tag:theconversation.com,2011:/ca/topics/habitable-zone-10054/articlesHabitable zone – The Conversation2023-11-30T19:06:13Ztag:theconversation.com,2011:article/2178612023-11-30T19:06:13Z2023-11-30T19:06:13ZMassive planet too big for its own sun pushes astronomers to rethink exoplanet formation<figure><img src="https://images.theconversation.com/files/562186/original/file-20231128-23-oz4tck.jpg?ixlib=rb-1.1.0&rect=0%2C8%2C1997%2C1488&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">LHS 3154b, a newly discovered massive planet that should be too big to exist. </span> <span class="attribution"><span class="source">The Pennsylvania State University</span></span></figcaption></figure><p>Imagine you’re a farmer searching for eggs in the chicken coop – but instead of a chicken egg, you find an ostrich egg, much larger than anything a chicken could lay.</p>
<p>That’s a little how <a href="https://scholar.google.com/citations?user=cSnTlM4AAAAJ&hl=en">our team</a> <a href="https://scholar.google.com/citations?user=lN5yvjMAAAAJ&hl=en">of astronomers</a> <a href="https://science.psu.edu/astrp/people/mmd6393">felt when we</a> <a href="https://doi.org/10.1126/science.abo0233">discovered a massive planet</a>, more than 13 times heavier than Earth, around a cool, dim red star, nine times less massive than Earth’s Sun, in 2023. </p>
<p>The smaller star, called an M star, is not only smaller than the Sun in Earth’s solar system, but it’s 100 times less luminous. Such a star should not have the necessary amount of material in its planet-forming disk to birth such a massive planet.</p>
<h2>The Habitable Zone Planet Finder</h2>
<p>Over the past decade, our team designed and built a new instrument at Penn State capable of detecting the light from these dim, cool stars at wavelengths beyond the sensitivity of the human eye – in the near-infrared – where such cool stars <a href="https://www.e-education.psu.edu/astro801/book/export/html/1755">emit most of their light</a>. </p>
<p>Attached to the 10-meter Hobby-Eberly Telescope in West Texas, our instrument, dubbed the <a href="https://hpf.psu.edu/">Habitable Zone Planet Finder</a>, can measure the subtle change in a star’s velocity as a planet gravitationally tugs on it. This technique, called the Doppler radial velocity technique, is <a href="https://theconversation.com/rarity-of-jupiter-like-planets-means-planetary-systems-exactly-like-ours-may-be-scarce-52116">great for detecting exoplanets</a>. </p>
<p>“<a href="https://exoplanets.nasa.gov/">Exoplanet</a>” is a combination of the words extrasolar and planet, so the term applies to any planet-sized body in orbit around a star that isn’t Earth’s Sun.</p>
<p>Thirty years ago, Doppler radial velocity observations enabled the discovery of <a href="https://exoplanets.nasa.gov/exoplanet-catalog/7001/51-pegasi-b/">51 Pegasi b</a>, the first known exoplanet orbiting a Sunlike star. In the ensuing decades, astronomers like us have improved this technique. These <a href="https://noirlab.edu/public/projects/neid/">increasingly more precise</a> measurements have an important goal: to enable the discovery of rocky planets in <a href="https://exoplanets.nasa.gov/search-for-life/habitable-zone/">habitable zones</a>, the regions around stars where liquid water can be sustained on the planetary surface. </p>
<p>The Doppler technique doesn’t yet have the capabilities to discover habitable zone planets the mass of the Earth around stars the size of the Sun. But the cool and dim M stars show a larger Doppler signature for the same Earth-size planet. The lower mass of the star leads to it getting tugged more by the orbiting planet. And the lower luminosity leads to a <a href="https://exoplanets.nasa.gov/resources/2255/what-is-the-habitable-zone/">closer-in habitable zone</a> and a shorter orbit, which also makes the planet easier to detect. </p>
<p>Planets around these smaller stars were the planets our team designed the Habitable Zone Planet Finder to discover. Our new discovery, <a href="https://doi.org/10.1126/science.abo0233">published in the journal Science</a>, of a massive planet orbiting closely around the cool dim M star LHS 3154 – the ostrich egg in the chicken coop – came as a real surprise.</p>
<h2>LHS 3154b: The planet that should not exist</h2>
<p><a href="https://theconversation.com/astronomers-have-learned-lots-about-the-universe-but-how-do-they-study-astronomical-objects-too-distant-to-visit-214320">Planets form in disks</a> composed of gas and dust. These disks pull together dust grains that grow into pebbles and eventually combine to form a solid planetary core. Once the core is formed, the planet can gravitationally pull in the solid dust, as well as surrounding gas such as hydrogen and helium. But it needs a lot of mass and materials to do this successfully. This way to form planets is called <a href="https://earthhow.com/planet-formation/">core accretion</a>.</p>
<p>A star as low mass as LHS 3154, nine times less massive than the Sun, should have a correspondingly low-mass planet forming disk. </p>
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<figcaption><span class="caption">An artist’s rendering of LHS 3154b. Video Credit: Abby Minnich.</span></figcaption>
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<p>A typical disk around such a low-mass star should simply not have enough solid materials or mass to be able to make a core heavy enough to create such a planet. From computer simulations our team conducted, we concluded that such a planet needs a disk at least 10 times more massive than typically assumed <a href="https://doi.org/10.48550/arXiv.1608.03621">from direct observations of planet-forming disks</a>.</p>
<p>A different planet formation theory, <a href="https://astrobites.org/2011/02/28/planet-formation-at-wide-orbits-through-gravitational-instability/">gravitational instability</a> – where gas and dust in the disk undergo a direct collapse to form a planet – also struggles to explain the formation of such a planet without a very massive disk.</p>
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<h2>Planets around the most common stars</h2>
<p>Cool, dim M stars are <a href="https://www.stsci.edu/contents/newsletters/2020-volume-37-issue-01/how-well-do-we-understand-m-dwarfs">the most common stars in our galaxy</a>. In DC comics lore, <a href="https://theconversation.com/how-astronomers-could-find-the-real-planet-krypton-56646">Superman’s home world</a>, <a href="https://www.theguardian.com/culture/us-news-blog/2012/nov/05/neil-degrasse-tyson-superman-planet">planet Krypton, orbited an M dwarf star</a>. </p>
<p>Astronomers know, from discoveries made with Habitable Zone Planet Finder and other instruments, that giant planets in close-in orbits around the most massive M stars are <a href="https://doi.org/10.48550/arXiv.2303.00659">at least 10 times rarer</a> than those around Sunlike stars. And we know of no such massive planets in close orbits around the least massive M stars – until the discovery of LHS 3154b. </p>
<p>Understanding how planets form around our coolest neighbors will help us understand both how planets form in general and how rocky worlds around the most numerous types of stars form and evolve. This line of research could also help astronomers understand whether M stars are capable of supporting life.</p><img src="https://counter.theconversation.com/content/217861/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Suvrath Mahadevan receives external funding from NSF, NASA, and the Heising-Simons Foundation, as well as research funding and support from Penn State.
</span></em></p><p class="fine-print"><em><span>Guðmundur Kári Stefánsson receives funding from NSF, NASA and the Heising-Simons Foundation.</span></em></p><p class="fine-print"><em><span>Megan Delamer receives funding from NSF, NASA, and Heising-Simons Foundation. </span></em></p>A newly discovered planet that should be too big to have formed around a tiny star is throwing into question what researchers know about planet formation.Suvrath Mahadevan, Verne M. Willaman Professor of Astronomy & Astrophysics, Penn StateGuðmundur Kári Stefánsson, NASA Hubble Fellow, Department of Astrophysical Sciences, Princeton UniversityMegan Delamer, Graduate Student, Department of Astronomy, Penn StateLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1982742023-03-08T13:41:07Z2023-03-08T13:41:07ZDistant star TOI-700 has two potentially habitable planets orbiting it – making it an excellent candidate in the search for life<figure><img src="https://images.theconversation.com/files/512978/original/file-20230301-18-2jg7dp.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C1859%2C1034&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The TOI-700 star system is home to four planets, including two in its habitable zone that could host liquid water.</span> <span class="attribution"><a class="source" href="https://www.jpl.nasa.gov/spaceimages/images/largesize/PIA23408_hires.jpg">NASA's Goddard Space Flight Center</a></span></figcaption></figure><p>NASA recently announced the <a href="https://doi.org/10.3847/2041-8213/acb599">discovery of a new, Earth-sized planet</a> in the habitable zone of a nearby star called TOI-700. <a href="https://sites.google.com/site/josepherodriguezjr/">We are</a> <a href="https://avanderburg.github.io/">two of</a> the astronomers who led the discovery of this planet, called TOI-700 e. TOI-700 e is just over 100 light years from Earth – too far away for humans to visit – but we do know that it is similar in size to the Earth, likely rocky in composition and could potentially support life.</p>
<p>You’ve probably heard about some of the <a href="https://exoplanets.nasa.gov/trappist1/">many</a> <a href="https://www.eso.org/public/news/eso1629/">other</a> <a href="https://www.nasa.gov/press-release/nasa-kepler-mission-discovers-bigger-older-cousin-to-earth">exoplanet</a> <a href="https://www.nasa.gov/image-feature/kepler-1649c-earth-size-habitable-zone-planet-hides-in-plain-sight">discoveries</a> in <a href="https://www.nasa.gov/mission_pages/kepler/news/kepscicon-briefing.html">recent</a> years. In fact, TOI-700 e is one of two potentially habitable planets just in the TOI-700 star system. </p>
<p>Habitable planets are those that are just the right distance from their star to have a surface temperature that could sustain liquid water. While it is always exciting to find a new, potentially habitable planet far from Earth, the focus of exoplanet research is shifting away from simply discovering more planets. Instead, researchers are focusing their efforts on finding and studying systems most likely to answer key questions about how planets form, how they evolve, and whether life might exist in the universe. TOI-700 e stands out from many of these other planet discoveries because it is well suited for future studies that could help answer big question about the conditions for life outside the solar system. </p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/xNeRqbw18Jk?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Specific methods for detecting exoplanets, like the transit method, which looks for a dip in the light coming from a distant star as a planet passes in front of it, have led to an explosion in the number of known exoplanets.</span></figcaption>
</figure>
<h2>From 1 to 5,000</h2>
<p>Astronomers discovered the first exoplanet around a Sun-like star <a href="https://doi.org/10.1038/378355a0">in 1995</a>. The field of exoplanet discovery and research has been rapidly evolving ever since.</p>
<p>At first, astronomers were finding only a <a href="https://www.hughosborn.co.uk/2015/02/09/a-history-of-planet-detection-in-one-animation/">few exoplanets each year</a>, but the combination of new cutting-edge facilities focused on <a href="https://www.nasa.gov/tess-transiting-exoplanet-survey-satellite">exoplanet science</a> with improved detection sensitivity have led to astronomers’ discovering hundreds of exoplanets each year. As detection methods and tools have improved, the amount of information scientists can learn about these planets has increased. In 30 years, scientists have gone from barely being able to detect exoplanets to <a href="https://theconversation.com/to-search-for-alien-life-astronomers-will-look-for-clues-in-the-atmospheres-of-distant-planets-and-the-james-webb-space-telescope-just-proved-its-possible-to-do-so-184828">characterizing key chemical clues in their atmospheres</a>, like water, using facilities like the James Webb Space Telescope.</p>
<p>Today, there are more than <a href="https://exoplanetarchive.ipac.caltech.edu/">5,000 known exoplanets</a>, ranging from gas giants to small rocky worlds. And perhaps most excitingly, astronomers have now found about a dozen exoplanets that are likely rocky and orbiting within the habitable zones of their respective stars.</p>
<p>Astronomers have even discovered a few systems – like TOI-700 – that have more than one planet orbiting in the habitable zone of their star. We call these keystone systems.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/512975/original/file-20230301-24-8pqinb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A diagram showing a star with a green ring around it marking the habitable zone." src="https://images.theconversation.com/files/512975/original/file-20230301-24-8pqinb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/512975/original/file-20230301-24-8pqinb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=185&fit=crop&dpr=1 600w, https://images.theconversation.com/files/512975/original/file-20230301-24-8pqinb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=185&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/512975/original/file-20230301-24-8pqinb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=185&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/512975/original/file-20230301-24-8pqinb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=233&fit=crop&dpr=1 754w, https://images.theconversation.com/files/512975/original/file-20230301-24-8pqinb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=233&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/512975/original/file-20230301-24-8pqinb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=233&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 TOI-700 system has a large habitable zone, and the newly discovered TOI-700 e, not shown in this image, orbits the star along the inner edge of the habitable zone.</span>
<span class="attribution"><a class="source" href="https://www.jpl.nasa.gov/spaceimages/images/largesize/PIA23407_hires.jpg">NASA's Goddard Space Flight Center</a></span>
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<h2>A pair of habitable siblings</h2>
<p>TOI-700 first made headlines when our team announced the discovery of <a href="https://doi.org/10.3847/1538-3881/aba4b2">three small planets orbiting the star</a> in early 2020. Using a <a href="https://doi.org/10.3847/1538-3881/aba4b3">combination of observations</a> from NASA’s <a href="https://exoplanets.nasa.gov/tess/">Transiting Exoplanet Surveying Satellite</a> mission and the <a href="https://www.nasa.gov/mission_pages/spitzer/main/index.html">Spitzer Space Telescope</a> we discovered these planets by measuring small dips in the amount of light coming from TOI-700. These dips in light are caused by planets passing in front of the small, cool, red dwarf star at the center of the system.</p>
<p>By taking precise measurements of the changes in light, we were able to determine that at least three small planets are in the TOI-700 system, with hints of a possible fourth. We could also determine that the third planet from the star, TOI-700 d, orbits within its star’s habitable zone, where the temperature of the planet’s surface could allow for liquid water. </p>
