tag:theconversation.com,2011:/au/topics/brown-dwarf-10210/articlesBrown dwarf – The Conversation2022-01-14T10:47:26Ztag:theconversation.com,2011:article/1746222022-01-14T10:47:26Z2022-01-14T10:47:26ZRogue planets: how wandering bodies in interstellar space ended up on their own<figure><img src="https://images.theconversation.com/files/440049/original/file-20220110-21-1dex06f.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C1280%2C1241&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The locations of 115 candidate free floating planets in the region between Upper Scorpius and Ophiuchus</span> <span class="attribution"><a class="source" href="https://www.eso.org/public/images/eso2120c/">European Southern Observatory</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>We now know of almost 5,000 planets outside the Solar System. If you were to picture what it would be like on one of these distant worlds, <a href="https://theconversation.com/explainer-how-do-you-find-exoplanets-24153">or exoplanets</a>, your mental image would probably include a parent star – or more than one, especially if you’re a Star Wars fan.</p>
<p>But scientists have <a href="https://www.eso.org/public/archives/releases/sciencepapers/eso2120/eso2120a_en.pdf">recently discovered</a> that more planets than we thought are floating through space all by themselves – unlit by a friendly stellar companion. These are icy “free-floating planets”, or FFPs. But how did they end up all on their own and what can they tell us about how such planets form?</p>
<p>Finding more and more exoplanets to study has, as we might have expected, widened our understanding of what a planet is. In particular, the line between planets and “<a href="https://theconversation.com/exoplanet-discovery-blurs-the-line-between-large-planets-and-small-stars-124150">brown dwarfs</a>” – cool stars that can’t fuse hydrogen like other stars – has become increasingly blurred. What dictates whether an object is a planet or a brown dwarf has long been <a href="https://theconversation.com/how-can-some-planets-be-hotter-than-stars-weve-started-to-unravel-the-mystery-156427">the subject of debate</a> – is it a question of mass? Do objects cease to be planets if they are undergoing nuclear fusion? Or is the way in which the object was formed most important?</p>
<p>While about half of stars and brown dwarfs exist in isolation, with the rest in multiple star systems, we typically think of planets as subordinate objects in orbit around a star. More recently, however, improvements in telescope technology have enabled us to see smaller and cooler isolated objects in space, including FFPs – objects that have too low a mass or temperature to be considered brown dwarfs.</p>
<figure class="align-center ">
<img alt="Infographic about the differences between planets, brown dwarfs and low mass stars. " src="https://images.theconversation.com/files/440048/original/file-20220110-25-7f3zuu.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/440048/original/file-20220110-25-7f3zuu.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/440048/original/file-20220110-25-7f3zuu.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/440048/original/file-20220110-25-7f3zuu.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/440048/original/file-20220110-25-7f3zuu.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/440048/original/file-20220110-25-7f3zuu.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/440048/original/file-20220110-25-7f3zuu.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The different characteristics of free floating planets, brown dwarfs and low mass stars. 13 Jupiter masses is often used to distinguish planets from brown dwarfs. Please note that size and mass are different entities – brown dwarfs are roughly the size of Jupiter although more massive.</span>
<span class="attribution"><span class="source">Author's own work</span></span>
</figcaption>
</figure>
<p>What we still don’t know is exactly how these objects formed. Stars and brown dwarfs form when a region of dust and gas in space starts to fall in on itself. This region becomes denser, so more and more material falls onto it (due to gravity) in a process dubbed gravitational collapse.</p>
<p>Eventually this ball of gas becomes dense and hot enough for nuclear fusion to start – hydrogen burning in the case of stars, deuterium (a type of hydrogen with an additional particle, a neutron, in the nucleus) burning for brown dwarfs. FFPs may form in the same way, but just never get big enough for fusion to start. It’s also possible such a planet could start off life in orbit around a star, but at some point get kicked out into interstellar space. </p>
<h2>How to spot a wandering planet</h2>
<p>Rogue planets are difficult to spot because they are relatively small and cold. Their only source of internal heat is the remaining energy left over from the collapse that resulted in their formation. The smaller the planet, the quicker that heat will be radiated away. </p>
<p>Cold objects in space emit less light, and the light they do emit is redder. A star like the Sun has its peak emission in the visible range; the peak for an FFP is instead in the infrared. Because it’s challenging to see them directly, many such planets have been found using the indirect method of “gravitational microlensing”, when a distant star is in just the right position for its light to be gravitationally distorted by the FFP.</p>
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Read more:
<a href="https://theconversation.com/rogue-planets-hunting-the-galaxys-most-mysterious-worlds-149588">Rogue planets: hunting the galaxy's most mysterious worlds</a>
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<p>However, detecting planets via a single, unique event comes with the disadvantage that we can’t ever observe that planet again. We also don’t see the planet in context with its surroundings, so we’re missing some vital information. </p>
