplanets of TRAPPIST-1. </span> <span class="attribution"><span class="source">NASA/JPL-Caltech</span></span></figcaption></figure><p>An international team of astronomers has found that a nearby star is accompanied by a swarm of at least seven small, rocky worlds. </p>
<p>One of the eyecatching claims in the work, <a href="http://nature.com/articles/doi:10.1038/nature21360">published today in Nature</a>, is that in the appropriate circumstances, there is a chance that any (or all) the planets could host the right conditions for liquid water to exist on their surface.</p>
<p>The orbits of several of the planets also appear locked in a delicate dance that could explain how such planetary systems form.</p>
<p>The host of this newly uncovered planetary system is a dim little object called <a href="http://www.trappist.one/">TRAPPIST-1</a>, which lies almost 40 light years from our Sun. </p>
<p>TRAPPIST-1 is so tiny that it only barely counts as a star. It has just 8% of the mass of the Sun and lies <a href="http://www.slate.com/blogs/bad_astronomy/2014/06/11/the_brown_dwarf_limit_astronomers_have_found_the_smallest_star_known.html">right on the boundary between normal stars and brown dwarfs</a>.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/157819/original/image-20170222-20310-vh6bnu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/157819/original/image-20170222-20310-vh6bnu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=420&fit=crop&dpr=1 600w, https://images.theconversation.com/files/157819/original/image-20170222-20310-vh6bnu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=420&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/157819/original/image-20170222-20310-vh6bnu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=420&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/157819/original/image-20170222-20310-vh6bnu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=528&fit=crop&dpr=1 754w, https://images.theconversation.com/files/157819/original/image-20170222-20310-vh6bnu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=528&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/157819/original/image-20170222-20310-vh6bnu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=528&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">TRAPPIST-1 is a tiny ultracool red dwarf, far smaller than the Sun.</span>
<span class="attribution"><a class="source" href="http://www.eso.org/public/images/eso1615e/">ESO</a></span>
</figcaption>
</figure>
<p>Were this tiny star slightly less massive, it would be too lightweight to fire nuclear fusion in its core, and it would be a <a href="http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/brown_dwarfs.html">failed star</a>. </p>
<p>If TRAPPIST-1 was dropped into our Solar System in the place of the Sun, it would shine in our sky only marginally more brightly than the full Moon. It would be deep red in colour, with a surface temperature of just over 2,200°C.</p>
<p>In this hypothetical scenario, the Solar System would be a lifeless place. Earth’s oceans would freeze, and then our atmosphere would do the same.</p>
<p>To be warm enough to potentially host liquid water, any planets around this dimly glowing ember would have to <a href="https://theconversation.com/for-life-to-form-on-a-planet-it-needs-to-orbit-the-right-kind-of-star-33477">huddle close and tight</a>. </p>
<h2>Three little worlds</h2>
<p>In May last year, a team of astronomers using the TRansiting Planets and PlanetesImals Small Telescope (<a href="http://www.trappist.ulg.ac.be/cms/c_3300885/en/trappist-portail">TRAPPIST</a>) announced their <a href="http://www.nature.com/nature/journal/v533/n7602/full/nature17448.html">first exoplanet discovery</a>. </p>
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<a href="https://images.theconversation.com/files/157810/original/image-20170222-20302-1f8vw6d.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/157810/original/image-20170222-20302-1f8vw6d.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/157810/original/image-20170222-20302-1f8vw6d.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=420&fit=crop&dpr=1 600w, https://images.theconversation.com/files/157810/original/image-20170222-20302-1f8vw6d.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=420&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/157810/original/image-20170222-20302-1f8vw6d.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=420&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/157810/original/image-20170222-20302-1f8vw6d.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=528&fit=crop&dpr=1 754w, https://images.theconversation.com/files/157810/original/image-20170222-20302-1f8vw6d.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=528&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/157810/original/image-20170222-20302-1f8vw6d.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=528&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">An artist’s impression of the three planets discovered orbiting TRAPPIST-1 in May 2016.</span>
<span class="attribution"><span class="source">ESO/M Kornmesser/N Risinger (skysurvey.org)</span></span>
</figcaption>
</figure>
<p>That team had found striking evidence that a dim, ultracool dwarf star was host to <a href="https://theconversation.com/ultracool-dwarf-star-hosts-three-potentially-habitable-earth-sized-planets-just-40-light-years-away-58695">at least three small rocky planets</a>. Over a period of a 62 days, the star was observed to wink periodically, dimming slightly, then returning to its normal brightness. </p>
<p>These winks were the <a href="https://theconversation.com/explainer-how-to-find-an-exoplanet-part-1-56682">telltale sign of planetary companions</a>. </p>
<p>As this was the telescope’s first discovery, the star was named TRAPPIST-1, and the planets TRAPPIST-1b, c and d.</p>
<p>The inner two planets were found to orbit the star every 1.5 and 2.4 days, respectively. They were small, only very slightly larger than Earth. </p>
<p>Because they huddled in so close to the star (within 2 million km) they would be warm. Most likely, they would be too warm to host liquid water on their surface. But if conditions were just right, then there might, just might, be an outside chance that they could be both warm and wet.</p>
<h2>The seven new ‘Earths’</h2>
<p>With today’s announcement, the TRAPPIST-1 system has suddenly became much more interesting. The same team has invested a huge amount of time following up on their discovery, using telescopes across the planet. </p>
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<p>Those observations have borne great fruit, with the capture of a large sample of transits. The result is the confirmation of the three previously known planets and the discovery of four more. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/157805/original/image-20170222-20326-145bgyw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/157805/original/image-20170222-20326-145bgyw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/157805/original/image-20170222-20326-145bgyw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=300&fit=crop&dpr=1 600w, https://images.theconversation.com/files/157805/original/image-20170222-20326-145bgyw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=300&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/157805/original/image-20170222-20326-145bgyw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=300&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/157805/original/image-20170222-20326-145bgyw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=377&fit=crop&dpr=1 754w, https://images.theconversation.com/files/157805/original/image-20170222-20326-145bgyw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=377&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/157805/original/image-20170222-20326-145bgyw.jpg?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"></a>
<figcaption>
<span class="caption">An artist’s conception shows what the TRAPPIST-1 planetary system may look like.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech</span></span>
</figcaption>
</figure>