<p>The Transiting Exoplanet Surveying Satellite observed TOI-700 for another year, from July 2020 through May 2021, and using these observations <a href="https://doi.org/10.3847/2041-8213/acb599">our team found the fourth planet, TOI-700 e</a>. TOI-700 e is 95% the size of the Earth and, much to our surprise, orbits on the inner edge of the star’s habitable zone, between planets c and d. Our discovery of this planet makes TOI-700 one of only a few known systems with two Earth-sized planets orbiting in the habitable zone of their star. The fact that it is relatively close to Earth also makes it one of the most accessible systems in terms of future characterization.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/512981/original/file-20230301-24-v7fzr8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="James Webb Space Telescope against the backdrop of space." src="https://images.theconversation.com/files/512981/original/file-20230301-24-v7fzr8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/512981/original/file-20230301-24-v7fzr8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=467&fit=crop&dpr=1 600w, https://images.theconversation.com/files/512981/original/file-20230301-24-v7fzr8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=467&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/512981/original/file-20230301-24-v7fzr8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=467&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/512981/original/file-20230301-24-v7fzr8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=586&fit=crop&dpr=1 754w, https://images.theconversation.com/files/512981/original/file-20230301-24-v7fzr8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=586&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/512981/original/file-20230301-24-v7fzr8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=586&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">New tools, like the James Webb Space Telescope, can provide clues about life on distant planets, but with thousands of scientific questions to answer, efficient use of time is key.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:James_Webb_Space_Telescope.jpg#/media/File:James_Webb_Space_Telescope.jpg">Bricktop/Wikimedia Commons</a></span>
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<h2>The bigger questions and tools to answer them</h2>
<p>With the successful launch of the James Webb Space Telescope, astronomers are now able to start <a href="https://www.nasa.gov/feature/goddard/2022/nasa-s-webb-reveals-an-exoplanet-atmosphere-as-never-seen-before">characterizing the atmospheric chemistry</a> of exoplanets and search for <a href="https://theconversation.com/to-search-for-alien-life-astronomers-will-look-for-clues-in-the-atmospheres-of-distant-planets-and-the-james-webb-space-telescope-just-proved-its-possible-to-do-so-184828">clues about whether life exists</a> on them. In the near future, a number of massive, ground-based telescopes will also help reveal further details about the composition of planets far from the solar system. </p>
<p>But even with powerful new telescopes, collecting enough light to learn these details requires pointing the telescope at a system for a <a href="https://arxiv.org/abs/1708.04239">long period of time</a>. With thousands of <a href="https://www.stsci.edu/jwst/science-execution/approved-programs/cycle-1-go">valuable scientific questions to answer</a>, astronomers need to know where to look. And that is the goal of our team, to find the most interesting and promising exoplanets to study with the Webb telescope and future facilities.</p>
<p>Earth is currently the only data point in the search for life. It is possible alien life could be vastly different from life as we know it, but for now, places similar to the home of humanity with liquid water on the surface offer a good starting point. We believe that keystone systems with multiple planets that are likely candidates for hosting life – like TOI-700 – offer the best use of observation time. By further studying TOI-700, our team will be able to learn more about what makes a planet habitable, how rocky planets similar to Earth form and evolve, and the mechanisms that shaped the solar system. The more astronomers know about how star systems like TOI-700 and our own solar system work, the better the chances of detecting life out in the cosmos.</p><img src="https://counter.theconversation.com/content/198274/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Joseph Rodriguez receives funding from the National Aeronautics and Space Administration and Michigan State University. </span></em></p><p class="fine-print"><em><span>Andrew Vanderburg receives funding from the National Aeronautics and Space Administration and the Massachusetts Institute of Technology. </span></em></p>With more than 5,000 known exoplanets, astronomers are shifting their focus from discovering additional distant worlds to identifying which are good candidates for further study.Joey Rodriguez, Assistant Professor of Physics and Astronomy, Michigan State UniversityAndrew Vanderburg, Assistant Professor of Physics, Massachusetts Institute of Technology (MIT)Licensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1640622021-07-19T12:06:44Z2021-07-19T12:06:44ZAre there any planets outside of our solar system?<figure><img src="https://images.theconversation.com/files/410219/original/file-20210707-25-2zom23.jpg?ixlib=rb-1.1.0&rect=31%2C15%2C5161%2C2903&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Artist illustration of an exoplanet.
</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/beautiful-exoplanet-with-exo-moons-orbiting-an-royalty-free-image/873145010?adppopup=true">dottedhippo/iStock via Getty Images</a></span></figcaption></figure><figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=293&fit=crop&dpr=1 600w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=293&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=293&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=368&fit=crop&dpr=1 754w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=368&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=368&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<p><em><a href="https://theconversation.com/us/topics/curious-kids-us-74795">Curious Kids</a> is a series for children of all ages. If you have a question you’d like an expert to answer, send it to <a href="mailto:curiouskidsus@theconversation.com">curiouskidsus@theconversation.com</a>.</em></p>
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<blockquote>
<p><strong>Are there any planets outside of our solar system? - Eli W., age 8, Baton Rouge, Louisiana</strong></p>
</blockquote>
<hr>
<p>This is a question that human beings have wondered about for thousands of years. </p>
<p>Here’s how the ancient Greek mathematician <a href="https://en.wikipedia.org/wiki/Metrodorus_of_Chios">Metrodorus</a> (400-350 B.C.) put it: A universe where Earth is “the only world,” he said, is about as believable as a “large field containing a single stalk.” </p>
<p>About 2,000 years later, in the 16th century, the Italian philosopher <a href="https://www.britannica.com/biography/Giordano-Bruno">Giordano Bruno</a> suggested something similar. </p>
<p>“Countless suns and countless earths” existed elsewhere, he said, all rotating “round their suns in exactly the same way as the planets of our system.” </p>
<p>Scientists now know that both Metrodorus and Bruno were essentially correct. Today, <a href="https://scholar.google.com/citations?hl=en&user=VRJuiHUAAAAJ">astronomers like me</a> are still exploring this question, using new tools. </p>
<figure class="align-center ">
<img alt="An exoplanet orbiting a red dwarf star." src="https://images.theconversation.com/files/410231/original/file-20210707-6685-1pr3hko.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/410231/original/file-20210707-6685-1pr3hko.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=350&fit=crop&dpr=1 600w, https://images.theconversation.com/files/410231/original/file-20210707-6685-1pr3hko.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=350&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/410231/original/file-20210707-6685-1pr3hko.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=350&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/410231/original/file-20210707-6685-1pr3hko.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=439&fit=crop&dpr=1 754w, https://images.theconversation.com/files/410231/original/file-20210707-6685-1pr3hko.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=439&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/410231/original/file-20210707-6685-1pr3hko.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=439&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">An exoplanet orbiting a red dwarf, a star that is dimmer than our Sun and about half the size.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/illustration/artwork-of-gliese-887-b-and-c-royalty-free-illustration/1271698835">Mark Garlick/Science Photo Library via Getty Images</a></span>
</figcaption>
</figure>
<h2>The exoplanets</h2>
<p>There is now evidence that demonstrates the existence of “exoplanets” – that is, planets orbiting stars other than our Sun. </p>
<p>That evidence is based on the discoveries made by the <a href="https://kidsdiscover.com/spotlight/kepler/">Kepler space telescope</a>, launched by NASA in 2009.</p>
<p>For four years, the telescope stared continuously at a single region of space within the <a href="https://kids.kiddle.co/Cygnus_(constellation)">constellation Cygnus</a>.</p>
<p>Looking from Earth, it’s an area that takes up less than 1% of your view of the sky. </p>
<figure class="align-center ">
<img alt="An illustration shows the Kepler telescope in space, next to a star and its planet." src="https://images.theconversation.com/files/410236/original/file-20210707-25-1fxhfsx.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/410236/original/file-20210707-25-1fxhfsx.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=350&fit=crop&dpr=1 600w, https://images.theconversation.com/files/410236/original/file-20210707-25-1fxhfsx.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=350&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/410236/original/file-20210707-25-1fxhfsx.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=350&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/410236/original/file-20210707-25-1fxhfsx.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=440&fit=crop&dpr=1 754w, https://images.theconversation.com/files/410236/original/file-20210707-25-1fxhfsx.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=440&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/410236/original/file-20210707-25-1fxhfsx.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=440&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Artist illustration of NASA’s Kepler space telescope.</span>
<span class="attribution"><a class="source" href="https://exoplanets.nasa.gov/news/1526/latest-on-the-kepler-spacecraft/">NASA Images</a></span>
</figcaption>
</figure>
<h2>How the telescope worked</h2>
<p>Kepler had 42 cameras on board, similar to the kind of smartphone camera that you use to take pictures. In that one region, the telescope detected more than 150,000 stars. </p>
<p>About every half-hour it observed the amount of light radiating from each star. Back here on Earth, a team of Kepler scientists analyzed the data.</p>
<p>For most stars, the amount of light stayed pretty much the same. </p>
<p>But for about 3,000 stars, the amount of light repeatedly decreased, by small amounts and for several hours. These drops in brightness happened at regular intervals, like clockwork. </p>
<p>The drops, astronomers concluded, were caused by a planet orbiting its star, periodically blocking some of the light that Kepler’s cameras would otherwise detect. </p>
<p>This event – when a planet passes between a star and its observer – is known as a <a href="https://exoplanets.nasa.gov/faq/31/whats-a-transit/">transit</a>.</p>
<p>And that means that in that one speck of space the Kepler telescope found 3,000 planets.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/BFi4HBUdWkk?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">NASA Video: Animation of a exoplanet transiting its star.</span></figcaption>
</figure>
<h2>That’s only the beginning</h2>
<p>Although 3,000 planets sounds like a lot, it’s certain many others within that area remain undetected. </p>
<p>That’s because their orbits never blocked the light as seen by Kepler. After all, planetary orbits aren’t all the same; they’re randomly oriented. </p>
<p>But because of the number of transits observed by Kepler, and astronomers’ knowledge of geometry, we can make a good guess on the total number of exoplanets out there.</p>
<p>And after making those calculations, scientists now think, on average, <a href="https://www.popsci.com/science/article/2012-01/new-exoplanet-analysis-determines-planets-are-more-common-stars-milky-way/">that every star has at least one planet</a>. </p>
<p>This discovery has revolutionized astronomy and our view of the universe. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/4IXYp9Fse44?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">NASA Video: Weird and Wondrous Worlds.</span></figcaption>
</figure>
<h2>100 billion stars, 100 billion planets</h2>
<p>For instance, our Milky Way galaxy has at least 100 billion stars; that means it has at least 100 billion planets too.</p>
<p>But remember: The universe holds up to 2 trillion galaxies. That’s 2,000,000,000,000! And each galaxy contains tens or even hundreds of billions of stars. </p>
<p>So the number of planets in the universe is truly astronomical, roughly equivalent to the <a href="https://www.universetoday.com/106725/are-there-more-grains-of-sand-than-stars/">number of grains of dry sand</a> on every beach on Earth. </p>
<p>Some of those planets are gas giants, like <a href="https://spaceplace.nasa.gov/all-about-jupiter/en/">Jupiter</a> in our solar system. Others are boiling hot, like <a href="https://spaceplace.nasa.gov/all-about-venus/en/">Venus</a>. Others may be <a href="https://www.nasa.gov/specials/ocean-worlds/">water worlds</a> or <a href="https://spaceplace.nasa.gov/ice-on-other-planets/en/">ice planets</a>. And some are Earth-like.</p>
<p>In fact, the Kepler team calculated the abundance of Earth-like planets in the “habitable zone,” a sector of space around each star where a world might have moderate temperatures and liquid water. </p>
<p>They found approximately <a href="https://www.nasa.gov/feature/ames/kepler-occurrence-rate">50% of Sun-like stars in the Milky Way</a> host an Earth-like planet in the habitable zone. </p>
<p>That adds up to <a href="https://www.space.com/habitable-planets-common-sunlike-stars-milky-way">billions of potentially habitable worlds</a> just in our galaxy.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/J04YN9azln8?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">NASA/JPL-Caltech Video: What is the “Habitable Zone”?</span></figcaption>
</figure>
<h2>Could life exist elsewhere?</h2>
<p>Although scientists haven’t found proof yet, many – <a href="https://seti.ucla.edu/jlm/">including me</a> – now think it’s unlikely that Earth is the only planet where life evolved. That would be as surprising as a large field containing a single stalk.</p>
<p>When will humans detect life elsewhere? Will it be intelligent life? Will people ever receive a message from another civilization?</p>
<p>Today, hundreds of scientists around the world are trying to answer those questions.</p>
<hr>
<p><em>Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to <a href="mailto:curiouskidsus@theconversation.com">CuriousKidsUS@theconversation.com</a>.