<p>To observe FFPs directly, the best strategy is to catch them while they are young. That means there is still a reasonable amount of heat left over from their formation, so they are at their brightest. In the recent study, researchers did just that. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/440701/original/file-20220113-21-6dzabs.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Cloud in the Upper Scorpius." src="https://images.theconversation.com/files/440701/original/file-20220113-21-6dzabs.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/440701/original/file-20220113-21-6dzabs.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=900&fit=crop&dpr=1 600w, https://images.theconversation.com/files/440701/original/file-20220113-21-6dzabs.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=900&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/440701/original/file-20220113-21-6dzabs.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=900&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/440701/original/file-20220113-21-6dzabs.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1131&fit=crop&dpr=1 754w, https://images.theconversation.com/files/440701/original/file-20220113-21-6dzabs.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1131&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/440701/original/file-20220113-21-6dzabs.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1131&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Image of a cloud in the Upper Scorpius.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>The team combined images from a large number of telescopes in order to find the faintest objects within a group of young stars, in a region called <a href="https://apod.nasa.gov/apod/ap210715.html">Upper Scorpius</a>. </p>
<p>They used data from large, general purpose surveys combined with more recent observations of their own to generate detailed visible and infrared maps of the area of sky covering a 20-year period. They then looked for faint objects moving in a way that indicated they were members of the group of stars (rather than background stars much further away). </p>
<p>The group found between 70 and 170 FFPs in the Upper Scorpius region, making their sample the largest directly identified so far – though the number has significant uncertainty.</p>
<h2>Rejected planets</h2>
<p>Based on our current understanding of gravitational collapse, there seems to be too many FFPs in this group of stars for them all to have formed in that way. The study authors conclude that at least 10% of them must have started out life as part of a star system, <a href="https://exoplanets.nasa.gov/faq/43/how-do-planets-form/">forming in a disk of dust</a> and dust around a young star rather than through gravitational collapse. At some point, however, a planet could get ejected due to interactions with other planets. In fact, the authors suggest that these “rejected” planets may be just as common as planets that have been alone from the beginning. </p>
<p>If you’re panicking about Earth suddenly spinning off into the depths of space, you probably don’t need to worry – these events are far more likely early on in the formation of a planetary system when there are a lot of planets jostling for position. But it’s not impossible – if something external to an established planetary system, <a href="https://www.space.com/rogue-star-kick-earth-out-solar-system.html">such as another star</a>, were to disrupt it, then a planet could still be detached from its sunny home.</p>
<p>While we still have a long way to go to fully understand these wandering planets, studies like this one are valuable. The planets can be revisited for further, more detailed investigation as new telescope technology becomes available, which might reveal more about the origins of these strange worlds.</p><img src="https://counter.theconversation.com/content/174622/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Joanna Barstow receives funding from the Science and Technology Facilities Council. She is also a Councillor and trustee for the Royal Astronomical Society. </span></em></p>Some planets are rejected by the Solar System that gave birth to them.Joanna Barstow, Ernest Rutherford Fellow, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1241502019-09-26T18:03:27Z2019-09-26T18:03:27ZExoplanet discovery blurs the line between large planets and small stars<figure><img src="https://images.theconversation.com/files/294148/original/file-20190925-51414-15r5rmu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Dome at Calar Alto Observatory. </span> <span class="attribution"><span class="source">Pedro Amado/Marco Azzaro - IAA/CSIC</span></span></figcaption></figure><p>The discovery of yet another exoplanet <a href="https://theconversation.com/more-than-1-000-new-exoplanets-discovered-but-still-no-earth-twin-59274">is no longer news</a>. More than 4,000 planets around other stars have now been found since the detection of the first one in 1995. As astronomers long suspected, or at least hoped, it seems that planets are ubiquitous in stellar systems and there are probably more planets than stars in our galaxy. </p>
<p>But a new discovery of a large planet orbiting the small star GJ3512 is worth noting. The paper, <a href="https://science.sciencemag.org/cgi/doi/10.1126/science.aay7775">published in Science</a>, challenges our understanding of how planets form – and further blurs the line between small, cool stars <a href="https://en.wikipedia.org/wiki/Brown_dwarf">known as brown dwarfs and planets</a>.</p>
<p>The star itself is a <a href="http://astronomy.swin.edu.au/cosmos/R/Red+Dwarf">red dwarf</a>, about 30 light years away, with a luminosity less than 0.2% that of the sun. It has around 12% of the sun’s mass and 14% of its radius. Such cool, dim stars are in fact the most common stars in the galaxy, but only one in ten of the known exoplanets have been found to orbit red dwarfs. </p>