<iframe src="https://datawrapper.dwcdn.net/lk2KO/1/" frameborder="0" allowtransparency="true" allowfullscreen="allowfullscreen" webkitallowfullscreen="webkitallowfullscreen" mozallowfullscreen="mozallowfullscreen" oallowfullscreen="oallowfullscreen" msallowfullscreen="msallowfullscreen" width="100%" height="450"></iframe>
<p>The TRAPPIST-1 planetary system is amazingly compact, with all the planets orbiting within 10 million km of their host star. To put that into perspective, Earth orbits almost 150 million km from the Sun.</p>
<p>So the scale of the TRAPPIST-1 system is dramatically different to that of our Solar system. But a better analogue lies close at hand – the satellites of <a href="http://solarsystem.nasa.gov/planets/jupiter">Jupiter</a>.</p>
<h2>A Galilean solar system?</h2>
<p>The scale of the TRAPPIST-1 system is strikingly similar to that of the Jovian system. Jupiter’s four most massive moons – Io, Europa, Ganymede and Callisto – all cluster within 2 million km of the giant planet. Known as the <a href="http://lasp.colorado.edu/education/outerplanets/moons_galilean.php">Galilean satellites</a>, their orbital periods span the same relative range as TRAPPIST-1’s planets.</p>
<p>The inner three Galilean satellites are trapped in what is known as a Laplace resonance, with Io completing four laps of Jupiter in the time it takes Europa to complete two, and Ganymede to complete one.</p>
<p>Their tightly packed nature and orbital resonance gives an important clue as to their formation. The idea is that those moons formed further from Jupiter than their current orbits, and migrated inwards, becoming trapped in the resonance as they went. </p>
<p>Which brings us back to TRAPPIST-1’s planetary system. Where three of Jupiter’s moons lie trapped in mutual resonance, the TRAPPIST-1 system takes things to a whole new level. The six inner planets all lie in near-resonance, with periods in the ratio 24:15:9:6:4:3. </p>
<p>This is the first time such a long near-resonant chain of orbits has ever been found. It remains possible that, when more data are available for the outermost planet, it will add to the chain.</p>
<p>This striking similarity to the Galilean satellites suggests a similar formation process. It hints that the planets orbiting TRAPPIST-1 may have begun their formation further from the star, before migrating inwards to their current orbits. </p>
<h2>Could there be water?</h2>
<p>Based on their distances from TRAPPIST-1, the authors make a first rough attempt to calculate whether the planets could host liquid water. </p>
<p>The innermost should be too warm (unless it is highly reflective, absorbing little of the light that falls upon it). The outermost should be too cold (unless it has a thick, insulating atmosphere). </p>
<p>For the others, things seem a little more promising. In terms of the amount of energy they receive from their star, planets c, d and f seem strikingly similar to Venus, Earth and Mars.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/157807/original/image-20170222-20343-sxdwzg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/157807/original/image-20170222-20343-sxdwzg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/157807/original/image-20170222-20343-sxdwzg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/157807/original/image-20170222-20343-sxdwzg.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/157807/original/image-20170222-20343-sxdwzg.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/157807/original/image-20170222-20343-sxdwzg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/157807/original/image-20170222-20343-sxdwzg.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/157807/original/image-20170222-20343-sxdwzg.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 seven planets of TRAPPIST-1 with their orbital periods, distances from their star, radii and masses as compared to those of Earth. The bottom row shows data about Mercury, Venus, Earth and Mars.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech</span></span>
</figcaption>
</figure>
<p>In the Solar system, it seems likely that Venus <a href="https://climate.nasa.gov/news/2475/nasa-climate-modeling-suggests-venus-may-have-been-habitable/">was once warm and wet</a>, before a runaway greenhouse led to the ocean’s boiling and the planet becoming inhospitable. </p>
<p>Mars was once <a href="https://theconversation.com/the-lost-ocean-of-mars-38739">warm and wet</a> before its atmosphere bled to space and was chemically drawn down into the surface, causing the planet to become frigid and barren.</p>
<p>The idea that TRAPPIST-1’s planets might have migrated from further out in the system could add weight to the possibility of their having water. </p>
<p>Farther from TRAPPIST-1, temperatures are colder, which would mean that in the <a href="https://theconversation.com/giant-impacts-planet-formation-and-the-search-for-life-elsewhere-33478">nebula from which the planets formed</a>, there should have been plenty of water ice for the planets to acquire as they accreted, giving the possibility that these are wet worlds.</p>
<p>But there’s a great difference between “maybe” and “yes”. As it stands, we have no evidence that these planets are wet. They could just as easily be <a href="http://geosci.uchicago.edu/%7Ekite/doc/Catling2009.pdf">barren, airless worlds, stripped of their atmospheres</a> by TRAPPIST-1’s youthful exuberance. </p>
<p>Or, given their resonant nature, they could be volcanic hellholes, just like Jupiter’s Io, with <a href="http://www.geology.sdsu.edu/how_volcanoes_work/io.html">continual and fiery volcanic activity</a>, <a href="https://www.nasa.gov/topics/solarsystem/features/io-volcanoes-displaced.html">driven by tidal interactions</a> with one another. We simply don’t know. But that could change in the coming years with new and better observations.</p>
<p>Until then, we can only speculate on what the planets around this ultracool, dim ember might be like.</p><img src="https://counter.theconversation.com/content/73352/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jonti Horner does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Several of the newly-discovered exoplanets orbiting a small star appear to be locked in an intricate dance that hints at how such planetary systems can form.Jonti Horner, Vice Chancellor's Senior Research Fellow, University of Southern QueenslandLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/620342016-08-15T20:07:07Z2016-08-15T20:07:07ZIt’s all in the atmosphere: exploring planets orbiting distant stars<figure><img src="https://images.theconversation.com/files/133500/original/image-20160809-20932-13mrsdm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The ruddy hue of our moon in a total lunar eclipse.</span> <span class="attribution"><span class="source">Shutterstock/Chris Collins </span></span></figcaption></figure><p><em>The second of a two part series that looks at what astronomers can find out about the planets that are discovered orbiting other stars in our galaxy.</em></p>
<p>When the Moon passes through the Earth’s shadow during a total lunar eclipse, it isn’t plunged into darkness, but instead takes on a ruddy hue. This is caused by the sunlight passing through our planet’s atmosphere, and being refracted into Earth’s shadow. </p>
<p>When a planet transits its host star, the main bulk of the planet obscures the star’s light, <a href="https://theconversation.com/explainer-how-to-find-an-exoplanet-part-1-56682">causing the wink used to detect it</a>. As with the total lunar eclipse, a small proportion of the star’s light will pass through the planet’s outer atmosphere, en-route to Earth.</p>
<p>As a result, we can use that transmitted light to reveal the nature of the planet’s outer atmosphere.</p>
<p>As the star’s light passes through the exoplanet’s atmosphere, the molecules that make up that atmosphere absorb some of the light. Each molecule absorbs only at certain specific frequencies, giving a unique spectral fingerprint.</p>
<p>But the contribution of the planet’s atmosphere is tiny, almost imperceptible. To reveal it, astronomers obtain spectra of the star before, during, and after the transit. </p>