Please tell us your name, age and the city where you live.</em></p>
<p><em>And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.</em></p><img src="https://counter.theconversation.com/content/164062/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jean-Luc Margot receives funding from the National Aeronautics and Space Administration, the National Science Foundation, and philanthropists.</span></em></p>Billions of galaxies are in the universe, with billions of stars in every galaxy. Could billions of planets be out there too?Jean-Luc Margot, Professor of Earth, Planetary, and Space Sciences, University of California, Los AngelesLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/936882018-04-09T10:33:24Z2018-04-09T10:33:24ZGoodbye Kepler, hello TESS: Passing the baton in the search for distant planets<figure><img src="https://images.theconversation.com/files/213635/original/file-20180406-5578-h4ba8e.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Imagined view from Kepler-10b, a planet that orbits one of the 150,000 stars that the Kepler spacecraft is monitoring.</span> <span class="attribution"><a class="source" href="https://www.nasa.gov/mission_pages/kepler/multimedia/images/kepler10_5.html">NASA/Kepler Mission/Dana Berry</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>For centuries, human beings have wondered about the possibility of other Earths orbiting distant stars. Perhaps some of these alien worlds would harbor strange forms of life or have unique and telling histories or futures. But it was only in 1995 that astronomers spotted the first planets orbiting sunlike stars outside of our solar system. </p>
<p>In the last decade, in particular, the number of planets known to orbit distant stars grew from under 100 to well over 2,000, with another 2,000 likely planets awaiting confirmation. Most of these new discoveries are due to a single endeavor — <a href="https://www.nasa.gov/mission_pages/kepler/overview/index.html">NASA’s Kepler mission</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/213631/original/file-20180406-5572-1e5h7cx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/213631/original/file-20180406-5572-1e5h7cx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/213631/original/file-20180406-5572-1e5h7cx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/213631/original/file-20180406-5572-1e5h7cx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/213631/original/file-20180406-5572-1e5h7cx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/213631/original/file-20180406-5572-1e5h7cx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/213631/original/file-20180406-5572-1e5h7cx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/213631/original/file-20180406-5572-1e5h7cx.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">Number of confirmed exoplanets continues to grow.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/image-feature/ames/planetary-systems-by-number-of-known-planets">NASA/Ames Research Center/Wendy Stenzel and The University of Texas at Austin/Andrew Vanderburg</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/213633/original/file-20180406-5597-1xrglfy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/213633/original/file-20180406-5597-1xrglfy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/213633/original/file-20180406-5597-1xrglfy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=255&fit=crop&dpr=1 600w, https://images.theconversation.com/files/213633/original/file-20180406-5597-1xrglfy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=255&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/213633/original/file-20180406-5597-1xrglfy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=255&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/213633/original/file-20180406-5597-1xrglfy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=321&fit=crop&dpr=1 754w, https://images.theconversation.com/files/213633/original/file-20180406-5597-1xrglfy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=321&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/213633/original/file-20180406-5597-1xrglfy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=321&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Scientists can determine the size or radius of a planet by measuring the depth of the dip in brightness and knowing the size of the star.</span>
<span class="attribution"><span class="source">NASA Ames</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Kepler is a spacecraft housing a 1-meter telescope that illuminates a 95 megapixel digital camera the size of a cookie sheet. The instrument detected tiny variations in the brightness of 150,000 distant stars, looking for the telltale sign of a planet blocking a portion of the starlight as it transits across the telescope’s line of sight. It’s so sensitive that it could detect a fly buzzing around a single streetlight in Chicago from an orbit above the Earth. It can see stars shake and vibrate; it can see starspots and flares; and, in favorable situations, it can see planets as small as the moon.</p>
<p>Kepler’s thousands of discoveries revolutionized our understanding of planets and planetary systems. Now, however, the spacecraft has run out of its hydrazine fuel and officially entered retirement. Luckily for planet hunters, NASA’s TESS mission launched in April and will take over the exoplanet search.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/213634/original/file-20180406-5581-17gqrvc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/213634/original/file-20180406-5581-17gqrvc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/213634/original/file-20180406-5581-17gqrvc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=402&fit=crop&dpr=1 600w, https://images.theconversation.com/files/213634/original/file-20180406-5581-17gqrvc.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=402&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/213634/original/file-20180406-5581-17gqrvc.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=402&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/213634/original/file-20180406-5581-17gqrvc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=505&fit=crop&dpr=1 754w, https://images.theconversation.com/files/213634/original/file-20180406-5581-17gqrvc.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=505&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/213634/original/file-20180406-5581-17gqrvc.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=505&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Prepping the Kepler spacecraft pre-launch in 2009.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/mission_pages/kepler/multimedia/09-02-03.html">NASA/Tim Jacobs</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Kepler’s history</h2>
<p>The Kepler mission was conceived in the early 1980s by NASA scientist <a href="https://www.nasa.gov/mission_pages/kepler/team/william_borucki.html">Bill Borucki</a>, with later help from <a href="https://www.nasa.gov/mission_pages/kepler/team/David_Koch.html">David Koch</a>. At the time, there were no known planets outside of the solar system. Kepler was eventually assembled in the 2000s and launched in March of 2009. <a href="https://scholar.google.com/citations?user=pyXWfSgAAAAJ&hl=en&oi=ao">I joined</a> the Kepler Science Team in 2008 (as a wide-eyed rookie), eventually co-chairing the group studying the motions of the planets with <a href="https://www.nasa.gov/centers/ames/research/2007/lissauer.html">Jack Lissauer</a>.</p>
<p>Originally, the mission was planned to last for three and a half years with possible extensions for as long as the fuel, or the camera, or the spacecraft lasted. As time passed, portions of the camera began to fail but the mission has persisted. However, in 2013 <a href="https://www.nature.com/news/the-wheels-come-off-kepler-1.13032">when two of its four stabilizing gyros</a> (technically “reaction wheels”) stopped, the original Kepler mission effectively ended.</p>
<figure class="align-center zoomable">
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<figcaption>
<span class="caption">NASA scientists figured out how to use solar pressure to stabilize Kepler.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/kepler/keplers-second-light-how-k2-will-work">NASA Ames/W Stenzel</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Even then, with some ingenuity, NASA was able to use <a href="https://www.nasa.gov/kepler/keplers-second-light-how-k2-will-work">reflected light from the Sun to help steer the spacecraft</a>. The mission was rechristened as K2 and continued finding planets for another half decade. Now, with the fuel gauge near empty, the business of planet hunting is winding down and the spacecraft will be left adrift in the solar system. The final catalog of planet candidates from the original mission was completed late last year and the last observations of K2 are wrapping up.</p>
<h2>Kepler’s science</h2>
<p>Squeezing what knowledge we can from those data will continue for years to come, but what we’ve seen thus far has amazed scientists across the globe.</p>
<p>We have seen some planets that orbit their host stars in only a few hours and are so hot that the surface rock <a href="https://www.nasa.gov/content/disintegrating-super-mercury-size-planet">vaporizes and trails behind the planet</a> like a comet tail. Other systems have <a href="http://www.washington.edu/news/2012/06/21/astronomers-spy-two-planets-in-tight-quarters-as-they-orbit-a-distant-star/">planets so close together</a> that if you were to stand on the surface of one, the second planet would appear larger than 10 full moons. One system is so packed with planets that <a href="https://www.nasa.gov/image-feature/ames/kepler-90-planets-orbit-close-to-their-star">eight of them are closer to their star</a> than the Earth is to the Sun. <a href="https://www.nasa.gov/ames/kepler/nasa-keplers-hall-of-fame-small-habitable-zone-exoplanets">Many have planets</a>, and sometimes multiple planets, orbiting within the <a href="https://www.nasa.gov/ames/kepler/habitable-zones-of-different-stars">habitable zone</a> of their host star, where liquid water may exist on their surfaces.</p>
<p>As with any mission, the Kepler package came with trade-offs. It needed to stare at a single part of the sky, blinking every 30 minutes, for four straight years. In order to study enough stars to make its measurements, the stars had to be quite distant – just as when you stand in the middle of a forest, there are more trees farther from you than right next to you. Distant stars are dim, and their planets are hard to study. Indeed, one challenge for astronomers who want to study the properties of Kepler planets is that Kepler itself is often the best instrument to use. High quality data from ground-based telescopes requires long observations on the largest telescopes – precious resources that limit the number of planets that can be observed.</p>
<p>We now know that there are at least as many planets in the galaxy as there are stars, and many of those planets are quite unlike what we have here in the solar system. Learning the characteristics and personalities of the wide variety of planets requires that astronomers investigate the ones orbiting brighter and closer stars where more instruments and more telescopes can be brought to bear.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/213636/original/file-20180406-5603-e2bdy9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/213636/original/file-20180406-5603-e2bdy9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/213636/original/file-20180406-5603-e2bdy9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=357&fit=crop&dpr=1 600w, https://images.theconversation.com/files/213636/original/file-20180406-5603-e2bdy9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=357&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/213636/original/file-20180406-5603-e2bdy9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=357&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/213636/original/file-20180406-5603-e2bdy9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=449&fit=crop&dpr=1 754w, https://images.theconversation.com/files/213636/original/file-20180406-5603-e2bdy9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=449&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/213636/original/file-20180406-5603-e2bdy9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=449&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Once launched, TESS will identify exoplanets orbiting the brightest stars just outside our solar system.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/image-feature/goddard/2016/tess-artist-concept">NASA's Goddard Space Flight Center</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Enter TESS</h2>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/213679/original/file-20180407-5569-5e4hvm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/213679/original/file-20180407-5569-5e4hvm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/213679/original/file-20180407-5569-5e4hvm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=548&fit=crop&dpr=1 600w, https://images.theconversation.com/files/213679/original/file-20180407-5569-5e4hvm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=548&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/213679/original/file-20180407-5569-5e4hvm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=548&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/213679/original/file-20180407-5569-5e4hvm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=689&fit=crop&dpr=1 754w, https://images.theconversation.com/files/213679/original/file-20180407-5569-5e4hvm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=689&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/213679/original/file-20180407-5569-5e4hvm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=689&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Duration of TESS’ observations on the celestial sphere, taking into account the overlap between sectors.</span>
<span class="attribution"><a class="source" href="https://tess.gsfc.nasa.gov/science.html">NASA</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p><a href="https://tess.gsfc.nasa.gov">NASA’s Transiting Exoplanet Survey Satellite mission</a>, led by MIT’s <a href="https://tess.gsfc.nasa.gov/dr-george-ricker-bio.html">George Ricker</a>, is searching for planets using the same detection technique that Kepler used. TESS’ orbit, rather than being around the Sun, has a close relationship with the Moon: TESS orbits the Earth twice for each lunar orbit. TESS’ observing pattern, rather than staring at a single part of the sky, will scan nearly the entire sky with overlapping fields of view (much like the petals on a flower).</p>
<p>Given what we learned from Kepler, astronomers expect TESS to find thousands more planetary systems. By surveying the whole sky, we will find systems that orbit stars 10 times closer and 100 times brighter than those found by Kepler – opening up new possibilities for measuring planet masses and densities, studying their atmospheres, characterizing their host stars, and establishing the full nature of the systems in which the planets reside. This information, in turn, will tell us more about our own planet’s history, how life may have started, what fates we avoided and what other paths we could have followed.</p>