<p>This is likely to be a selection effect. Red dwarfs are so dim that it is hard to detect their planets with the “<a href="https://theconversation.com/explainer-how-do-you-find-exoplanets-24153">Doppler shift method”</a>. This relies on detecting how the wavelength of the starlight gets periodically shifted (to blue or red) by a tiny amount as the unseen planet orbits, tugging the star to and fro. Several of the other planets that have been discovered orbiting red dwarf stars have instead been found by the transit method – looking at how a star’s light dims as a planet passes in front of it.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/294151/original/file-20190925-51452-wrm207.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/294151/original/file-20190925-51452-wrm207.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=436&fit=crop&dpr=1 600w, https://images.theconversation.com/files/294151/original/file-20190925-51452-wrm207.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=436&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/294151/original/file-20190925-51452-wrm207.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=436&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/294151/original/file-20190925-51452-wrm207.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=548&fit=crop&dpr=1 754w, https://images.theconversation.com/files/294151/original/file-20190925-51452-wrm207.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=548&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/294151/original/file-20190925-51452-wrm207.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=548&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Comparison of GJ 3512 to the Solar System and other nearby red-dwarf planetary systems.</span>
<span class="attribution"><span class="source">Guillem Anglada-Escude - IEEC, SpaceEngine.org</span></span>
</figcaption>
</figure>
<p>What makes the new discovery stand out is that the planet, dubbed GJ3512b, is a gas giant in a 204-day elliptical orbit. The planet has a mass of at least half that of Jupiter and its diameter is likely to be around 70% that of the star it orbits. It is therefore one of the largest planets known to be orbiting such a small star in such a wide orbit – and this poses a problem for understanding how it formed. </p>
<h2>Planet formation</h2>
<p>Our solar system was born out of a “<a href="https://theconversation.com/a-disc-of-dust-and-gas-found-around-a-newborn-planet-could-be-the-birthplace-of-moons-118260">protoplanetary disc</a>” – a cloud containing dense gas and dust surrounding our newly formed sun. </p>
<p>The most commonly accepted explanation for how the gas giant planets formed is that rocky icy cores were created by the accumulation of smaller bodies in the outer regions of the disc. This went on until these cores had built up to around ten Earth masses. At this point, they were able to gather a hydrogen and helium envelope before the planets migrated to the inner edge of the disc, or the disc dispersed. </p>
<p>This is how gas giant planets are believed to form in most exoplanetary systems, including so-called <a href="https://theconversation.com/uk-satellite-twinkle-will-reveal-atmospheres-of-distant-exoplanets-44945">“hot-Jupiters” discovered</a> in close, orbits around their stars. But it’s hard to see how planets could form in this way around a low mass star – the disc would not be massive enough. </p>
<p>An alternative scenario is likely to have happened in the case of GJ3512b – and potentially many other planetary systems out there. Here, it seems the planet may have formed by direct fragmentation of the protoplanetary disc. That means part of the disc collapsed and condensed (changing from gas to a liquid and thereafter solid) into a large body, without the need to build up by accumulation of smaller rocks. This is similar to the way in which <a href="https://www.scientificamerican.com/article/how-is-a-star-born/">stars themselves normally form</a>. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/294153/original/file-20190925-51401-nivgg8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/294153/original/file-20190925-51401-nivgg8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/294153/original/file-20190925-51401-nivgg8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/294153/original/file-20190925-51401-nivgg8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/294153/original/file-20190925-51401-nivgg8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/294153/original/file-20190925-51401-nivgg8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/294153/original/file-20190925-51401-nivgg8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">3.5-m telescope at the Calar Alto observatory where the CARMENES spectrograph is installed.</span>
<span class="attribution"><span class="source">Pedro Amado/Marco Azzaro - IAA/CSIC</span></span>
</figcaption>
</figure>
<p>The team behind the new study report further evidence for this formation route from hints of a second giant exoplanet in the system (tentatively called GJ3512c) with an orbital period in excess of 1,400 days. This might also explain the unusually eccentric orbit of GJ3512b, which may have resulted from interactions between the two planets soon after the planets formed. This process would have ejected a third planet from the system. And if three large planets once existed around such a small star, the only way they could have formed is by direct fragmentation of the disc.</p>
<h2>Star versus planet</h2>
<p>The discovery of this system also has implications <a href="https://www.space.com/42790-brown-dwarfs-coolest-stars-hottest-planets.html">for the debate</a> over what constitutes a <a href="https://en.wikipedia.org/wiki/Brown_dwarf">brown dwarf star</a> and what constitutes a planet. Brown dwarfs are stars that failed to initiate nuclear fusion in their cores, and so have a mass below about 8% that of the sun or roughly 85 Jupiter masses. </p>
<p>The lowest mass brown dwarfs known have masses as small as 12 times that of Jupiter, while the highest mass planets known have masses up to 30 times that of Jupiter. So, if the most massive planets are heavier than the least massive stars – what is it that distinguishes a star from a planet?</p>