<p>They can then subtract the out-of-transit spectrum from the in-transit spectrum, removing the star’s contribution, and leaving just that of the planet itself. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/127512/original/image-20160621-13008-h2ev5f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/127512/original/image-20160621-13008-h2ev5f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/127512/original/image-20160621-13008-h2ev5f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=480&fit=crop&dpr=1 600w, https://images.theconversation.com/files/127512/original/image-20160621-13008-h2ev5f.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=480&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/127512/original/image-20160621-13008-h2ev5f.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=480&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/127512/original/image-20160621-13008-h2ev5f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=603&fit=crop&dpr=1 754w, https://images.theconversation.com/files/127512/original/image-20160621-13008-h2ev5f.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=603&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/127512/original/image-20160621-13008-h2ev5f.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"></a>
<figcaption>
<span class="caption">(Star + Planet) - (Star) = Planet.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech/R. Hurt (SSC/Caltech)</span></span>
</figcaption>
</figure>
<p>This technique is called <a href="http://nexsci.caltech.edu/workshop/2012/talks/HeatherKnutson_124.pdf">transmission spectroscopy</a>, and has already proved hugely successful. It has allowed astronomers to detect a wide variety of chemical species in the atmospheres of hot Jupiters (including water and carbon dioxide). </p>
<p>The spectra also carry hints as to the structure of the planetary atmosphere, allowing astronomers to find evidence of <a href="http://www.exoclimes.com/news/recent-results/the-diversity-of-hot-jupiter-atmospheres/">haze, cloud layers</a>, and even temperature inversions on these distant worlds.</p>
<h2>Studying planets by the light</h2>
<p>As well as studying the atmospheres of planets by transmitted light, it is also possible to study the light the planets reflect back from their host stars. And some planets are even hot enough that we can detect the light they emit themselves. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/127476/original/image-20160621-13022-11guo9a.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/127476/original/image-20160621-13022-11guo9a.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/127476/original/image-20160621-13022-11guo9a.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/127476/original/image-20160621-13022-11guo9a.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/127476/original/image-20160621-13022-11guo9a.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/127476/original/image-20160621-13022-11guo9a.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/127476/original/image-20160621-13022-11guo9a.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/127476/original/image-20160621-13022-11guo9a.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">An artist’s impression of a hot Jupiter, Upsilon Andromedae b, about to pass behind the disk of its host star.</span>
<span class="attribution"><span class="source">NASA/JPL-Caltech/R. Hurt (SSC)</span></span>
</figcaption>
</figure>
<p>To observe these thermal emissions, and starlight reflected from a planet’s atmosphere, astronomers employ a technique called emission or occultation spectroscopy.</p>
<p>Here, instead of observing spectral absorption features in the starlight after it has passed through the planet’s atmosphere, astronomers directly measure the infrared light being emitted and reflected by the planet.</p>
<p>To measure the thermal emissions, a star is observed just before its planet moves into its secondary eclipse (when the planet goes behind its host). It is then observed during the secondary eclipse, and finally after the secondary eclipse is complete.</p>
<p>Astronomers then subtract the in-eclipse observations from those taken before and after. </p>
<p>So when we subtract the light observed during the eclipse from that observed just before or after, we are left with just the component reflected, or emitted (if the planet is hot enough) by the planet itself. </p>
<p>As was the case with transmission spectroscopy, the emission spectrum of a planet can reveal its atmospheric composition. Even the colour of an exoplanet tells us something. Observations of the hot Jupiter HD 189733b, for example, revealed the planet was a <a href="http://science.nasa.gov/science-news/science-at-nasa/2013/11jul_cobaltblue/">spectacular cobalt blue</a>. </p>
<p>Just like the Moon and Venus, these planets exhibit phases. Thus, if we extend this technique away from the time just before and after secondary eclipse, we can learn even more about the planet. As it moves around its orbit, a planet will turn from showing its night side to its day side.</p>
<p>So if we can measure the planet’s temperature (its brightness, in the infrared) as it orbits its star, we can map the distribution of day-night temperatures across its atmosphere.</p>
<p>Such temperature maps can reveal whether heat from the day-side is being redistributed efficiently to the night-side of a tidally locked hot Jupiter, <a href="http://science.nasa.gov/science-news/science-at-nasa/2013/24may_hotjupiters/">revealing the planet’s weather</a>. </p>
<p>Some hot Jupiters have been shown to have large temperature contrasts between their day and night sides, suggesting inefficient heat redistribution.</p>
<h2>The search for life</h2>
<p>To date, most of the atmospheres we have studied are those of the easiest targets, the bloated hot Jupiters. But as techniques have improved, and new instruments have come online, astronomers are probing <a href="https://www.spacetelescope.org/news/heic1603/">smaller and smaller worlds</a>. </p>
<p>In doing so, they are preparing the tools we will one day employ to <a href="https://theconversation.com/cloudy-with-a-chance-of-life-how-to-find-alien-life-on-distant-exoplanets-50603">search for evidence of life beyond the Solar system</a>.</p>
<p>In the coming years, it is likely that we will discover the first truly Earth-like planets orbiting other stars, and the search for life upon them will begin in earnest.</p>
<p>The first targets will be small, rocky worlds. They will orbit at just the right distance from their star to potentially host liquid water on their surfaces. By the time we’re ready to search for signs of life upon them, we will have found tens, or hundreds of possible targets. </p>
<p>Those planets will then be whittled down to the most promising few on the basis of a wide variety of factors, from <a href="https://theconversation.com/for-life-to-form-on-a-planet-it-needs-to-orbit-the-right-kind-of-star-33477">the star they orbit</a> and <a href="https://theconversation.com/impacts-extinctions-and-climate-in-the-search-for-life-elsewhere-34791">the system in which they move</a> to <a href="https://theconversation.com/what-makes-one-earth-like-planet-more-habitable-than-another-33479">the precise nature of the planet itself</a>.</p>
<p>Once the best targets have been chosen, the observations will begin. To determine whether the targets are truly habitable will require their atmospheres to be studied in detail. By using the same techniques that currently reveal the nature of hot Jupiters, astronomers will uncover their composition and climate, before going on to search for any evidence of life (biosignatures). </p>
<p>To carry out these observations of such tiny and distant worlds presents an enormous observational challenge for astronomers. At visible wavelengths, an Earth-like planet would be ten billion times fainter than its host star. Even in the infrared, where things are a bit better, the planet would still be ten million times fainter.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/116512/original/image-20160328-17835-1b3iek7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/116512/original/image-20160328-17835-1b3iek7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/116512/original/image-20160328-17835-1b3iek7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=480&fit=crop&dpr=1 600w, https://images.theconversation.com/files/116512/original/image-20160328-17835-1b3iek7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=480&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/116512/original/image-20160328-17835-1b3iek7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=480&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/116512/original/image-20160328-17835-1b3iek7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=603&fit=crop&dpr=1 754w, https://images.theconversation.com/files/116512/original/image-20160328-17835-1b3iek7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=603&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/116512/original/image-20160328-17835-1b3iek7.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"></a>