<p>The quest to find our place in the universe continues as Kepler finishes its leg of the journey and TESS takes the baton.</p><img src="https://counter.theconversation.com/content/93688/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jason Steffen receives funding from NASA. </span></em></p>When NASA first started planning the Kepler mission, no one knew if the universe held any planets outside our solar system. Thousands of exoplanets later, the search enters a new phase as Kepler retires.Jason Steffen, Assistant Professor of Physics and Astronomy, University of Nevada, Las VegasLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/724252017-02-08T07:39:05Z2017-02-08T07:39:05ZUntil we get better tools, excited reports of ‘habitable planets’ need to come back down to Earth<p>In 1950, physicist <a href="https://commons.wikimedia.org/wiki/File:%22Where_is_Everybody%3F%22_An_Account_of_Fermi%27s_Question.pdf">Enrico Fermi famously asked</a>, “Where are they?” as a kind of lament about the lack of observational evidence for alien intelligence in our universe. Today, the question is still asked in the context of the always-hoped-for discovery of other worlds like our own, with the thought that maybe, just maybe, we will finally find those aliens.</p>
<p>Against this backdrop, advances that have occurred in the past 20 years in the field of exoplanet discovery have excited the imaginations of scientists and enthusiasts alike.</p>
<p>When, the question goes, will we finally discover a planet that can sustain life? When will we discover Earth 2.0?</p>
<p>The impatience associated with this question has led many in the media and even some in the scientific community to <a href="http://gizmodo.com/this-is-the-most-earth-like-planet-ever-discovered-1719724534">make premature declarations</a> that an “Earth analogue” has been discovered. But when exoplanets are discovered, claims that they are similar to Earth are based on, at best, optimistic modelling and, at worst, sensationalism.</p>
<p>Many such claims have been made <a href="http://www.nature.com/articles/s41550-017-0042">on the basis of invented ranking systems</a> that use the observed properties of exoplanets to extrapolate how Earth-like a planet might be. Unfortunately, these systems must make extremely simple – and almost certainly incorrect – assumptions about the characteristics of the planets they are trying to describe. </p>
<h2>Ranking habitability: not so easy</h2>
<p>Before any exoplanets had been discovered at all, certain astrophysicists proposed that each star had an associated zone around it that came to be known as the “<a href="http://www.as.utexas.edu/astronomy/education/spring02/scalo/kasting.pdf">habitable zone</a>”. This zone is at a distance from every star where a hypothetical Earth twin would have an average surface temperature between the freezing and boiling point of water. Too close and you exceed 100°C; too far and you drop below 0°C. </p>
<p>But if a planet has an atmospheric composition different from Earth’s, its true surface temperature is likely to be completely different. </p>
<p>Gaseous planets do not even have a well-defined surface to consider. And, for rocky planets, a thinner atmosphere can make them much colder (especially at night) while a thicker atmosphere can make them much hotter. </p>
<p>One of the most dramatic examples of this problem is Venus. Owing to its thick atmosphere and <a href="http://www.esa.int/Our_Activities/Space_Science/Venus_Express/Greenhouse_effects_also_on_other_planets">a runaway greenhouse effect</a>, the planet has a temperature of a whopping 450°C — far higher than than the 25°C you would calculate given an Earth-like atmosphere. Although Venus lies within our Sun’s nominal habitable zone, it is surely not accurate to call it habitable.</p>
<p>The two most fruitful methods for discovering exoplanets (the “<a href="http://www.planetary.org/explore/space-topics/exoplanets/transit-photometry.html">transit method</a>” and the “<a href="https://lco.global/spacebook/radial-velocity-method/">radial velocity method</a>”) both give a <a href="http://www.calctool.org/CALC/phys/astronomy/planet_orbit">straightforward way to determine the distance between a star and an exoplanet</a>. That, together with our knowledge of how much heat is given off by the star, lets us calculate whether the planet is in the star’s habitable zone. But, as we have seen, that is not the same thing as discovering a habitable planet.</p>
<p>Nevertheless, discoveries of planets in the habitable zones of other stars have been identified in the media and even in press releases of scientific institutions as <a href="https://www.nasa.gov/ames/kepler/nasas-kepler-discovers-first-earth-size-planet-in-the-habitable-zone-of-another-star">discoveries of second Earths</a>. Since we do not know the surface temperatures of any exoplanet, whether they are actual Earth analogues can only be guessed at using other lines of evidence. </p>
<h2>Learning more about exoplanets</h2>
<p>Our best understanding to date is that surface temperatures are strongly dependent on atmospheric composition and planet density. The density of a planet is dependent on both its mass and volume, but the two detection methods only allow for one or the other of the two complementary characteristics to be directly measured. </p>
<p>The transit method detects the shadow a planet casts on the star it is orbiting, allowing the planet’s area (and, <a href="https://www.scientificamerican.com/article/why-are-planets-round/">because planets are spheres</a>, its volume) to be measured. What is not directly discoverable from this method, however, is the planet’s mass. </p>
<p>Alternatively, the radial velocity method detects a planet via a wobble in the star’s motion that can be used to infer a minimum possible planetary mass tugging on the star with its gravity. In many cases, the tug is being done at an angle so <a href="http://spiff.rit.edu/classes/phys301/lectures/mass_ii/mass_ii.html">we see a reduced effect</a>, which makes us infer a mass that is smaller than the actual mass of planet. Aside from this potential confusion, there is no way via the radial velocity method alone to determine a planet’s volume.</p>
<p>Astrophysicists who model planet formation and composition have proposed a <a href="https://www.cfa.harvard.edu/%7Elzeng/planetmodels.html">variety of models</a> that offer possible relationships between the volumes and masses of planets depending on planet compositions. </p>
<p>The smallest planets in our own solar system are rocky and the largest planets are gaseous, but we see a number of exoplanets whose sizes lie between the smallest gaseous planet (Neptune) and the largest rocky planet (Earth). We have models that can accommodate “<a href="http://www.universetoday.com/108452/what-is-a-super-earth/">super-Earths</a>” that are rocky or “<a href="http://www.space.com/23079-alien-planets-super-earth-mini-neptune.html">mini-Neptunes</a>” that are gaseous and <a href="http://www.nature.com/articles/s41550-017-0043">all manner of hybrids in between</a>. </p>
<p>These varied models can accommodate a range of atmospheres, and the exoplanets will have very different surface temperatures depending on all of this. It is therefore of some importance that we learn more about exoplanet atmospheres directly using better telescopes and more sensitive techniques. </p>
<p><a href="http://iopscience.iop.org/article/10.1088/0004-637X/814/2/91/pdf">Some astronomers have proposed a scheme</a> to decide which exoplanets are most likely to have their atmospheres directly detected – the obvious next step in working towards determining a planet’s surface temperature and ultimately whether it is habitable.</p>
<h2>Jumping the gun</h2>
<p>A regrettable tendency right now is to jump the gun. Planets have been discovered in the habitable zones of other stars with <a href="https://www.eso.org/public/chile/news/eso0019/">radial-velocity-measured minimum masses that are similar to Earth’s</a> and <a href="https://www.nasa.gov/ames/kepler/a-keplers-dozen-small-habitable-zone-planets">transit-measured surface areas that are not much larger than Earth’s</a>. </p>
<p>Crucially, none of these “analogues” has yet been measured in both ways. But almost every time such planets are discovered, <a href="http://thetechnews.com/2017/01/21/could-wolf-1061c-be-the-next-earth/">breathless</a> <a href="https://phys.org/news/2016-08-earth-like-planet-proxima-centauri.html">reports</a> <a href="http://www.bbc.com/news/science-environment-33641648">of</a> <a href="https://www.thesun.co.uk/news/2295568/astronomers-spot-mysterious-second-earth-and-its-got-perfect-conditions-for-alien-life/">their</a> <a href="https://www.inverse.com/article/26715-wolf-1061-planet-sustain-life">possible</a> <a href="http://nypost.com/2017/01/21/scientists-searching-for-life-on-nearby-super-earth/">import</a> <a href="http://fox13now.com/2016/08/24/new-earth-like-planet-found-that-could-potentially-support-life/">are</a> <a href="http://www.space.com/30030-earth-cousin-kepler-452b-exoplanet-details-infographic.html">generated</a>. </p>
<p>While discoveries of exoplanets are exciting, it is definitely premature to try to decide how Earth-like any planet is or is not on the basis of the scant data we are now able to gather. The best we can hope to do at this time is collate a list of possible targets for future observation.</p>
<p>Someday, we may discover definitive proof that another Earth is out there. But that day has not yet arrived – despite the excited headlines.</p><img src="https://counter.theconversation.com/content/72425/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Joshua Tan receives funding from the Chilean government's Comisión Nacional de Investigación Científica y Tecnológica under a grant from the Fondo Nacional de Desarrollo Científico y Tecnológico; Project Number 3150484.</span></em></p>Over the last 20 years, advances in the field of exoplanet discovery have excited the imaginations of scientists and enthusiasts alike. But we’re in position to know yet whether a planet is habitable.Joshua Tan, Astronomer at the Instituto de Astrofísica, Universidad Católica de ChileLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/643212016-08-24T17:01:07Z2016-08-24T17:01:07ZPossibly habitable planet found around our nearest neighbour star<figure><img src="https://images.theconversation.com/files/135138/original/image-20160823-30231-tng3zh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Artist's impression of the surface of the planet Proxima b, orbiting Proxima Centauri.</span> <span class="attribution"><span class="source">ESO/M. Kornmesser</span></span></figcaption></figure><p>With thousands of exoplanets discovered to date, it’s no wonder that we regularly come across <a href="https://theconversation.com/the-five-most-earth-like-exoplanets-so-far-50669">“Earth-like worlds”</a> around distant stars. But let’s face it: while it’s exciting that there are planets out there that may be able to harbour life, they are so far away that we will not be able to visit them anytime soon.</p>
<p>So wouldn’t it be amazing if we discovered a planet just like the Earth – in our own neighbourhood? Well, a new study led by some of my colleagues at Queen Mary University of London has finally done just that. Proxima Centauri – a “<a href="http://www%20.space.com/23772-red-dwarf-stars.html">red dwarf</a>” star that’s some 14% the size of the sun and around half the temperature – is the closest star to our solar system at 4.24 light years away. Until now, we weren’t sure if it had any planets in orbit, but the new study, <a href="http://nature.com/articles/doi:10.1038/nature19106">published in Nature</a>, reports the discovery of a potentially habitable world that we may actually be able to send <a href="https://theconversation.com/why-sailing-to-the-stars-has-suddenly-become-a-realistic-goal-57762">tiny robots</a> to in the next few decades.</p>
<p>The team behind the discovery is called Pale Red Dot – an observational campaign of the <a href="http://www.eso.org/sci/facilities/lasilla/instruments/harps.html">High Accuracy Radial velocity Planet Searcher (HARPS)</a>, an instrument on the European Southern Observatory’s 3.6-metre <a href="http://www.eso.org/public/unitedkingdom/teles-instr/lasilla/">La Silla telescope </a> in Chile’s Atacama Desert. It measured the <a href="https://theconversation.com/explainer-seeing-the-universe-through-spectroscopic-eyes-37759">spectra of light</a> from Proxima, essentially the fingerprints that reveal what the star is made of, and looked for changes in the frequency of those lines. Small shifts in this starlight can be used to work out tiny movements of the star in response to an orbiting planet’s gravitational pull.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/135140/original/image-20160823-30238-13grujs.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/135140/original/image-20160823-30238-13grujs.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=636&fit=crop&dpr=1 600w, https://images.theconversation.com/files/135140/original/image-20160823-30238-13grujs.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=636&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/135140/original/image-20160823-30238-13grujs.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=636&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/135140/original/image-20160823-30238-13grujs.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=799&fit=crop&dpr=1 754w, https://images.theconversation.com/files/135140/original/image-20160823-30238-13grujs.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=799&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/135140/original/image-20160823-30238-13grujs.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=799&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A view of the southern skies with images of the stars Proxima Centauri (lower-right) and the double star Alpha Centauri AB (lower-left) from the NASA/ESA Hubble Space Telescope.</span>
<span class="attribution"><span class="source">Y. Beletsky (LCO)/ESO/ESA/NASA/M. Zamani</span></span>
</figcaption>
</figure>
<p>Pale Red Dot’s measurements were made each night for around three months at the beginning of 2016. And the results revealed the tell-tale signature of a planet, now labelled “Proxima b” as per naming conventions.</p>
<h2>Chances of finding life on Proxima b</h2>
<p>From the data gathered, the team has determined quite a lot about the planet’s properties. It orbits Proxima every 11.2 days, which places it a tenth of the distance from its star as Mercury is from the sun. While that would be an extremely unpleasant place to be in our solar system, at Proxima that’s just within the estimated “habitable zone” – an area where it is plausible that liquid water could exist on the surface of a planet. Proxima b is also at least 30% heavier than our world and if it is in fact rocky its surface gravity might only be 10% more than we’re used to. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/zQLL9AheEsY?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Exciting discovery.</span></figcaption>
</figure>
<p>So what are the possibilities for life on Proxima b? That’s hard to say just yet. We don’t know if the planet has an atmosphere at all, let alone what it might be made of. Over the next few years we might be able to figure that out using Hubble or the upcoming <a href="https://theconversation.com/how-hubbles-successor-will-give-us-a-glimpse-into-the-very-first-galaxies-45970">James Webb Space Telescope</a>. </p>
<p>These might also reveal if any of the ingredients for life are present. But Proxima b may not be quite as hospitable as Earth. Red dwarf stars are incredibly violent so Proxima Centauri could bombard the planet with radiation. The proximity of Proxima b to the star could also mean that the planet is “tidally locked”, with one side always facing the star in perpetual day and the other in unending darkness. This would result in extremes of temperature: hot desert and barren rock versus frozen wasteland.</p>