<p>One answer is to say that stars form like stars do, and planets form like planets do, so mass is to some extent irrelevant. The problem is that normally we cannot tell how an individual planet or brown dwarf formed. In the case of GJ3512b, the likely formation method is more like that of a star than that of a planet.</p>
<p>So the picture is even more confused than it was before, and may only be solved by future discoveries. Increasing the census of planetary systems will ultimately show which formation mechanisms are most common.</p><img src="https://counter.theconversation.com/content/124150/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andrew Norton has previously received funding from the Science & Technology Facilities Council (STFC). </span></em></p>The discovery of a huge planet orbiting a small star challenges our understanding of planet formation.Andrew Norton, Professor of Astrophysics Education, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1046092018-10-10T09:04:37Z2018-10-10T09:04:37ZHow we solved a centuries-old mystery by discovering a rare form of star collision<figure><img src="https://images.theconversation.com/files/239858/original/file-20181009-72100-kg229j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The ALMA telescope has seen tantalising hints of a violent event.</span> <span class="attribution"><span class="source">ESO/B. Tafreshi/TWAN (twanight.org)</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span></figcaption></figure><p>A bright new star appeared in the sky in June, 1670. It was seen by the Carthusian monk Père Dom Anthelme in Dijon, France, and astronomer <a href="https://en.wikipedia.org/wiki/Johannes_Hevelius">Johannes Hevelius</a> in Gdansk, Poland. Over the next few months, it slowly faded to invisibility. But in March 1671, it reappeared – now even more luminous and among the 100 brightest stars in the sky. Again it faded, and by the end of the summer it was gone. Then in 1672, it put in a third appearance, now only barely visible to the naked eye. After a few months it was gone again and hasn’t been seen since. </p>
<p>This has always seemed to be an odd event. For centuries, astronomers regarded it as the <a href="https://en.wikipedia.org/wiki/CK_Vulpeculae">oldest known nova</a> – a type of star explosion. But this explanation became untenable in the 20th century. A nova is a fairly common event, when hydrogen ignites in an otherwise extinct star causing a thermonuclear runaway reaction. Stars can also explode as supernovae, following an implosion of their core. However, we know now that neither would give the kind of repeated appearance seen in this event. </p>
<p>So what was it? Our new research, published in the <a href="https://doi.org/10.1093/mnras/sty2554">Monthly Notices of the Royal Astronomical Society</a>, offers a whole new explanation. </p>
<p>In 1982, the American astronomer <a href="http://www.astro.columbia.edu/profile?uid=mshara">Mike Shara</a> found a nebula – an interstellar cloud of dust, hydrogen, helium and other gases – at the position of the missing star, which had since acquired the name <a href="https://en.wikipedia.org/wiki/CK_Vulpeculae">CK Vul</a> in between. This proved that something had indeed happened here. Astronomers later noted that the nebula was expanding, and that the expansion had started around 300 years ago. But the star itself couldn’t be seen.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/239793/original/file-20181008-72106-a7p3kt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/239793/original/file-20181008-72106-a7p3kt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=593&fit=crop&dpr=1 600w, https://images.theconversation.com/files/239793/original/file-20181008-72106-a7p3kt.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=593&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/239793/original/file-20181008-72106-a7p3kt.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=593&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/239793/original/file-20181008-72106-a7p3kt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=745&fit=crop&dpr=1 754w, https://images.theconversation.com/files/239793/original/file-20181008-72106-a7p3kt.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=745&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/239793/original/file-20181008-72106-a7p3kt.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=745&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">This chart of the position of the ‘nova’ (marked in red) was recorded by the famous astronomer Hevelius and was published by the Royal Society in England.</span>
<span class="attribution"><a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Things became even stranger when the astronomer <a href="https://www.researchgate.net/profile/Tomasz_Kaminski2">Tomasz Kamiński</a> <a href="https://arxiv.org/abs/1807.10647">discovered</a> that the nebula contained a most unusual mix of elements, being very abundant in two isotopes (elements with a different number of neutrons in their nucleus compared to the “normal” atom): a type of nitrogen (15N) and radioactive aluminium (26Al). These require very high temperatures to form. Whatever happened, this had been a high-energy event. </p>
<h2>New observations</h2>
<p>We observed the location of the star with <a href="https://www.almaobservatory.org/en/home/">ALMA observatory in Chile.</a>. This spectacular-looking telescope uses 64 separate dishes, and observes in the microwave region of light. It is particularly good at detecting radiation from molecules in space. What we found is that the debris from the event is visible as two rings of dust, resembling an hourglass. This hourglass is embedded within a larger hourglass seen in previous observations, and itself contains other structures – nested like a Russian doll. </p>