<figcaption>
<span class="caption">Artist impression of the James Webb Space Telescope.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>The James Webb Space Telescope (<a href="http://www.jwst.nasa.gov/">JWST</a>), scheduled for launch in 2018, should enable astronomers to study the atmospheres of nearby super-Earths (planets a bit more massive than the Earth), another step along the road to truly habitable planets. </p>
<p>But to characterise a true Earth analogue will require a different approach. At the moment, it seems likely that direct imaging will be the only viable option. And as of yet, such observations are beyond us.</p>
<hr>
<p><em>See also, part 1: <a href="https://theconversation.com/its-all-in-the-rotation-exploring-planets-orbiting-distant-stars-59593">It’s all in the rotation: exploring planets orbiting distant stars</a></em></p><img src="https://counter.theconversation.com/content/62034/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 red hue of the moon during a total lunar eclipse gives astronomers at cue on how to find out more about the planets being discovered around other stars.Jonti Horner, Vice Chancellor's Senior Research Fellow, University of Southern QueenslandBrett Addison, Postdoc astrophysicist, Mississippi State UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/586952016-05-02T15:04:20Z2016-05-02T15:04:20Z‘Ultracool’ dwarf star hosts three potentially habitable Earth-sized planets just 40 light-years away<figure><img src="https://images.theconversation.com/files/120774/original/image-20160501-28141-1wqjepx.jpg?ixlib=rb-1.1.0&rect=0%2C269%2C1768%2C1233&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Imagined view from the surface of one of the newly discovered planets, with ultracool dwarf star TRAPPIST-1 in the background.</span> <span class="attribution"><span class="source">ESO/M. Kornmesser</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>The search for Earth-like planets – and life – beyond the solar system has long been the stuff of science fiction and fantasy. But today’s ground and space telescopes, high-precision instruments and advanced analysis techniques have made this search an active area of real scientific research. <a href="http://exoplanets.org/">Hundreds of terrestrial worlds</a> have been found over the past several years, including a handful at the right distance from their host star to have conditions amenable to liquid water on their rocky surfaces. Astronomers focus on planets in these “<a href="https://en.wikipedia.org/wiki/Circumstellar_habitable_zone">habitable zones</a>” in the search for life beyond Earth.</p>
<p><a href="http://dx.doi.org/10.1038/nature17448">Now for the first time</a>, our international team has found Earth-sized planets around a type of star so extreme it’s referred to as an “ultracool dwarf.” This is the first time planets have been found around the lowest-mass stars, and indicates that they may be the ideal hunting grounds for habitable worlds beyond the solar system. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/120877/original/image-20160502-19542-sruze8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/120877/original/image-20160502-19542-sruze8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/120877/original/image-20160502-19542-sruze8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/120877/original/image-20160502-19542-sruze8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/120877/original/image-20160502-19542-sruze8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/120877/original/image-20160502-19542-sruze8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/120877/original/image-20160502-19542-sruze8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/120877/original/image-20160502-19542-sruze8.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">Size comparison of the Sun, an ultracool dwarf star and the planet Jupiter.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Sol_Cha-110913-773444_Jupiter.jpg">Chaos syndrome</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>Shifting the focus in the search</h2>
<p>Astronomers have recently started focusing their search for Earth-like planets away from bright, Sun-like stars to dimmer, cooler, low-mass stars called M dwarfs. These stars, while far more numerous in the Milky Way, are too faint to be seen with the naked eye.</p>
<p>Yet their relatively small diameters – less than one-half the width of the Sun – make it easier to detect Earth-sized planets orbiting them using a common technique called the <a href="https://lcogt.net/spacebook/transit-method/">transit method</a>. A transit occurs when a planet passes between us and its host star, resulting in a very slight apparent dimming of the star as the planet blocks a portion of its light.</p>
<p>The alignment of the planet and star must be just right for a transit to be seen, so the probability of this happening is small, and usually only happens if the planet orbits very close to its star. Fortunately, the habitable zone around a cool M dwarf is also closer in than it is around a hotter Sun-like star, so transiting Earth-like planets in these systems have a greater chance of having the conditions necessary for liquid water on their surfaces.</p>
<p>Unfortunately, the feeble amount of light emitted by M dwarfs restricts the search for planetary transits to those stars closest to the Sun, and requires larger telescopes.</p>
<h2>TRAPPIST-1 and its planets</h2>
<p>It is a technical and scientific feat, then, that our international team of astronomers has found the first Earth-like planets around one of the coolest and smallest M dwarfs near the Sun. These “ultracool dwarf” stars are a mere tenth of the diameter of the Sun and 2,000 times fainter.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/120878/original/image-20160502-19546-1r7tjp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/120878/original/image-20160502-19546-1r7tjp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/120878/original/image-20160502-19546-1r7tjp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=401&fit=crop&dpr=1 600w, https://images.theconversation.com/files/120878/original/image-20160502-19546-1r7tjp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=401&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/120878/original/image-20160502-19546-1r7tjp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=401&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/120878/original/image-20160502-19546-1r7tjp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/120878/original/image-20160502-19546-1r7tjp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/120878/original/image-20160502-19546-1r7tjp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">TRAPPIST telescope, ESO La Silla Observatory in Chile.</span>
<span class="attribution"><a class="source" href="http://www.ati.ulg.ac.be/TRAPPIST/Trappist_main/Gallery/Pages/April_2010_4__TRAPPIST_by_night.html#3">TRAPPIST</a></span>
</figcaption>
</figure>