<p>The true test would be to go there. Using conventional space technology (either manned or unmanned) and some clever slingshot manoeuvres, it would take at least 15,000 years to reach Proxima Centauri. But the <a href="https://theconversation.com/why-sailing-to-the-stars-has-suddenly-become-a-realistic-goal-57762">ambitious Starshot Project</a> aims to send tiny robots to this star system, propelled by powerful Earth-based lasers. They are estimating that it would only take about 20 years to get there in this manner, travelling at a speed of approximately 60,000 km per second (or 135m miles per hour). Those robots could relay back data about the system, and potentially even closeup pictures of Proxima b.</p>
<p>It certainly seems possible that we could find something out of this world within our lifetime.</p><img src="https://counter.theconversation.com/content/64321/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Martin Archer 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>Scientists have finally found an Earth-like planet we may actually be able to visit.Martin Archer, Space Plasma Physicist, Queen Mary University of LondonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/554242016-07-05T01:57:30Z2016-07-05T01:57:30ZPlate tectonics: new findings fill out the 50-year-old theory that explains Earth’s landmasses<figure><img src="https://images.theconversation.com/files/129030/original/image-20160701-18317-xgbe00.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Satellite image of California's San Andreas fault, where two continental plates come together.</span> <span class="attribution"><a class="source" href="http://photojournal.jpl.nasa.gov/catalog/PIA14555">NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>Fifty years ago, there was a seismic shift away from the longstanding belief that Earth’s continents were permanently stationary. </p>
<p>In 1966, <a href="https://www.geolsoc.org.uk/Plate-Tectonics/Chap1-Pioneers-of-Plate-Tectonics/John-Tuzo-Wilson">J. Tuzo Wilson</a> published <a href="http://fossilhub.org/wp-content/uploads/2012/10/Wilson1966_did_Atlantic_reopen.pdf">Did the Atlantic Close and then Re-Open?</a> in the journal Nature. The Canadian author introduced to the mainstream the idea that continents and oceans are in continuous motion over our planet’s surface. Known as <a href="https://en.wikipedia.org/wiki/Plate_tectonics">plate tectonics</a>, the theory describes the large-scale motion of the outer layer of the Earth. It explains tectonic activity (things like earthquakes and the building of mountain ranges) at the edges of continental landmasses (for instance, the San Andreas Fault in California and the Andes in South America). </p>
<p>At 50 years old, with a surge of interest in where the surface of our planet has been and where it’s going, scientists are reassessing what plate tectonics does a good job of explaining – and puzzling over where new findings might fit in.</p>
<h2>Evidence for the theory</h2>
<p>Although the widespread acceptance of the theory of plate tectonics is younger than Barack Obama, German scientist <a href="https://en.wikipedia.org/wiki/Alfred_Wegener">Alfred Wegener</a> first advanced the hypothesis back in 1912.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/125595/original/image-20160607-15061-lvdpu.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/125595/original/image-20160607-15061-lvdpu.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/125595/original/image-20160607-15061-lvdpu.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=707&fit=crop&dpr=1 600w, https://images.theconversation.com/files/125595/original/image-20160607-15061-lvdpu.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=707&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/125595/original/image-20160607-15061-lvdpu.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=707&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/125595/original/image-20160607-15061-lvdpu.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=888&fit=crop&dpr=1 754w, https://images.theconversation.com/files/125595/original/image-20160607-15061-lvdpu.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=888&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/125595/original/image-20160607-15061-lvdpu.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=888&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A map of the original supercontinent, Pangaea, with modern continent outlines.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Pangaea_continents.svg">Kieff</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>He noted that the Earth’s current landmasses could fit together like a jigsaw puzzle. After analyzing fossil records that showed similar species once lived in now geographically remote locations, meteorologist Wegener proposed that the continents had <a href="https://en.wikipedia.org/wiki/Continental_drift">once been fused</a>. But without a mechanism to explain how the continents could actually “drift,” most geologists dismissed his ideas. His “amateur” status, combined with <a href="http://www.smithsonianmag.com/science-nature/when-continental-drift-was-considered-pseudoscience-90353214/?no-ist">anti-German sentiment</a> in the period after World War I, meant his hypothesis was deemed speculative at best.</p>
<p>In 1966, Tuzo Wilson built on earlier ideas to provide a missing link: the Atlantic ocean had opened and closed at least once before. By studying rock types, he found that parts of New England and Canada were of European origin, and that parts of Norway and Scotland were American. From this evidence, Wilson showed that the Atlantic Ocean had opened, closed and re-opened again, taking parts of its neighboring landmasses with it.</p>
<p>And there it was: proof our planet’s continents were not stationary.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/129045/original/image-20160701-18294-p1ohek.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/129045/original/image-20160701-18294-p1ohek.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/129045/original/image-20160701-18294-p1ohek.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=409&fit=crop&dpr=1 600w, https://images.theconversation.com/files/129045/original/image-20160701-18294-p1ohek.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=409&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/129045/original/image-20160701-18294-p1ohek.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=409&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/129045/original/image-20160701-18294-p1ohek.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=515&fit=crop&dpr=1 754w, https://images.theconversation.com/files/129045/original/image-20160701-18294-p1ohek.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=515&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/129045/original/image-20160701-18294-p1ohek.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=515&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The 15 major plates on our planet’s surface.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Plates_tect2_en.svg">USGS</a></span>
</figcaption>
</figure>
<h2>How plate tectonics works</h2>
<p>Earth’s crust and top part of the mantle (the next layer in toward the core of our planet) run about 150 km deep. Together, they’re called the <a href="https://www.geolsoc.org.uk/Plate-Tectonics/Chap2-What-is-a-Plate/Mechanical-properties-lithosphere-and-asthenosphere">lithosphere</a> and make up the “plates” in plate tectonics. We now know there are 15 major plates that cover the planet’s surface, moving at around the speed at which our fingernails grow.</p>
<p>Based on <a href="https://www.youtube.com/watch?v=phZeE7Att_s">radiometric dating</a> of rocks, we know that no ocean is more than 200 million years old, though our continents are much older. The oceans’ opening and closing process – called the <a href="https://www.youtube.com/watch?v=I_q3sAcuzIY">Wilson cycle</a> – explains how the Earth’s surface evolves. </p>
<p>A continent breaks up due to changes in the way molten rock in the Earth’s interior is flowing. That in turn acts on the lithosphere, changing the direction plates move. This is how, for instance, South America broke away from Africa. The next step is continental drift, sea-floor spreading, ocean formation – and hello, Atlantic Ocean. In fact, the Atlantic is still opening, generating new plate material in the middle of the ocean and making the flight from New York to London a few inches longer each year. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/125567/original/image-20160607-15024-19y7pwa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/125567/original/image-20160607-15024-19y7pwa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=464&fit=crop&dpr=1 600w, https://images.theconversation.com/files/125567/original/image-20160607-15024-19y7pwa.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=464&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/125567/original/image-20160607-15024-19y7pwa.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=464&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/125567/original/image-20160607-15024-19y7pwa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=583&fit=crop&dpr=1 754w, https://images.theconversation.com/files/125567/original/image-20160607-15024-19y7pwa.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=583&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/125567/original/image-20160607-15024-19y7pwa.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=583&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A simplified ‘Wilson Cycle’.</span>
<span class="attribution"><span class="source">Philip Heron</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Oceans close when their tectonic plate sinks beneath another, a process geologists call subduction. Off the Pacific Northwest coast of the United States, the ocean is slipping under the continent and into the mantle below the lithosphere, creating in slow motion Mount St Helens and the Cascade mountain range. </p>
<p>In addition to undergoing spreading (construction) and subduction (destruction), plates can simply rub up against each other - usually generating large earthquakes. These interactions, also discovered by Tuzo Wilson back in the 1960s, are termed “conservative.” All three processes occur at the edges of plate boundaries.</p>
<p>But the conventional theory of plate tectonics stumbles when it tries to explain some things. For example, what produces mountain ranges and earthquakes that occur within continental interiors, far from plate boundaries?</p>
<h2>Gone but not forgotten</h2>
<p>The answer may lie in a <a href="http://doi.org/10.1038/ncomms11834">map of ancient continental collisions</a> my colleagues and I assembled. </p>
<p>Over the past 20 years, improved computer power and mathematical techniques have allowed researchers to more clearly look below the Earth’s crust and explore the deeper parts of our plates. Globally, we find many instances of scarring left over from the ancient collisions of continents that formed our present-day continental interiors.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/129046/original/image-20160701-18291-vfhlh0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/129046/original/image-20160701-18291-vfhlh0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/129046/original/image-20160701-18291-vfhlh0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=354&fit=crop&dpr=1 600w, https://images.theconversation.com/files/129046/original/image-20160701-18291-vfhlh0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=354&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/129046/original/image-20160701-18291-vfhlh0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=354&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/129046/original/image-20160701-18291-vfhlh0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=445&fit=crop&dpr=1 754w, https://images.theconversation.com/files/129046/original/image-20160701-18291-vfhlh0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=445&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/129046/original/image-20160701-18291-vfhlh0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=445&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Present day plate boundaries (white) with hidden ancient plate boundaries that may reactivate to control plate tectonics (yellow). Regions where anomalous scarring beneath the crust are marked by yellow crosses.</span>
<span class="attribution"><a class="source" href="http://www.nature.com/ncomms/2016/160610/ncomms11834/fig_tab/ncomms11834_F1.html">Philip Heron</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>A map of ancient continental collisions may represent regions of hidden tectonic activity. These old impressions below the Earth’s crust may still govern surface processes – despite being so far beneath the surface. If these deep scarred structures (more than 30 km down) were reactivated, they would cause devastating new tectonic activity. </p>
<p>It looks like previous plate boundaries (of which there are many) may never really disappear. These <a href="https://eos.org/articles/tiny-mineral-grains-could-drive-plate-tectonics">inherited structures contribute to geological evolution</a>, and may be why we see geological activity within current continental interiors.</p>
<h2>Mysterious blobs 2,900 km down</h2>
<p>Modern geophysical imaging also shows <a href="https://en.wikipedia.org/wiki/Large_low-shear-velocity_provinces">two chemical “blobs”</a>
at the boundary of Earth’s core and mantle – thought to possibly stem from our planet’s formation. </p>
<p>These hot, dense piles of material lie beneath Africa and the Pacific. Located more than 2,900 km below the Earth’s surface, they’re difficult to study. And nobody knows where they came from or what they do. When these blobs of anomalous substance interact with cold ocean floor that has subducted from the surface down to the deep mantle, they generate hot plumes of mantle and blob material that cause <a href="https://philheron.com/lips/">super-volcanoes at the surface</a>. </p>
<p>Does this mean plate tectonic processes control how these piles behave? Or is it that the deep blobs of the unknown are actually controlling what we see at the surface, by releasing hot material to break apart continents? </p>
<p><a href="http://doi.org/10.1038/ngeo2733">Answers to these questions</a> have the potential to shake the very foundations of plate tectonics.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/125581/original/image-20160607-15041-th83ur.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/125581/original/image-20160607-15041-th83ur.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=159&fit=crop&dpr=1 600w, https://images.theconversation.com/files/125581/original/image-20160607-15041-th83ur.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=159&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/125581/original/image-20160607-15041-th83ur.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=159&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/125581/original/image-20160607-15041-th83ur.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=200&fit=crop&dpr=1 754w, https://images.theconversation.com/files/125581/original/image-20160607-15041-th83ur.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=200&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/125581/original/image-20160607-15041-th83ur.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=200&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Arizona State seismology expert Ed Garnero’s summary of how far we have come in over 100 years of studying the interior of the Earth.</span>