<p>Such hourglass lobes indicate the presence of jets coming from the centre, which blow out the opposing bubbles. But the hourglasses are at slightly different angles. This suggests that the originating structure was spinning, and this requires a protracted process. Whatever happened, it was not just a single explosion. The ejection must have taken some time. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/239791/original/file-20181008-72100-1tqlccm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/239791/original/file-20181008-72100-1tqlccm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=325&fit=crop&dpr=1 600w, https://images.theconversation.com/files/239791/original/file-20181008-72100-1tqlccm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=325&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/239791/original/file-20181008-72100-1tqlccm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=325&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/239791/original/file-20181008-72100-1tqlccm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=408&fit=crop&dpr=1 754w, https://images.theconversation.com/files/239791/original/file-20181008-72100-1tqlccm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=408&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/239791/original/file-20181008-72100-1tqlccm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=408&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Dust rings seen by ALMA.</span>
<span class="attribution"><a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>But if it wasn’t an explosion, what happened? The alternative to a stellar explosion is a collision between two stars. These are rare events which have caught much attention in recent years. In 2008, a collision <a href="https://www.aanda.org/articles/aa/abs/2011/04/aa16221-10/aa16221-10.html">was caught near the centre of our galaxy</a>. The colliding stars circled each other closely, before finally merging.</p>
<p>During the event, the stars became 100 times brighter than before, and over the next two years they faded again. A similar event may have happened in the year 2000, when a star called <a href="https://en.wikipedia.org/wiki/V838_Monocerotis">V838 Mon</a> suddenly brightened and then slowly faded. </p>
<p>CK Vul could be the result of a merger between two normal stars. But this didn’t seem to fit. Luckily, though, there is a complete zoo of possible collisions, as stars come in many types. We have now worked out that two stars from the opposite side of the stellar spectrum could have produced the pattern seen in the sky. </p>
<p>The main actor would have been a <a href="https://imagine.gsfc.nasa.gov/science/objects/dwarfs2.html">white dwarf</a>, a dense remnant left after a star like the sun reaches the end of its life. The supporting actor would have been a <a href="https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question62.html">brown dwarf</a>, an object in the twilight zone between stars and planets: too light to produce the hydrogen fusion which normally takes place at the centre of a stars, but too heavy to be a planet. They are 10 to 80 times heavier than Jupiter. Brown dwarfs are probably quite common, but they are hard to find because they are so faint. </p>
<p>A collision between a white dwarf and a brown dwarf would be spectacular. The brown dwarf would be shredded by the much heavier and denser white dwarf. Some of the shredded dwarf would rain down on the white dwarf and provide the fuel for a thermonuclear reaction. The rest of the brown dwarf would be swept up in the debris from the outburst. </p>
<p>Unlike a normal star, white dwarfs can be extremely faint, and after the merger and thermonuclear explosion, would eventually have returned to this brightness. The remaining dust shells may also have contributed, making it opaque to visible light. A merger of normal stars would have left a star of normal luminosity, and even if obscured could still have been seen in the infrared.</p>
<p>Is this what actually happened? We have made a plausible model but further tests would be required to produce conclusive evidence. For example, would this collision provide the right conditions to form radioactive aluminium? Upcoming observations could look at the details of the innermost region of the hourglass structure to find out.</p>
<p>Our discovery represents the first ever detection of a collision between a white and a brown dwarf. Once confirmed, we can use it to look for other events like it. Astronomy is an adventure: a beautiful mix of physics and discovery. We are still learning.</p><img src="https://counter.theconversation.com/content/104609/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Albert Zijlstra receives funding from the UK Science and Technology Facility Council (STFC) </span></em></p>The ‘oldest known nova’ (a star explosion) in the sky was actually not a nova, astronomers show.Albert Zijlstra, Professor of Astrophysics, University of ManchesterLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/586952016-05-02T15:04:20Z2016-05-02T15:04:20Z‘Ultracool’ dwarf star hosts three potentially habitable Earth-sized planets just 40 light-years away<figure><img src="https://images.theconversation.com/files/120774/original/image-20160501-28141-1wqjepx.jpg?ixlib=rb-1.1.0&rect=0%2C269%2C1768%2C1233&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Imagined view from the surface of one of the newly discovered planets, with ultracool dwarf star TRAPPIST-1 in the background.</span> <span class="attribution"><span class="source">ESO/M. Kornmesser</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>The search for Earth-like planets – and life – beyond the solar system has long been the stuff of science fiction and fantasy. But today’s ground and space telescopes, high-precision instruments and advanced analysis techniques have made this search an active area of real scientific research. <a href="http://exoplanets.org/">Hundreds of terrestrial worlds</a> have been found over the past several years, including a handful at the right distance from their host star to have conditions amenable to liquid water on their rocky surfaces. Astronomers focus on planets in these “<a href="https://en.wikipedia.org/wiki/Circumstellar_habitable_zone">habitable zones</a>” in the search for life beyond Earth.</p>