<p>The planets were found by <a href="https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets#Transit_photometry">the transit method</a>, using a facility called <a href="http://www.ati.ulg.ac.be/TRAPPIST/Trappist_main/Home.html">TRAPPIST</a> (TRAnsiting Planets and PlanetesImals Small Telescope), a 60-cm telescope at La Silla Observatory in Chile, optimized to search for small variations in the dim light emitted by ultracool dwarfs. The trick is to monitor them in near-infrared light, a form of radiation with wavelengths longer than the visible light our eyes can perceive (infrared radiation is often used for television remote controls). Over the past year, my colleagues on the TRAPPIST team have monitored several dozen ultracool dwarfs to search for the faint transit signals characteristic of an Earth-sized planet, a mere one percent dip in the already faint light they emit. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/120774/original/image-20160501-28141-1wqjepx.jpg?ixlib=rb-1.1.0&rect=0%2C269%2C1768%2C1233&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/120774/original/image-20160501-28141-1wqjepx.jpg?ixlib=rb-1.1.0&rect=0%2C269%2C1768%2C1233&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/120774/original/image-20160501-28141-1wqjepx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=848&fit=crop&dpr=1 600w, https://images.theconversation.com/files/120774/original/image-20160501-28141-1wqjepx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=848&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/120774/original/image-20160501-28141-1wqjepx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=848&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/120774/original/image-20160501-28141-1wqjepx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1066&fit=crop&dpr=1 754w, https://images.theconversation.com/files/120774/original/image-20160501-28141-1wqjepx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1066&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/120774/original/image-20160501-28141-1wqjepx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1066&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Imagined view from the surface of one of the newly discovered planets, with ultracool dwarf star TRAPPIST-1 in the background.</span>
<span class="attribution"><span class="source">ESO/M. Kornmesser</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>In September 2015, they found their first signal from a star they’ve dubbed TRAPPIST-1, located just 40 light-years away from us. Over the next several months they found more. In total, the astronomers have inferred the presence of three Earth-sized planets, all on very close orbits around the star, with orbital periods (“years”) ranging from 1.5 days to 73 days.</p>
<p>To have such short orbital periods, the planets must be extremely close to their star, between 1/100th and 1/10th the distance between the Sun and the Earth. This is closer than Mercury is to the Sun, and such a small orbit would scorch a planet in our solar system. However, around TRAPPIST-1 these orbits are in and around the habitable zone.</p>
<p>The inner two planets receive two and four times more light energy from their star than the Earth receives from the Sun, and while highly reflective surfaces might make these worlds cool enough for liquid water, they are probably more like Venuses – hot planets in which the water has evaporated into the atmosphere – than Earths. But the third planet, TRAPPIST-1d, receives between 20 percent and 100 percent of the starlight that Earth does from our Sun (<a href="http://www.geog.ucsb.edu/ideas/Insolation.html">insolation</a>), so it orbits at the right distance to have liquid water on its surface, and is potentially an Earth-like world.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/120884/original/image-20160502-19517-5kty3s.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/120884/original/image-20160502-19517-5kty3s.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/120884/original/image-20160502-19517-5kty3s.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=420&fit=crop&dpr=1 600w, https://images.theconversation.com/files/120884/original/image-20160502-19517-5kty3s.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=420&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/120884/original/image-20160502-19517-5kty3s.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=420&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/120884/original/image-20160502-19517-5kty3s.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=528&fit=crop&dpr=1 754w, https://images.theconversation.com/files/120884/original/image-20160502-19517-5kty3s.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=528&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/120884/original/image-20160502-19517-5kty3s.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=528&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">An imagined view from close to one of the three planets orbiting TRAPPIST-1. These worlds have sizes and temperatures similar to those of Venus and Earth – but that’s not all it takes to support life.</span>
<span class="attribution"><span class="source">ESO/M. Kornmesser</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Filling out the planetary picture</h2>
<p>Being at the right distance to have surface liquid water does not guarantee that an Earth-sized planet is truly Earth-like. </p>
<p>First, the proximity of these planets to their host star means they are likely “tidally locked,” forced to rotate at the same rate as they orbit the star, so that one side of the planet is in perpetual day and one side in perpetual night. (Tidal locking is why we always see the same face of the moon from Earth.) While it has long been held that this configuration would prevent the existence of surface liquid water, recent work suggests that such worlds <a href="http://dx.doi.org/10.1051/0004-6361/201321042">may still have regions of habitability</a>.</p>
<p>The composition and circulation of an atmosphere, if it exists, also plays a major role in habitability, either by reflecting stellar light or trapping heat through the greenhouse effect. </p>
<p>Finally, both tectonic activity and the existence of a protective planetary magnetic field can play roles. Tectonic forces are of particular interest for the innermost planet, TRAPPIST-1b, which may be squeezed and stretched by tidal forces from the host star, heating it from the inside and producing the kind of extensive volcanism we see on Jupiter’s moon Io.</p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/120879/original/image-20160502-19557-k7ily2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/120879/original/image-20160502-19557-k7ily2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/120879/original/image-20160502-19557-k7ily2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=684&fit=crop&dpr=1 600w, https://images.theconversation.com/files/120879/original/image-20160502-19557-k7ily2.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=684&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/120879/original/image-20160502-19557-k7ily2.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=684&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/120879/original/image-20160502-19557-k7ily2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=859&fit=crop&dpr=1 754w, https://images.theconversation.com/files/120879/original/image-20160502-19557-k7ily2.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=859&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/120879/original/image-20160502-19557-k7ily2.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=859&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Technicians continue to work on the Webb telescope’s instrumentation in advance of its launch in 2018.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/nasawebbtelescope/11210557006">NASA/Chris Gunn</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span>
</figcaption>
</figure>
<p>The observations obtained by TRAPPIST cannot tell us anything about these planetary details, but the <a href="http://jwst.nasa.gov/index.html">James Webb Space Telescope</a> should tell us more when it is launched in 2018. This advanced replacement to the Hubble Space Telescope will have the sensitivity to detect the even smaller <a href="http://www.exoclimes.com/topics/transmission-spectroscopy/">signal of absorption</a> by the planets during their transit. Imprinted on this signal will be the chemical absorption patterns of the gases present in the atmosphere, which may include biogenic gases such as oxygen, methane and nitrous oxide, or volcanic gases such as sulfur dioxide. </p>