<span class="attribution"><a class="source" href="http://garnero.asu.edu/research_images/images_interp.html">Ed Garnero</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Plate tectonics in other times and places</h2>
<p>And the biggest question of all remains unsolved: How did plate tectonics even begin?</p>
<p>The early Earth’s interior <a href="http://www.livescience.com/42373-early-earth-crust-dripped.html">had significantly hotter temperatures</a> – and therefore different physical properties – than current conditions. Plate tectonics then may not be the same as what our conventional theory dictates today. What we understand of today’s Earth may have little bearing on its earliest beginnings; we might as well be thinking about <a href="https://theconversation.com/keep-a-lid-on-it-the-controversy-over-earths-oldest-rocks-19825">an entirely different world</a>.</p>
<p>In the coming years, we may be able to apply what we discover about how plate tectonics got started here to actual other worlds – the billions of exoplanets found in the <a href="http://www.bbc.co.uk/science/space/universe/sights/habitable_zones">habitable zone</a> of our universe. </p>
<figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/125589/original/image-20160607-15045-7a0xna.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/125589/original/image-20160607-15045-7a0xna.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=480&fit=crop&dpr=1 600w, https://images.theconversation.com/files/125589/original/image-20160607-15045-7a0xna.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=480&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/125589/original/image-20160607-15045-7a0xna.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=480&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/125589/original/image-20160607-15045-7a0xna.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=603&fit=crop&dpr=1 754w, https://images.theconversation.com/files/125589/original/image-20160607-15045-7a0xna.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=603&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/125589/original/image-20160607-15045-7a0xna.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=603&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Venus has some geologic features, but not plate tectonics.</span>
<span class="attribution"><a class="source" href="http://photojournal.jpl.nasa.gov/catalog/PIA00254">NASA/JPL</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>So far, amazingly, Earth is the only planet we know of that has plate tectonics. In our solar system, for example, <a href="https://en.wikipedia.org/wiki/Venus">Venus</a> is often considered Earth’s twin - just with a hellish climate and complete <a href="http://arstechnica.com/science/2014/04/venus-crust-heals-too-fast-for-plate-tectonics/">lack of plate tectonics</a>.</p>
<p>Incredibly, the ability of a planet to generate complex life is <a href="https://theconversation.com/does-a-planet-need-plate-tectonics-to-develop-life-61303">inextricably linked to plate tectonics</a>. A gridlocked planetary surface has helped produce Venus’ inhabitable toxic atmosphere of 96 percent CO₂. On Earth, <a href="http://doi.org/10.1038/nature13072">subduction helps push carbon down into the planet’s interior</a> and out of the atmosphere.</p>
<p>It’s still difficult to explain how <a href="https://en.wikipedia.org/wiki/Cambrian_explosion">complex life exploded all over our world 500 million years ago</a>, but the processes of removing carbon dioxide from the atmosphere is further <a href="http://www.astrobio.net/news-brief/earths-breathable-atmosphere-tied-plate-tectonics/">helped by continental coverage</a>. An exceptionally slow process starts with carbon dioxide mixing with rain water to wear down continental rocks. This combination can form carbon-rich limestone that subsequently washes away to the ocean floor. The long removal processes (even for geologic time) eventually could create a more breathable atmosphere. It just took 3 billion years of plate tectonic processes to get the right carbon balance for life on Earth.</p>
<h2>A theory works now, but what’s in the future?</h2>
<p>Fifty years on from Wilson’s 1966 paper, geophysicists have progressed from believing continents never moved to thinking that every movement may leave a lasting memory on our Earth. </p>
<p>Life here would be vastly different if plate tectonics changed its style – as we know it can. A changing mantle temperature may affect the interaction of our lithosphere with the rest of the interior, <a href="https://www.sciencenews.org/article/plate-tectonics-just-stage-earth%E2%80%99s-life-cycle">stopping plate tectonics</a>. Or those continent-sized chemical blobs could move from their relatively stable state, causing <a href="http://es.ucsc.edu/%7Ethorne/TL.pdfs/GLM_P4.pdf">super-volcanoes as they release material</a> from their deep reservoirs.</p>
<p>It’s hard to understand what our future holds if we don’t understand our beginning. By discovering the secrets of our past, we may be able to predict the motion of our plate tectonic future.</p><img src="https://counter.theconversation.com/content/55424/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Philip Heron receives funding from Natural Sciences and Engineering Research Council of Canada. He works for the University of Toronto. </span></em></p>Fifty years on from a groundbreaking paper, geophysicists have progressed from believing continents never moved to thinking that every movement may leave a lasting memory on our planet.Philip Heron, Postdoctoral Fellow in Geodynamics, University of TorontoLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/570922016-04-14T10:06:14Z2016-04-14T10:06:14ZHow could we build an invisibility cloak to hide Earth from an alien civilization?<figure><img src="https://images.theconversation.com/files/118208/original/image-20160411-21965-h6pv1u.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A laser could hide – or broadcast – our existence.</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/esoastronomy/8497613660">European Southern Observatory</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>What would it take to hide an entire planet? It sounds more like a question posed in an episode of “Star Trek” than in academic discourse, but sometimes the bleeding edge of science blurs with themes found in science fiction.</p>
<p>Of course we’ve been leaking our own position to distant stars via radio and television signals for six decades now, largely ignorant of the cosmic implications. But several notable scientists, <a href="http://www.hawking.org.uk/life-in-the-universe.html">such as Stephen Hawking</a>, have <a href="http://setiathome.berkeley.edu/meti_statement_0.html">publicly voiced concerns</a> about revealing our presence to other civilizations. These concerns largely draw from the darker chapters of our own history, when a more advanced civilization would subjugate and displace a less advanced one.</p>
<p>It might be too late for us to withdraw back into invisibility, but maybe not for other intelligent alien civilizations out there. A far-off planet’s inhabitants might prefer to hide from the likes of us. In 2016, my graduate student <a href="http://www.coolworlds.nyc">Alex Teachey and I</a> published a paper that <a href="http://doi.org/10.1093/mnras/stw672">proposes a way to cloak planets</a>, as well as a way to broadcast a civilization’s existence. Even if we’re not manipulating our own signal in this way, it doesn’t mean other planets out there aren’t. It’s possible what we see as we scan the universe for other habitable planets has been engineered to disguise or highlight the existence of other civilizations.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/118197/original/image-20160411-21959-10r7w7l.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/118197/original/image-20160411-21959-10r7w7l.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/118197/original/image-20160411-21959-10r7w7l.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=255&fit=crop&dpr=1 600w, https://images.theconversation.com/files/118197/original/image-20160411-21959-10r7w7l.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=255&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/118197/original/image-20160411-21959-10r7w7l.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=255&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/118197/original/image-20160411-21959-10r7w7l.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=321&fit=crop&dpr=1 754w, https://images.theconversation.com/files/118197/original/image-20160411-21959-10r7w7l.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=321&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/118197/original/image-20160411-21959-10r7w7l.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=321&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">When a planet passes between us and its star, the star’s light seems to dim.</span>
<span class="attribution"><a class="source" href="http://www.nasa.gov/mission_pages/kepler/multimedia/images/transit-light-curve.html">NASA Ames</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Tracking transits to find other planets</h2>
<p>Before we talk about how to hide a planet from distant voyeurs, consider the best way we’ve figured out to find one.</p>
<p>Humanity’s most successful technique for detecting other planets is the transit method. A transit occurs when a planet appears to pass in front of its parent sun, blocking out some of its starlight for a few hours. So if we have our telescopes trained at one part of the universe and a star seems to fade out for part of a day, that tells us that a planet has temporarily come between us as it goes about its orbit.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/118219/original/image-20160411-21963-76ym4b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/118219/original/image-20160411-21963-76ym4b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/118219/original/image-20160411-21963-76ym4b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/118219/original/image-20160411-21963-76ym4b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/118219/original/image-20160411-21963-76ym4b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/118219/original/image-20160411-21963-76ym4b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=565&fit=crop&dpr=1 754w, https://images.theconversation.com/files/118219/original/image-20160411-21963-76ym4b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=565&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/118219/original/image-20160411-21963-76ym4b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=565&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Kepler Mission identified 4,696 planet candidates by July 2015.</span>
<span class="attribution"><a class="source" href="http://www.nasa.gov/ames/kepler/kepler-planet-candidates-july-2015">NASA Ames/W. Stenzel</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Using this technique, <a href="http://www.nasa.gov/mission_pages/kepler/overview/index.html">NASA’s Kepler Mission</a> has <a href="http://kepler.nasa.gov/Mission/discoveries/">discovered several thousand planets</a>.</p>
<p>It seems likely that any advanced civilization would be aware of this simple method. Each time a planet transits its star, its existence is essentially being advertised to all points lying along the same plane as the planet and star.</p>
<p>An advanced civilization might be okay having its planet’s location, size and even <a href="http://www.exoclimes.com/topics/transmission-spectroscopy/">atmospheric chemistry</a> advertised across the cosmos. Or it might wish to conceal its presence. If the latter, it might choose to build a cloak.</p>
<h2>A planetary invisibility cloak</h2>
<p>It turns out that hiding planets from the transit method would be surprisingly easy, so easy that we earthlings could do it right now, if we chose. Since transits appear as a brightness decrease of a distant star, our hypothetical cloak simply produces the opposite brightness increase.</p>
<p>Lasers provide an efficient means of countering that dip in brightness. All a laser’s power is contained in a relatively narrow beam, as opposed to spreading out in all directions like starlight does. Due to the way light spreads as it travels – called diffraction – the laser beam would spread to encompass entire solar systems after journeying many light years across space, bathing that distant planetary system within the cloaking beam. No dip in brightness makes it look like there’s no planet there at all.</p>
<p>A laser cloak capable of hiding the Earth from an alien version of NASA’s Kepler Mission would require 30 megawatts of power at peak intensity, approximately equivalent to 10 wind turbines worth of power output.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/z1Pqqf_6J9w?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Alex Teachey describes how a cloaking system would work.</span></figcaption>
</figure>
<p>While Kepler sees light in only one color, advanced civilizations might use more sophisticated detectors capable of collecting light at all wavelengths. Here too, our current technology could cloak us using <a href="https://www.rp-photonics.com/supercontinuum_generation.html">modern tunable lasers</a>, for a cost of about 10 times more power overall. More advanced civilizations might be able to detect other fine details of the light’s properties, betraying the cloak. But here too there’s no reason why with a little bit of work we couldn’t engineer solutions, leading to a near perfect cloak which could be targeted at distant stars where we suspect someone might be home.</p>
<h2>Why choose to hide</h2>
<p>So yes, it sounds like science fiction, but even current technology could do a fine job of cloaking the Earth’s transit signature.</p>
<p>Forget the Earth though; we never really thought of this as something humanity should or should not do. Instead, we posit that if our rudimentary human technology can build such an effective transit cloak at relatively little economic cost, then more advanced civilizations may be able to hide from us with respect to all detection techniques. The universe might not be all that it seems.</p>
<p>Why might a civilization choose to wrap itself in invisibility? It could be a sort of insurance policy: find the nearby planets with potential for supporting life and turn on a targeted cloak – just in case a civilization ever emerges. Such a policy effectively buys them time to reveal their presence when they see fit.</p>
<p>Given how cheap such a cloak would be, an insurance policy for your home planet is perhaps not as strange as it seems. It’s certainly not implausible a civilization might want to bide its time – surveilling the neighbors for a while before rolling out the intergalactic welcome mat. But there’s a flip side to this technology that could turn it from an invisibility cloak into more of a we-are-here spotlight.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/118216/original/image-20160411-21965-1l0nhwy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/118216/original/image-20160411-21965-1l0nhwy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/118216/original/image-20160411-21965-1l0nhwy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/118216/original/image-20160411-21965-1l0nhwy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/118216/original/image-20160411-21965-1l0nhwy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/118216/original/image-20160411-21965-1l0nhwy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/118216/original/image-20160411-21965-1l0nhwy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">We think we know what it means when we see certain transit signals….</span>