<p><a href="http://dx.doi.org/10.1038/nature17448">Now for the first time</a>, our international team has found Earth-sized planets around a type of star so extreme it’s referred to as an “ultracool dwarf.” This is the first time planets have been found around the lowest-mass stars, and indicates that they may be the ideal hunting grounds for habitable worlds beyond the solar system. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/120877/original/image-20160502-19542-sruze8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/120877/original/image-20160502-19542-sruze8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/120877/original/image-20160502-19542-sruze8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/120877/original/image-20160502-19542-sruze8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/120877/original/image-20160502-19542-sruze8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/120877/original/image-20160502-19542-sruze8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/120877/original/image-20160502-19542-sruze8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/120877/original/image-20160502-19542-sruze8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Size comparison of the Sun, an ultracool dwarf star and the planet Jupiter.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Sol_Cha-110913-773444_Jupiter.jpg">Chaos syndrome</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>Shifting the focus in the search</h2>
<p>Astronomers have recently started focusing their search for Earth-like planets away from bright, Sun-like stars to dimmer, cooler, low-mass stars called M dwarfs. These stars, while far more numerous in the Milky Way, are too faint to be seen with the naked eye.</p>
<p>Yet their relatively small diameters – less than one-half the width of the Sun – make it easier to detect Earth-sized planets orbiting them using a common technique called the <a href="https://lcogt.net/spacebook/transit-method/">transit method</a>. A transit occurs when a planet passes between us and its host star, resulting in a very slight apparent dimming of the star as the planet blocks a portion of its light.</p>
<p>The alignment of the planet and star must be just right for a transit to be seen, so the probability of this happening is small, and usually only happens if the planet orbits very close to its star. Fortunately, the habitable zone around a cool M dwarf is also closer in than it is around a hotter Sun-like star, so transiting Earth-like planets in these systems have a greater chance of having the conditions necessary for liquid water on their surfaces.</p>
<p>Unfortunately, the feeble amount of light emitted by M dwarfs restricts the search for planetary transits to those stars closest to the Sun, and requires larger telescopes.</p>
<h2>TRAPPIST-1 and its planets</h2>
<p>It is a technical and scientific feat, then, that our international team of astronomers has found the first Earth-like planets around one of the coolest and smallest M dwarfs near the Sun. These “ultracool dwarf” stars are a mere tenth of the diameter of the Sun and 2,000 times fainter.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/120878/original/image-20160502-19546-1r7tjp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/120878/original/image-20160502-19546-1r7tjp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/120878/original/image-20160502-19546-1r7tjp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=401&fit=crop&dpr=1 600w, https://images.theconversation.com/files/120878/original/image-20160502-19546-1r7tjp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=401&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/120878/original/image-20160502-19546-1r7tjp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=401&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/120878/original/image-20160502-19546-1r7tjp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/120878/original/image-20160502-19546-1r7tjp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/120878/original/image-20160502-19546-1r7tjp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">TRAPPIST telescope, ESO La Silla Observatory in Chile.</span>
<span class="attribution"><a class="source" href="http://www.ati.ulg.ac.be/TRAPPIST/Trappist_main/Gallery/Pages/April_2010_4__TRAPPIST_by_night.html#3">TRAPPIST</a></span>
</figcaption>
</figure>
<p>The planets were found by <a href="https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets#Transit_photometry">the transit method</a>, using a facility called <a href="http://www.ati.ulg.ac.be/TRAPPIST/Trappist_main/Home.html">TRAPPIST</a> (TRAnsiting Planets and PlanetesImals Small Telescope), a 60-cm telescope at La Silla Observatory in Chile, optimized to search for small variations in the dim light emitted by ultracool dwarfs. The trick is to monitor them in near-infrared light, a form of radiation with wavelengths longer than the visible light our eyes can perceive (infrared radiation is often used for television remote controls). Over the past year, my colleagues on the TRAPPIST team have monitored several dozen ultracool dwarfs to search for the faint transit signals characteristic of an Earth-sized planet, a mere one percent dip in the already faint light they emit. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/120774/original/image-20160501-28141-1wqjepx.jpg?ixlib=rb-1.1.0&rect=0%2C269%2C1768%2C1233&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/120774/original/image-20160501-28141-1wqjepx.jpg?ixlib=rb-1.1.0&rect=0%2C269%2C1768%2C1233&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/120774/original/image-20160501-28141-1wqjepx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=848&fit=crop&dpr=1 600w, https://images.theconversation.com/files/120774/original/image-20160501-28141-1wqjepx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=848&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/120774/original/image-20160501-28141-1wqjepx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=848&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/120774/original/image-20160501-28141-1wqjepx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1066&fit=crop&dpr=1 754w, https://images.theconversation.com/files/120774/original/image-20160501-28141-1wqjepx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1066&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/120774/original/image-20160501-28141-1wqjepx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1066&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Imagined view from the surface of one of the newly discovered planets, with ultracool dwarf star TRAPPIST-1 in the background.</span>