<p>The TRAPPIST team will soon be starting the next phase of its search for Earth-like worlds around ultracool dwarfs with the <a href="http://www.orca.ulg.ac.be/SPECULOOS/Speculoos_main/Home.html">SPECULOOS</a> (Search for habitable Planets EClipsing ULtra-cOOl Stars) survey. This program will monitor 500 of the nearest ultracool dwarfs using four 1-meter robotic telescopes in Cerro Paranal, Chile. Construction of the site is already underway, and the team is looking forward to expanding our census of nearby habitable worlds around the smallest stars.</p><img src="https://counter.theconversation.com/content/58695/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Adam Burgasser receives funding from the National Aeronautics and Space Administration (NASA) and the National Science Foundation (NSF). </span></em></p>We don’t need to look for Earth-like planets exclusively around Sun-like stars. Tiny, dim TRAPPIST-1 has only 11 percent the diameter of the Sun and is much redder.Adam Burgasser, Professor of Physics, University of California, San DiegoLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/547992016-02-18T10:45:09Z2016-02-18T10:45:09ZEying exomoons in the search for E.T.<figure><img src="https://images.theconversation.com/files/111700/original/image-20160216-30543-q8x12.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">If you're looking for life, you'd do well to look for some moons.</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/mualphachi/10353071926">Maxwell Hamilton</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>When I was young, the only planets we knew about were the ones in our own solar system.</p>
<p>Astronomers presumed that many of the other stars in the night sky had planets too, but this was sheer speculation. We could never know for sure, the thinking went, because such planets were ridiculously small and faint. To ever see or study them seemed a complete impossibility. “Extrasolar planets,” or “exoplanets,” were a <a href="http://starchive.cs.umanitoba.ca/?locations/planets-moons">staple of science fiction</a>, but not of professional astrophysics.</p>
<p>It’s hard to believe that there was once such a simple time. The first <a href="http://www2.astro.psu.edu/users/alex/pulsar_planets.htm">definitive detection of an exoplanet</a> was in 1991, identified by the tiny wobbles experienced by the parent star as its exoplanet swung around it. Since then, the <a href="https://commons.wikimedia.org/wiki/File:Exoplanet_Discovery_Methods_Bar.png">field has exploded</a>. There are now around <a href="http://exoplanets.org/">1,600 confirmed exoplanets</a>, with almost 4,000 other known candidates. There are exoplanets <a href="http://www.nasa.gov/mission_pages/kepler/news/kepler-37b.html">smaller than Mercury</a>, and others <a href="http://news.nationalgeographic.com/news/2009/08/090817-new-planet-orbits-backward.html">many times bigger than Jupiter</a>. Their orbits around their parent stars range from <a href="http://www.space.com/22451-fastest-earth-size-lava-planet-kepler78b.html">a few hours</a> to <a href="http://www.space.com/20231-giant-exoplanets-hr-8799-atmosphere-infographic.html">hundreds of years</a>. And the ones we know about are just a tiny fraction of the <a href="http://www.space.com/19103-milky-way-100-billion-planets.html">more than 100 billion exoplanets</a> we now believe are spread throughout our Milky Way galaxy.</p>
<p>But while the golden age of exoplanets has barely begun, an exciting additional chapter is also taking shape: the hunt for exomoons.</p>
<h2>Beyond Earth-like planets to exomoons</h2>
<p>An exomoon is a moon orbiting a planet, which in turn is orbiting another star. You may not have ever heard of exomoons before now. But if you’re a fan of films such as “<a href="http://www.avatarmovie.com">Avatar</a>,” “<a href="http://www.starwars.com/films/star-wars-episode-vi-return-of-the-jedi">Return of the Jedi</a>” or “<a href="http://www.projectprometheus.com/">Prometheus</a>,” this should be familiar territory: in all three cases, most of the action takes place on an exomoon.</p>
<p>But what about real life? How many exomoons do we know of? At the moment, zero.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/111701/original/image-20160216-19232-10h7to6.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/111701/original/image-20160216-19232-10h7to6.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/111701/original/image-20160216-19232-10h7to6.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/111701/original/image-20160216-19232-10h7to6.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/111701/original/image-20160216-19232-10h7to6.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/111701/original/image-20160216-19232-10h7to6.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/111701/original/image-20160216-19232-10h7to6.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/111701/original/image-20160216-19232-10h7to6.jpeg?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">Endor: not all exomoons come with ewoks.</span>
<span class="attribution"><a class="source" href="http://www.starwars.com/databank/ewok">Star Wars: Episode VI Return of the Jedi</a></span>
</figcaption>
</figure>
<p>But <a href="http://www.space.com/28918-exomoons-alien-life-search.html">the race is on</a> to find the real-life analogs of <a href="http://starwars.wikia.com/wiki/Endor">Endor</a> and <a href="http://james-camerons-avatar.wikia.com/wiki/Pandora">Pandora</a>.</p>
<p>You might think searching for tiny rocks orbiting distant planets around faint stars hundreds or thousands of light years away is the ultimate example of an obscure academic pursuit. But exomoons are poised to become a big deal.</p>
<p>The whole reason exoplanets are exciting is that they’re a path to answering one of the grandest questions of all: “<a href="https://www.ted.com/playlists/82/are_we_alone_in_the_universe">Are we alone?</a>” As we find more and more exoplanets, we eagerly ask <a href="http://www.universetoday.com/120982/what-is-the-habitable-zone/">whether life could exist there</a>, and whether <a href="https://en.wikipedia.org/wiki/Earth_Similarity_Index">this planet is anything like Earth</a>. However, so far we’ve yet to find an exact match to Earth, nor can we yet really know for sure whether any exoplanet, Earth-like or otherwise, hosts life.</p>
<h2>Enter exomoons in the search for life</h2>
<p>There are several reasons why exomoons, these little distant worlds, may be the key to finding life elsewhere in the universe.</p>
<p>First, there’s the stark reality that life on Earth may not have happened at all <a href="http://www.astrobio.net/topic/exploration/moon-to-mars/if-we-had-no-moon/">without the starring role played by our own moon</a>.</p>
<p>The Earth’s axis is tilted by 23.5 degrees relative to its motion around the sun. This tilt <a href="http://www.windows2universe.org/earth/climate/cli_seasons.html">gives us seasons</a>, and because this tilt is relatively small, seasons on Earth are mild: most places never get impossibly hot or unbearably cold. One thing that has been crucial for life is that this tilt has stayed the same for very long periods: for millions of years, the angle of tilt has <a href="http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/Beth_Caissie/obliquity.htm">varied by only a couple of degrees</a>.</p>
<p>What has kept the Earth so steady? The <a href="http://www.space.com/12464-earth-moon-unique-solar-system-universe.html">gravity of our moon</a>.</p>
<p>In contrast, Mars only has <a href="http://mars.nasa.gov/allaboutmars/extreme/moons/">two tiny moons</a>, which have negligible gravity. Without a stabilizing influence, Mars has gradually tumbled back and forth, its <a href="http://www.spacedaily.com/news/mars-water-science-00d.html">tilt ranging between 0 and 60 degrees</a> over millions of years. Extreme changes in climate have resulted. Any Martian life that ever existed would have found the need to continually adapt very challenging.</p>
<p>Without our moon, the Earth, too, would likely have been <a href="http://sciencenordic.com/what-would-we-do-without-moon">subject to chaotic climate conditions</a>, rather than the relative certainty of the seasons that stretches back deep into the fossil record.</p>