<span class="attribution"><a class="source" href="http://www.spacetelescope.org/images/opo1354a/">NASA Goddard Space Flight Center</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>The reverse: flick on the beacon</h2>
<p>Perhaps not all civilizations are xenophobic – some might want to talk. If you wanted to reveal your presence to other civilizations as cheaply and unambiguously as possible, how might you do it?</p>
<p>Imagine looking at some data of a distant planet – which has become a somewhat normal enterprise for astronomers – and noticing something weird. The signature of the planet has a strange shape – in fact, none of your models are able to explain it. It looks like someone has imprinted a series of spikes into the data, following the prime number series. Nothing in nature can do this – you have just detected another civilization’s beacon. Alternative use of the cloaking system’s laser could be to make a planet’s signal look highly artificial, instead of hidden. Now they don’t care about building the perfect cloak; they want to be found!</p>
<p>Could such signals be lurking in our existing measurements? Perhaps so. No one has ever looked, and we hope our work sparks efforts on that front. It may be a long shot, since to even get to this point we need to try to imagine how aliens might think – but given the scientific prize on offer it’s also worth it. If we identify a strange transit, it may well contain information encoded via laser light pulses. Huge volumes of information could be hidden within the transit signatures of other planets.</p>
<p>For us, this was an exercise in intellectual curiosity. We simply calculated how much energy it would take to either cloak or broadcast a planet’s existence. Whether we should seriously consider wrapping Earth in a protective cloak of invisibility – or conversely, getting serious about trumpeting our existence – via laser manipulations is something we should all decide together.</p><img src="https://counter.theconversation.com/content/57092/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Kipping does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>There are technological ways to hide a planet from intergalactic detection – as well as ways to signal that we’re just sitting here, eager for contact.David Kipping, Assistant Professor of Astronomy, Columbia UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/450012015-07-21T15:19:07Z2015-07-21T15:19:07ZIf we are to find life beyond Earth, we need to be explorers, not hunters<figure><img src="https://images.theconversation.com/files/89187/original/image-20150721-24261-h852n6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">What secrets will space reveal?</span> <span class="attribution"><a class="source" href="http://www.shutterstock.com/cat.mhtml?lang=en&language=en&ref_site=photo&search_source=search_form&version=llv1&anyorall=all&safesearch=1&use_local_boost=1&autocomplete_id=&search_tracking_id=7aHh5Z2HIn3Mj2zn8iaYvg&searchterm=through%20the%20telescope&show_color_wheel=1&orient=&commercial_ok=&media_type=images&search_cat=&searchtermx=&photographer_name=&people_gender=&people_age=&people_ethnicity=&people_number=&color=&page=1&inline=193325363">AstroStar</a></span></figcaption></figure><p>The <a href="http://www.theguardian.com/science/2015/jul/20/breakthrough-listen-massive-radio-wave-project-scan-far-regions-for-alien-life">news that</a> the <a href="http://www.seti.org">search for extraterrestrial intelligence</a> is to receive increased funding and data through the $100m (£64m) <a href="http://www.wired.co.uk/news/archive/2015-07/20/breakthrough-listen-project">Breakthrough Listen</a> project is welcome news for <a href="https://depts.washington.edu/astrobio/drupal/content/what-astrobiology">astrobiologists</a> like myself. Launched by Stephen Hawking, it particularly helps to allay growing concerns in the field about having too narrow a focus in our search for life in the universe.</p>
<p>Last week I attended the <a href="http://pathways2015.sciencesconf.org">Pathways Towards Habitable Planets</a> conference in Switzerland, where leading scientists in the search for habitable planets shared their results and ideas for the future. What was especially interesting was the relatively strong consensus on the problems with our definition of the <a href="http://www.bbc.co.uk/science/space/universe/sights/habitable_zones">habitable zone</a> – the area around a star which is neither too hot nor too cold for orbiting planets to support liquid water on the surface. Even its name is misleading, as we’ll see in a moment. If we aren’t careful, obsessing about this zone could prevent us from reaching our ultimate goal of finding extraterrestrial life. </p>
<p>For as long as we have considered planets orbiting other stars, we have speculated over their propensity to host living organisms in the way that the Earth does. The habitable zone concept has helped astronomers to define where, in all those quintillions of acres of galactic real estate, we should search for planets that might be inhabited.</p>
<p>It may seem sensible to look for extraterrestrial life in regions where any Earth-like planet would have liquid water on the surface. Liquid water is an essential solvent for the chemical reactions that Earth biology relies on. If we find planets with liquid water, they satisfy a key criterion for being conducive to life as we know it.</p>
<p>Yet being in the zone neither automatically means that a planet will have water, nor that it could support life. It needs to have a “healthy” atmospheric composition – usually assumed to mean similar to Earth’s – and ideally a healthy magnetic field to shield it from high-energy particles belched forth by its parent star. </p>
<p>We might also demand that the planet’s orbit and rotation is stable and that any planetary neighbours kindly leave it alone. We don’t have enough data on the planets we have found to date to know if they meet all these criteria. Even if we did, we would most likely have to run a sophisticated computer simulation to model their climate before we could determine what conditions were really like on the surface.</p>
<p>These difficulties with the definition of the habitable zone can lead to astronomers and astrobiologists coming a cropper when speaking to the press. When a press release announces the detection of “a planet in the habitable zone”, the general public reads “a habitable planet”. It’s this confusion that prompted the discussion at the Pathways conference on whether we should change the zone’s name to something else – perhaps the surface liquid water zone, or the temperate zone.</p>
<p>Beyond this, there are other problems with the concept. Perhaps life doesn’t require surface liquid water to survive at all. Some <a href="http://advances.sciencemag.org/content/1/1/e1400067">have speculated</a> that the liquid hydrocarbons on Titan, Saturn’s largest moon, could be a solvent for a very different form of life, for example. </p>
<p>Other moons in our solar system, <a href="http://www.nasa.gov/press/2015/march/spacecraft-data-suggest-saturn-moons-ocean-may-harbor-hydrothermal-activity">such as</a> Europa and Enceladus, meanwhile, appear
to have subsurface liquid water even though they reside outside the traditional habitable zone. The tidal heating they receive from their host planets is enough to make habitable zones beyond the habitable zone, if that’s not too confusing. The more we learn about other planets, the more the simplicity of the habitable zone’s definition begins to look dangerous. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/89188/original/image-20150721-24301-gm4bqq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/89188/original/image-20150721-24301-gm4bqq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/89188/original/image-20150721-24301-gm4bqq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=420&fit=crop&dpr=1 600w, https://images.theconversation.com/files/89188/original/image-20150721-24301-gm4bqq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=420&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/89188/original/image-20150721-24301-gm4bqq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=420&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/89188/original/image-20150721-24301-gm4bqq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=528&fit=crop&dpr=1 754w, https://images.theconversation.com/files/89188/original/image-20150721-24301-gm4bqq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=528&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/89188/original/image-20150721-24301-gm4bqq.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">Could Titan’s methane lakes host life?</span>
<span class="attribution"><a class="source" href="http://www.shutterstock.com/downloading_tips.mhtml?code=&id=161764496&size=medium&image_format=jpg&method=download&super_url=http%3A%2F%2Fdownload.shutterstock.com%2Fgatekeeper%2FW3siZSI6MTQzNzUwODY2MiwiYyI6Il9waG90b19zZXNzaW9uX2lkIiwiZGMiOiJpZGxfMTYxNzY0NDk2IiwiayI6InBob3RvLzE2MTc2NDQ5Ni9tZWRpdW0uanBnIiwibSI6IjEiLCJkIjoic2h1dHRlcnN0b2NrLW1lZGlhIn0sImp4bUZ6cHptU1ZaQXRMZkZ4Z0trTHRsVHVFVSJd%2Fshutterstock_161764496.jpg&racksite_id=ny&chosen_subscription=1&license=standard&src=H_vmISXeBm2ZqBNa8OEh1g-1-12&el_order_id=">manjik</a></span>
</figcaption>
</figure>
<h2>The need for focus</h2>
<p>So why have astronomers persisted with the concept? The real reason is target selection. There are lots of stars in the Milky Way – and we now know of lots of planets surrounding them. Astronomers have limited resources, and not all astronomers want to search for biospheres.</p>
<p>Because we can only observe a few targets, we choose the ones that we think have a higher chance of yielding signs of life. Depending on how you detect them, most candidates are merely silhouettes on a star’s surface or wobbles in a star’s orbit. If we’re lucky, some are both – or we have managed to discover some information about molecules in their atmospheres using <a href="http://www.nature.com/nature/journal/v505/n7481/full/nature12888.html">transit spectroscopy</a>, which is the study of the light the planet reflects from its parent star. </p>
<p><a href="http://tess.gsfc.nasa.gov">The</a> next <a href="http://cheops.unibe.ch">generation</a> of <a href="http://sci.esa.int/plato/">exoplanet observations</a> is <a href="https://www.eso.org/sci/facilities/develop/instruments/espresso.html">designed</a> to ensure we glean the maximum amount of information about as many planets as possible. This is in advance of the coming <a href="http://www.eso.org/public/unitedkingdom/teles-instr/e-elt/">extremely large telescopes</a>, which may be able to <a href="http://www.planetary.org/explore/space-topics/exoplanets/direct-imaging.html">directly image</a> any “Earth-like” planets nearby. </p>
<p>Yet this isn’t an excuse for going down the wrong path. It might be tempting to rush to the end of the search – hunting exclusively in the habitable zone – but we might be rushing to the wrong end. Consequently many scientists are saying we shouldn’t be looking for things that look like life, but merely things that look anomalous and can’t be explained by geochemical, non-biological processes. </p>
<p>The weird blooms of methane in Mars’ atmosphere <a href="http://www.researchgate.net/profile/M_Giuranna/publication/51366439_Detection_of_methane_in_the_atmosphere_of_Mars/links/00b7d532207f6d7510000000.pdf">pointed towards life</a>, for instance. This turned out to be something of a false alarm, since they can also be explained without requiring organisms, as can many other potential <a href="http://arxiv.org/abs/1407.2622">signs of life</a> on the planet. Frustrations aside, such anomalies are still worth exploring. The more we find, the more likely we are to find one caused by organisms.</p>
<p>I’m pleased to say that Breakthrough Listen is in the spirit of this approach. It will focus on sifting data from radio and infrared telescopes for signs of extraterrestrial intelligence. It will not restrict its focus to zones, particular conditions, or even planets at all, but scan more widely to look for signals that can’t be explained by natural phenomena. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/89189/original/image-20150721-24286-n2t3mc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/89189/original/image-20150721-24286-n2t3mc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/89189/original/image-20150721-24286-n2t3mc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=433&fit=crop&dpr=1 600w, https://images.theconversation.com/files/89189/original/image-20150721-24286-n2t3mc.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=433&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/89189/original/image-20150721-24286-n2t3mc.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=433&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/89189/original/image-20150721-24286-n2t3mc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=544&fit=crop&dpr=1 754w, https://images.theconversation.com/files/89189/original/image-20150721-24286-n2t3mc.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=544&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/89189/original/image-20150721-24286-n2t3mc.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=544&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">We should be exploring every kind of planet, not just hunting for ‘blue marbles’ like our own.</span>
<span class="attribution"><a class="source" href="http://www.shutterstock.com/cat.mhtml?lang=en&language=en&ref_site=photo&search_source=search_form&version=llv1&anyorall=all&safesearch=1&use_local_boost=1&autocomplete_id=&searchterm=planet%20earth&show_color_wheel=1&orient=&commercial_ok=&media_type=images&search_cat=&searchtermx=&photographer_name=&people_gender=&people_age=&people_ethnicity=&people_number=&color=&page=1&inline=132903536">Fisherss</a></span>
</figcaption>
</figure>
<p>Every planet we find and learn about – even hellish worlds such as Venus, or gas giants such as Jupiter – is a piece of the puzzle of how planets form and evolve. They all help us learn how biospheres are born and how common or rare we really are. As we put our blue marble into ever clearer context, it’s my fond hope it will help us appreciate and cherish our singular, complex, beautiful world all the more.</p>
<p>As <a href="http://www.u-bordeaux1.fr/red08/fiche_selsis.htm">Franck Selsis</a>, a leading figure in finding and characterising potentially habitable worlds, said at the Pathways conference, “Perhaps the best strategy is to have no strategy – except to simply explore”.</p><img src="https://counter.theconversation.com/content/45001/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Duncan is supported by the ECOGAL program, which is funded by a European Research Council Advanced Grant. He is a member of the Royal Society of Edinburgh's Young Academy of Scotland, and a founding member of the UK SETI Research Network.</span></em></p>Why the Breakthrough Listen project is a step in the right direction in our hunt for life beyond Earth.Duncan Forgan, Research Fellow, University of St AndrewsLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/289962014-07-09T15:04:39Z2014-07-09T15:04:39ZNo, that new exoplanet is not the best candidate to support life<figure><img src="https://images.theconversation.com/files/53425/original/j97cpkk6-1404907829.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Building a space colony is not easy.</span> <span class="attribution"><a class="source" href="http://en.wikipedia.org/wiki/OGLE-2005-BLG-390Lb#mediaviewer/File:OGLE-2005-BLG-390Lb_planet.jpg">NASA</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>In June, the <a href="http://www.huffingtonpost.com/2014/06/27/alien-planet-life-gliese-832c_n_5536601.html">popular press</a> <a href="http://www.iflscience.com/space/nearby-exoplanet-best-candidate-supporting-life">went wild</a>. A planet had been discovered that was so much like Earth it was heralded as our best bet for supporting life. Positioned 16 light years away, Gliese (or GJ) 832c was a mere hop from home and there were rumours a popular coffee shop chain had already applied for planning permission.</p>