<span class="attribution"><span class="source">ESO/M. Kornmesser</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>In September 2015, they found their first signal from a star they’ve dubbed TRAPPIST-1, located just 40 light-years away from us. Over the next several months they found more. In total, the astronomers have inferred the presence of three Earth-sized planets, all on very close orbits around the star, with orbital periods (“years”) ranging from 1.5 days to 73 days.</p>
<p>To have such short orbital periods, the planets must be extremely close to their star, between 1/100th and 1/10th the distance between the Sun and the Earth. This is closer than Mercury is to the Sun, and such a small orbit would scorch a planet in our solar system. However, around TRAPPIST-1 these orbits are in and around the habitable zone.</p>
<p>The inner two planets receive two and four times more light energy from their star than the Earth receives from the Sun, and while highly reflective surfaces might make these worlds cool enough for liquid water, they are probably more like Venuses – hot planets in which the water has evaporated into the atmosphere – than Earths. But the third planet, TRAPPIST-1d, receives between 20 percent and 100 percent of the starlight that Earth does from our Sun (<a href="http://www.geog.ucsb.edu/ideas/Insolation.html">insolation</a>), so it orbits at the right distance to have liquid water on its surface, and is potentially an Earth-like world.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/120884/original/image-20160502-19517-5kty3s.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/120884/original/image-20160502-19517-5kty3s.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/120884/original/image-20160502-19517-5kty3s.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=420&fit=crop&dpr=1 600w, https://images.theconversation.com/files/120884/original/image-20160502-19517-5kty3s.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=420&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/120884/original/image-20160502-19517-5kty3s.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=420&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/120884/original/image-20160502-19517-5kty3s.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=528&fit=crop&dpr=1 754w, https://images.theconversation.com/files/120884/original/image-20160502-19517-5kty3s.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=528&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/120884/original/image-20160502-19517-5kty3s.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=528&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">An imagined view from close to one of the three planets orbiting TRAPPIST-1. These worlds have sizes and temperatures similar to those of Venus and Earth – but that’s not all it takes to support life.</span>
<span class="attribution"><span class="source">ESO/M. Kornmesser</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Filling out the planetary picture</h2>
<p>Being at the right distance to have surface liquid water does not guarantee that an Earth-sized planet is truly Earth-like. </p>
<p>First, the proximity of these planets to their host star means they are likely “tidally locked,” forced to rotate at the same rate as they orbit the star, so that one side of the planet is in perpetual day and one side in perpetual night. (Tidal locking is why we always see the same face of the moon from Earth.) While it has long been held that this configuration would prevent the existence of surface liquid water, recent work suggests that such worlds <a href="http://dx.doi.org/10.1051/0004-6361/201321042">may still have regions of habitability</a>.</p>
<p>The composition and circulation of an atmosphere, if it exists, also plays a major role in habitability, either by reflecting stellar light or trapping heat through the greenhouse effect. </p>
<p>Finally, both tectonic activity and the existence of a protective planetary magnetic field can play roles. Tectonic forces are of particular interest for the innermost planet, TRAPPIST-1b, which may be squeezed and stretched by tidal forces from the host star, heating it from the inside and producing the kind of extensive volcanism we see on Jupiter’s moon Io.</p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/120879/original/image-20160502-19557-k7ily2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/120879/original/image-20160502-19557-k7ily2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/120879/original/image-20160502-19557-k7ily2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=684&fit=crop&dpr=1 600w, https://images.theconversation.com/files/120879/original/image-20160502-19557-k7ily2.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=684&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/120879/original/image-20160502-19557-k7ily2.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=684&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/120879/original/image-20160502-19557-k7ily2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=859&fit=crop&dpr=1 754w, https://images.theconversation.com/files/120879/original/image-20160502-19557-k7ily2.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=859&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/120879/original/image-20160502-19557-k7ily2.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=859&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Technicians continue to work on the Webb telescope’s instrumentation in advance of its launch in 2018.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/nasawebbtelescope/11210557006">NASA/Chris Gunn</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span>