<p>The gravity of the <a href="http://www.techtimes.com/articles/80715/20150831/moon-affects-tides.htm">moon also produces the Earth’s tides</a>. Billions of years ago, the ebb and flow of the oceans produced an alternating cycle of high and low salt content on ancient rocky shores. This recurring cycle <a href="https://www.newscientist.com/article/dn4786-no-moon-no-life-on-earth-suggests-theory/">could have enabled the unique chemical processes</a> needed to generate the first DNA-like molecules.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/111704/original/image-20160216-19232-15pbxgu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/111704/original/image-20160216-19232-15pbxgu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/111704/original/image-20160216-19232-15pbxgu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=535&fit=crop&dpr=1 600w, https://images.theconversation.com/files/111704/original/image-20160216-19232-15pbxgu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=535&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/111704/original/image-20160216-19232-15pbxgu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=535&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/111704/original/image-20160216-19232-15pbxgu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=672&fit=crop&dpr=1 754w, https://images.theconversation.com/files/111704/original/image-20160216-19232-15pbxgu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=672&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/111704/original/image-20160216-19232-15pbxgu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=672&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Moons might contribute to a planet’s habitability.</span>
<span class="attribution"><a class="source" href="http://www.nasa.gov/jpl/cassini/pia18322/triple-crescents">NASA/JPL-Caltech/Space Science Institute</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Exomoons might have Earth-like environments</h2>
<p>Overall, as we continue to hunt for another Earth somewhere out there, it seems likely that a twin of Earth, but without a moon accompanying it, would not look familiar. Finding exomoons is a key part of finding somewhere like here.</p>
<p>Meanwhile, we shouldn’t be discouraged by the fact that most exoplanets found so far are <a href="http://www.space.com/18522-super-jupiter-alien-planet-photo.html">bloated gaseous beasts</a>, with hostile environments unlikely to support life as we know it. What we don’t know yet, crucially, is whether these exoplanets have moons. This prospect is exciting, because exomoons are expected to be smaller rocky or icy bodies, <a href="http://www.astrobio.net/news-exclusive/exomoons-abundant-sources-habitability/">possibly hosting oceans and atmospheres</a>.</p>
<p>This is hardly speculation: Titan (a moon of Saturn) has <a href="http://saturn.jpl.nasa.gov/science/index.cfm?SciencePageID=75">a thick atmosphere</a> even denser than Earth’s, while <a href="http://www.earthmagazine.org/article/pair-moons-underground-oceans">underground oceans</a> are thought to exist on <a href="http://www.nasa.gov/press-release/cassini-finds-global-ocean-in-saturns-moon-enceladus/">Enceladus</a> (another moon of Saturn) and on <a href="http://www.bbc.com/earth/story/20150326-europa-may-be-home-to-alien-life">Europa</a> and <a href="http://www.space.com/28807-jupiter-moon-ganymede-salty-ocean.html">Ganymede</a> (both moons of Jupiter). Thus, if there is any other life out there somewhere, it may well not be found on a distant planet, but <a href="http://www.iflscience.com/space/exomoons-might-be-secret-alien-life">on a distant moon</a>.</p>
<p>The <a href="https://www.newscientist.com/article/dn27180-race-to-find-the-first-exomoon-heats-up/">hunt is on</a>. While exomoons are too faint to see directly, astronomers are deploying <a href="http://www.astrobio.net/news-exclusive/new-exomoon-hunting-technique-could-find-solar-system-like-moons/">ingenious indirect techniques</a> in their searches. Those moons are assuredly out there by the billions – and soon we will find them. It won’t be too much longer before these tiny worlds help us answer huge questions.</p><img src="https://counter.theconversation.com/content/54799/count.gif" alt="The Conversation" width="1" height="1" />
<h4 class="border">Disclosure</h4><p class="fine-print"><em><span>Bryan Gaensler receives funding from the Natural Sciences and Engineering Research Council of Canada.</span></em></p>As the list of known planets beyond our solar system grows, the search for their moons is intensifying. One reason: they might hold the key to finding life elsewhere in the universe.Bryan Gaensler, Director, Dunlap Institute for Astronomy and Astrophysics, University of TorontoLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/521162015-12-11T09:25:19Z2015-12-11T09:25:19ZRarity of Jupiter-like planets means planetary systems exactly like ours may be scarce<figure><img src="https://images.theconversation.com/files/105138/original/image-20151209-15588-1cyskbt.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Artist's depiction of the newly discovered Jupiter-like planet orbiting the star HD 32963. </span> <span class="attribution"><span class="source">Stefano Meschiari</span></span></figcaption></figure><p>Is our little corner of the galaxy a special place? As of this date, we’ve <a href="http://exoplanets.org">discovered more than 1,500 exoplanets</a>: planets orbiting stars other than our sun. Thousands more will be added to the list in the coming years as we confirm planetary candidates by alternative, independent methods.</p>
<p>In the hunt for other planets, we’re especially interested in those that might potentially host life. So we focus our modern exoplanet surveys on planets that might be similar to Earth: low-mass, rocky and with just the right temperature to allow for liquid water. But what about the other planets in the solar system? The <a href="https://en.wikipedia.org/wiki/Copernican_principle">Copernican principle</a> – the idea that the Earth and the solar system are not unique or special in the universe – suggests the architecture of our planetary system should be common. But it doesn’t seem to be.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/105306/original/image-20151210-7425-1kd373r.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/105306/original/image-20151210-7425-1kd373r.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/105306/original/image-20151210-7425-1kd373r.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=429&fit=crop&dpr=1 600w, https://images.theconversation.com/files/105306/original/image-20151210-7425-1kd373r.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=429&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/105306/original/image-20151210-7425-1kd373r.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=429&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/105306/original/image-20151210-7425-1kd373r.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=539&fit=crop&dpr=1 754w, https://images.theconversation.com/files/105306/original/image-20151210-7425-1kd373r.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=539&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/105306/original/image-20151210-7425-1kd373r.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=539&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A mass-period diagram. Each dot marks the mass and orbital period of a confirmed exoplanet.</span>
<span class="attribution"><span class="source">Stefano Meschiari</span></span>
</figcaption>
</figure>
<p>The figure above, called a <em>mass-period diagram</em>, provides a visual way to compare the planets of our solar system with those we’ve spotted farther away. It charts the orbital periods (the time it takes for a planet to make one trip around its central star) and the masses of the planets discovered so far, compared with the properties of solar system planets.</p>
<p>Planets like Earth, Jupiter, Saturn and Uranus occupy “empty” parts of the diagram – we haven’t found other planets with similar masses and orbits so far. At face value, this would indicate that the majority of planetary systems do not resemble our own solar system.</p>
<p>The solar system lacks close-in planets (planets with orbital periods between a few and a few tens of days) and super-Earths (a class of planets with masses a few times the mass of the Earth often detected in other planetary systems). On the other hand, it does feature several long-period gaseous planets with very nearly circular orbits (Jupiter, Saturn, Uranus and Neptune). </p>