<p>And this was all utterly wrong.</p>
<p>The <a href="http://adsabs.harvard.edu/abs/2014arXiv1406.5587W">journal article</a>, published by a team led by Robert Wittenmyer from the University of New South Wales in Australia, described a world boiling under its own stifling cloud cover. With an orbit that only skirted the region capable of maintaining water, and a mass sufficient to attract a thick atmosphere, the planet was designated a likely “super Venus” and unsuitable for life. Worse still, it was so close to its star that it risked being in a tidal lock, with one side doused in the heat of a perpetual day while its reverse remained shrouded in night. </p>
<p>GJ 832c is a planet to inspire thoughts of Dante’s Inferno and the facts were clearly laid out in the freely available journal article. How then did the press <a href="http://www.sci-news.com/astronomy/science-gliese832c-potentially-habitable-super-earth-02029.html">get it</a> so <a href="http://io9.com/this-potentially-habitable-super-earth-is-just-16-light-1595894609">completely</a> <a href="http://www.astrobio.net/news-brief/high-score-easy-scale-gliese-832c-potentially-planetary-habitability/">wrong</a>?</p>
<h2>Falling for a number</h2>
<p>The answer lies in the use of a quantity denoted as the “Earth Similarity Index” or <a href="http://phl.upr.edu/projects/earth-similarity-index-esi">ESI</a>. It is designed to indicate how “Earth-like” a planet might be. The problem is that the resulting number is a weak comparison between the two objects. It cannot be equated to how suitable the planet may be for supporting life.</p>
<p>Accessing the habitability of a world outside our solar system is no easy task. The information planetary scientists have about a planet is limited. They are typically working with only the mass (and sometimes radius) of the planet and the type of star it is orbiting. It is this data that ESI uses, and that is why it is fallacious.</p>
<p>The distance at which the planet is orbiting a particular star can be used to define the “Habitable Zone”. Coined in 1959 by scientist Su-Shu Huang working at the University of California at Berkeley, the Habitable Zone around a star is the location where water could exist on the surface of a planet, if that planet had a sufficient atmospheric pressure.</p>
<p>The caveats here are important: the Habitable Zone does not say there is water present, or that a planet exists that might be able to support life. It only marks out a region where the amount of stellar radiation would not boil nor freeze water. </p>
<p>In short, very few planets found in the Habitable Zone of their star will be suitable for life, but if another Earth-like planet existed, it would be there. This makes the region a prime search area for future missions targeted towards habitability.</p>
<p>In the case of GJ 832c, the planet orbits just inside the inner edge of its Habitable Zone, providing the location of its edge is extended to its most generous estimate. The generosity requires scientists to include the possibility of water that is only supported during the early evolution of the planet, as is thought to have been true for Mars and Venus.</p>
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<span class="attribution"><a class="source" href="http://phl.upr.edu/press-releases/gliese832">PHL @ UPR Arecibo</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
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<p>However, it is not its proximity to the edge of the Habitable Zone that kills the deal for GJ 832c. With a measured mass at least five times that of the Earth, GJ 832c is capable of attracting a thick atmosphere. Even if the gas constitution was similar to that on Earth, this would result in a high quantity of greenhouse gases that trap heat reflected from the planet’s surface, evaporating all the water, if any existed there. </p>
<p>If that is not unpleasant enough, the higher planet mass could result in hydrogen and helium being retained in the planet’s atmosphere – which otherwise escape Earth’s weak gravity – producing a terrible atmospheric mess from the perspective of humans someday living there.</p>
<p>Finally, GJ 832c is very close to its star, with an orbit of only 35.8 days. Since the star is small and relatively cool, the planet is saved from the cooked fate of Mercury, but the gravitational pull is likely to be sufficient to cause a tidal lock, much like our moon is locked with one side permanently facing the Earth. This causes further problems for life to exist.</p>
<h2>A number to rule them all</h2>
<p>The calculation of the ESI is based on four parameters: mean radius, bulk density, escape velocity and surface temperature. They are manipulated in way that on multiplication the value is between 0.0 and 1.0. Any value above 0.8 is considered a near-Earth match. GJ 832c has an ESI of 0.81.</p>
<p>The ESI value holds no water (pun intended). For instance Venus with its estimated temperature would give it an incredible 0.9 match with the Earth.</p>
<p>“All conversations regarding habitability are worthless if we ignore the limited data present within our own solar system,” Stephen Kane, a planetary scientist from San Francisco State University said. “If we start defining Venusian planets as being habitable then we are no longer doing science.”</p>
<p>While the mass, radius and temperature certainly have a bearing on the planet’s environment, they are overwhelmed by other factors that contribute to the support of life. For example, water is <a href="https://theconversation.com/looks-like-a-comet-but-feels-like-an-asteroid-thats-wild-20268">thought</a> to have been delivered to the Earth by ice-rich meteorites scattered inwards by the outer planets during its formation. A different system of planets could bypass this process, allowing Earth’s twin to form but be uninhabitable to all known forms of life. </p>
<p>Similarly, the Earth’s magnetic field protects it from harmful solar radiation, its distance from the sun allows it to rotate hundreds of times per orbit to ensure an even distribution of heat and the sun itself is a quiet star without violent radiation outbursts that could overwhelm the Earth’s defences. These are a small fraction of the processes that will determine a planet’s suitability for life and none of them are included in the ESI.</p>
<p>The upshot of this is that a planet with an ESI of 0.1 is just as likely to support life as one with ESI 0.99. The ESI is neither used nor referred to in scientific publications, but its tantalising one-number answer has caused it to propagate rapidly through the majority of science news sites. The result is articles that are, at best, misleading but, most of the time, just wrong.</p>
<p>Perhaps though, the most disappointing fact about the ESI is that it detracts from the real excitement of GJ 832c. With its high mass and thick atmosphere, the planet has been declared a “super Venus”, for which the only other example (Kepler 69c) was discovered last year. “Understanding how common they (Venus-like planets) are will help us to decode why the atmosphere of Venus so radically diverged from its sister planet, Earth,” Kane explained.</p>
<hr>
<p><em>This article is an edited version of a post that appeared on <a href="http://physicsfocus.org/uninhabitable-planet-debunking-esi/">Physics Focus</a>. Next, read this: <a href="https://theconversation.com/habitable-exoplanets-are-bad-news-for-humanity-25838">Habitable exoplanets are bad news for humanity</a></em></p><img src="https://counter.theconversation.com/content/28996/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Elizabeth Tasker 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>In June, the popular press went wild. A planet had been discovered that was so much like Earth it was heralded as our best bet for supporting life. Positioned 16 light years away, Gliese (or GJ) 832c was…Elizabeth Tasker, Assistant Professor, Hokkaido UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/258382014-04-23T11:43:24Z2014-04-23T11:43:24ZHabitable exoplanets are bad news for humanity<figure><img src="https://images.theconversation.com/files/46907/original/yzhm4pf8-1398247831.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Let's hope it's barren.</span> <span class="attribution"><span class="source">NASA Ames/SETI Institute/JPL-CalTech</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>Last week, scientists <a href="http://www.nasa.gov/ames/kepler/kepler-186f-the-first-earth-size-planet-in-the-habitable-zone/#.U1EqAuZdX5o">announced</a> the discovery of Kepler-186f, a planet 492 light years away in the Cygnus constellation. Kepler-186f is special because it marks the first planet almost exactly the same size as Earth orbiting in the “habitable zone” – the distance from a star in which we might expect liquid water, and perhaps life. </p>
<p>What did not make the news, however, is that this discovery also slightly increases how much credence we give to the possibility of near-term human extinction. This is because of a concept known as the <a href="http://hanson.gmu.edu/greatfilter.html">Great Filter</a>.</p>
<p>The Great Filter is an argument that attempts to resolve the <a href="http://www.seti.org/seti-institute/project/details/fermi-paradox">Fermi Paradox</a>: why have we not found aliens, despite the existence of hundreds of billions of solar systems in our galactic neighbourhood in which life might evolve? As the namesake physicist Enrico Fermi noted, it seems rather extraordinary that not a single extraterrestrial signal or <a href="http://www.aleph.se/Nada/dysonFAQ.html">engineering project</a> has been detected (UFO conspiracy theorists notwithstanding).</p>
<p>This apparent absence of thriving extraterrestrial civilisations suggests that at least one of the steps from humble planet to interstellar civilisation is exceedingly unlikely. The absence could be caused because either intelligent life is extremely rare or intelligent life has a tendency to go extinct. This bottleneck for the emergence of alien civilisations from any one of the many billions of planets is referred to as the Great Filter.</p>
<h2>Are we alone?</h2>
<p>What exactly is causing this bottleneck has been the subject of debate for more than 50 years. Explanations could include a paucity of Earth-like planets or self-replicating molecules. Other possibilities could be an improbable jump from simple prokaryotic life (cells without specialised parts) to more complex eukaryotic life – after all, this transition took well over a billion years on Earth.</p>
<p>Proponents of this “Rare Earth” hypothesis also argue that the evolution of complex life requires an exceedingly large number of perfect conditions. In addition to Earth being in the habitable zone of the sun, our star must be far enough away from the galactic centre to avoid destructive radiation, our gas giants must be massive enough to sweep asteroids from Earth’s trajectory, and our unusually large moon stabilises the axial tilt that gives us different seasons. </p>
<p>These are just a few prerequisites for complex life. The emergence of symbolic language, tools and intelligence could require other such “perfect conditions” as well.</p>
<h2>Or is the filter ahead of us?</h2>
<p>While emergence of intelligent life could be rare, the silence could also be the result of intelligent life emerging frequently but subsequently failing to survive for long. Might every sufficiently advanced civilisation stumble across a suicidal technology or unsustainable trajectory? We know that a Great Filter prevents the emergence of prosperous interstellar civilisations, but we don’t know whether or not it lies in humanity’s past or awaits us in the future.</p>
<p>For 200,000 years humanity has survived supervolcanoes, asteroid impacts, and naturally occurring pandemics. But our track record of survival is limited to just a few decades in the presence of nuclear weaponry. And we have no track record at all of surviving many of the radically novel technologies that are <a href="http://www.bbc.co.uk/news/business-22002530">likely to arrive this century</a>. </p>
<p>Esteemed scientists such as Astronomer Royal Martin Rees at the Cambridge Centre for the Study of Existential Risk <a href="https://theconversation.com/astronomer-royal-on-science-environment-and-the-future-18162">point</a> to advances in biotechnology as being potentially catastrophic. Others such as Stephen Hawking, Max Tegmark and Stuart Russell, also with the Cambridge Centre, have <a href="http://www.huffingtonpost.com/stephen-hawking/artificial-intelligence_b_5174265.html">expressed</a> serious concern about the exotic but understudied possibility of machine superintelligence.</p>
<h2>Let’s hope Kepler-186f is barren</h2>
<p>When the Fermi Paradox was initially proposed, it was thought that planets themselves were rare. Since then, however, the tools of astronomy have revealed the existence of hundreds of exoplanets. That just seems to be the tip of the iceberg.</p>
<p>But each new discovery of an Earth-like planet in the habitable zone, such as Kepler-186f, makes it less plausible that there are simply no planets aside from Earth that might support life. The Great Filter is thus more likely to be lurking in the path between habitable planet and flourishing civilisation. </p>
<p>If Kepler-186f is teeming with intelligent life, then that would be <a href="http://www.nickbostrom.com/extraterrestrial.pdf">really bad news</a> for humanity. For that fact would push back the Great Filter’s position further into the technological stages of a civilisation’s development. We might then expect that catastrophe awaits both our extraterrestrial companions and ourselves.</p>
<p>In the case of Kepler-186f, we still have many reasons to think intelligent life might not emerge. The atmosphere might be too thin to prevent freezing, or the planet <a href="http://www.nasa.gov/sites/default/files/files/kepler186_main_final.pdf">might</a> be tidally locked, causing a relatively static environment. Discovery of these hostile conditions should be cause for celebration. As philosopher Nick Bostrom <a href="http://www.nickbostrom.com/extraterrestrial.pdf">once said</a>:</p>
<blockquote>
<p>The silence of the night sky is golden … in the search for extraterrestrial life, no news is good news. It promises a potentially great future for humanity.</p>
</blockquote><img src="https://counter.theconversation.com/content/25838/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andrew Snyder-Beattie works at the Future of Humanity Institute at the University of Oxford.</span></em></p>Last week, scientists announced the discovery of Kepler-186f, a planet 492 light years away in the Cygnus constellation. Kepler-186f is special because it marks the first planet almost exactly the same…Andrew Snyder-Beattie, University of OxfordLicensed as Creative Commons – attribution, no derivatives.