<p>Part of this difference is due to selection effects: close-in, massive planets are easier to discover than far-out, low-mass planets. In light of this discovery bias, astronomers <a href="http://aasnova.org/2015/09/25/how-normal-is-our-solar-system/">Rebecca Martin and Mario Livio</a> convincingly argue that our solar system is actually <a href="http://dx.doi.org/10.1088/0004-637X/810/2/105">more typical than it seems at first glance</a>.</p>
<p>There is a sticking point, however: Jupiter still stands out. It’s an outlier based both on its orbital location (with a corresponding period of about 12 years) and its very-close-to-circular orbit. Understanding whether Jupiter’s relative uniqueness is a real feature, or another product of selection effects, has real implications for our understanding of exoplanets.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/3afEX8a2jPg?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Jupiter as seen by the Hubble Space Telescope.</span></figcaption>
</figure>
<h2>Throwing its weight around</h2>
<p>According to our understanding of how our solar system formed, Jupiter shaped much of the other planets’ early history. Due to its gravity, it influenced the <a href="http://www.sciencedaily.com/releases/2011/06/110605132437.htm">formation of Mars</a> and Saturn. It potentially facilitated the development of life by shielding Earth from cosmic collisions that would have delayed or extinguished it, and by funneling water-rich bodies towards it. And its gravity <a href="http://doi.org/10.1073/pnas.1423252112">likely swept the inner solar system of solid debris</a>. Thanks to this clearing action, Jupiter might have prevented the formation of super-Earth planets with massive atmospheres, thereby ensuring that the inner solar system is populated with small, rocky planets with thin atmospheres. </p>
<p>Without Jupiter, it looks unlikely that we’d be here. As a consequence, figuring out if Jupiter is a relatively common type of planet might be crucial to understanding whether terrestrial planets with a similar formation environment as Earth are abundant in the galaxy.</p>
<p>Despite their relative heft, it’s a challenge to discover Jupiter analogs – those planets with periods and masses similar to Jupiter’s. Astronomers typically discover them using an indirect detection technique called the <a href="https://en.wikipedia.org/wiki/Doppler_spectroscopy">Doppler radial velocity method</a>. The gravitational pull of the planet causes tiny shifts in the wavelength of features in the spectrum of the star, in a distinctive, periodic pattern. We can detect these shifts by periodically capturing the star’s light with a telescope and turning it into a spectrum with <a href="https://www2.keck.hawaii.edu/inst/hires/">a spectrograph</a>. This periodic signal, based on a planet’s long orbital period, can require monitoring a star over many years, even decades.</p>
<h1>Are Jupiter-like planets rare?</h1>
<p><a href="http://arxiv.org/abs/1512.00417">In a recent paper</a>, Dominick Rowan, a high school senior from New York, and his coauthors (including astronomers from the University of Texas, the University of California at Santa Cruz and me) analyzed the Doppler data for more than 1,100 stars. Each star was observed with the <a href="http://www.keckobservatory.org/">Keck Observatory telescope</a> in Hawaii; many of them had been monitored for a decade or more. To analyze the data, he used the <a href="https://www.r-project.org">open-source statistical environment R</a> together with a freely available application that I developed, called <a href="http://www.stefanom.org/systemic">Systemic</a>. Many universities use an <a href="http://www.stefanom.org/systemic-live">online version</a> to teach how to analyze astronomical data.</p>
<p>Our team studied the available data for each star and calculated the probability that a Jupiter-like planet could have been missed – either because not enough data are available, or because the data are not of high enough quality. To do this, we simulated hundreds of millions of possible scenarios. Each was created with a computer algorithm and represents a set of alternative possible observations. This procedure makes it possible to infer how many Jupiter analogs (both discovered and undiscovered) orbited the sample of 1,100 stars.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/105134/original/image-20151209-15552-1nqxyfv.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/105134/original/image-20151209-15552-1nqxyfv.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/105134/original/image-20151209-15552-1nqxyfv.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/105134/original/image-20151209-15552-1nqxyfv.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/105134/original/image-20151209-15552-1nqxyfv.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/105134/original/image-20151209-15552-1nqxyfv.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/105134/original/image-20151209-15552-1nqxyfv.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/105134/original/image-20151209-15552-1nqxyfv.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Orbit of the newly discovered Jupiter-mass planet orbiting the star HD 32963, compared to the orbits of Earth and Jupiter around the sun.</span>
<span class="attribution"><span class="source">Stefano Meschiari</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>While carrying out this analysis, we discovered a <a href="http://exoplanet.eu/catalog/hd_32963_b/">new Jupiter-like planet</a> orbiting HD 32963, which is a star very similar to the sun in terms of age and physical properties. To make this discovery, we analyzed each star with an automated algorithm that tried to uncover periodic signals potentially associated with the presence of a planet.</p>
<p>We pinpointed the frequency of Jupiter analogs across the survey at approximately 3%. This result is broadly consistent with previous estimates, which were based on a smaller set of stars or a different discovery technique. It greatly strengthens earlier predictions because we took <em>decades</em> of observations into account in the simulations. </p>
<p>This result has several consequences. First, the relative rarity of Jupiter-like planets indicates that true solar system analogs should themselves be rare. By extension, given the important role that Jupiter played at all stages of the formation of the solar system, Earth-like habitable planets with similar formation history to our solar system will be rare.</p>
<p>Finally, it also underscores that Jupiter-like planets do not form as readily around stars as other types of planets do. It could be because not enough solid material is available, or because these gas giants migrate closer to the central stars very efficiently. <a href="http://astrobites.org/2015/08/18/giant-planets-from-far-out-there/">Recent planet-formation simulations</a> tentatively bear out the latter explanation.</p>
<p>Long-running, ongoing surveys will continue to help us understand the architecture of the outer regions of planetary systems. Programs including the Keck planet search and the <a href="http://arxiv.org/abs/1512.02965">McDonald Planet Search</a> have been accumulating data for decades. Discovering ice giants similar to Uranus and Neptune will be even tougher than tracking down these Jupiter analogs. Because of their long orbital periods (84 and 164 years) and the very small Doppler shifts they induce on their central stars (tens of times smaller than a Jupiter-like planet), the detection of Uranus and Neptune analogs lies far in the future.</p><img src="https://counter.theconversation.com/content/52116/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Stefano Meschiari works for the University of Texas at Austin.</span></em></p>Jupiter had a big influence on how our solar system’s planets formed. New research – led by a high school student – tried to nail down how rare Jupiter analogs really are in other planetary systems.Stefano Meschiari, W J McDonald Postdoctoral Fellow, The University of Texas at AustinLicensed as Creative Commons – attribution, no derivatives.