</figcaption>
</figure>
<p>The observations obtained by TRAPPIST cannot tell us anything about these planetary details, but the <a href="http://jwst.nasa.gov/index.html">James Webb Space Telescope</a> should tell us more when it is launched in 2018. This advanced replacement to the Hubble Space Telescope will have the sensitivity to detect the even smaller <a href="http://www.exoclimes.com/topics/transmission-spectroscopy/">signal of absorption</a> by the planets during their transit. Imprinted on this signal will be the chemical absorption patterns of the gases present in the atmosphere, which may include biogenic gases such as oxygen, methane and nitrous oxide, or volcanic gases such as sulfur dioxide. </p>
<p>The TRAPPIST team will soon be starting the next phase of its search for Earth-like worlds around ultracool dwarfs with the <a href="http://www.orca.ulg.ac.be/SPECULOOS/Speculoos_main/Home.html">SPECULOOS</a> (Search for habitable Planets EClipsing ULtra-cOOl Stars) survey. This program will monitor 500 of the nearest ultracool dwarfs using four 1-meter robotic telescopes in Cerro Paranal, Chile. Construction of the site is already underway, and the team is looking forward to expanding our census of nearby habitable worlds around the smallest stars.</p><img src="https://counter.theconversation.com/content/58695/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Adam Burgasser receives funding from the National Aeronautics and Space Administration (NASA) and the National Science Foundation (NSF). </span></em></p>We don’t need to look for Earth-like planets exclusively around Sun-like stars. Tiny, dim TRAPPIST-1 has only 11 percent the diameter of the Sun and is much redder.Adam Burgasser, Professor of Physics, University of California, San DiegoLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/503412015-11-09T15:16:50Z2015-11-09T15:16:50ZWhat’s it like to see auroras on other planets?<figure><img src="https://images.theconversation.com/files/101275/original/image-20151109-29341-pe4xmd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Aalto University</span></span></figcaption></figure><p>Witnessing an aurora first-hand is a truly awe-inspiring experience. The natural beauty of the northern or southern lights captures the public imagination unlike any other aspect of space weather. But auroras aren’t unique to Earth and can be seen on several other planets in our solar system.</p>
<p><a href="https://theconversation.com/what-caused-those-spectacular-northern-lights-and-how-you-can-catch-them-next-time-39081">An aurora</a> is the impressive end result of a series of events that starts at the sun. The sun constantly emits a stream of charged particles known as the solar wind into the depths of the solar system. When these particles reach a planet, such as Earth, they interact with the magnetic field surrounding it (the magnetosphere), compressing the field into a teardrop shape and transferring energy to it.</p>
<p>Because of the way the lines of a magnetic field can change, the charged particles inside the magnetosphere can then be accelerated into the upper atmosphere. Here they collide with molecules such as nitrogen and oxygen, giving off energy in the form of light. This creates a ribbon of colour that can be seen across the sky close to the planet’s magnetic north and south poles – this is the aurora.</p>
<h2>Gas giant auroras</h2>
<p>Using measurements from spacecraft, such as <a href="http://www.esa.int/Our_Activities/Space_Science/Cassini-Huygens">Cassini</a>, or images from telescopes, such as the <a href="http://hubblesite.org/">Hubble Space Telescope</a>, space physicists have been able to verify that some of our closest neighbours have their own auroras. Scientists do this by studying the electromagnetic radiation received from the planets, and certain wavelength emissions are good indicators of the presence of auroras.</p>
<p>Each of the gas giants (Jupiter, Saturn, Uranus, and Neptune) has a strong magnetic field, a dense atmosphere and, as a result, its own aurora. The exact nature of these auroras is slightly different from Earth’s, since their atmospheres and magnetospheres are different. The colours, for example, depend on the gases in the planet’s atmosphere. But the fundamental idea behind the auroras is the same.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/101276/original/image-20151109-29326-1md6x8a.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/101276/original/image-20151109-29326-1md6x8a.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=342&fit=crop&dpr=1 600w, https://images.theconversation.com/files/101276/original/image-20151109-29326-1md6x8a.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=342&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/101276/original/image-20151109-29326-1md6x8a.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=342&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/101276/original/image-20151109-29326-1md6x8a.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=430&fit=crop&dpr=1 754w, https://images.theconversation.com/files/101276/original/image-20151109-29326-1md6x8a.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=430&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/101276/original/image-20151109-29326-1md6x8a.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=430&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Blue aurora on Jupiter.</span>
<span class="attribution"><span class="source">NASA/J Clarke</span></span>
</figcaption>
</figure>
<p>For example, several of Jupiter’s moons, including Io, Ganymede and Europa, affect the blue aurora created by the solar wind. Io, which is just a little larger than our own moon, is volcanic and spews out vast amounts of charged particles into Jupiter’s magnetosphere, <a href="http://www.space.com/29248-jupiter-auroras-volcanic-moon-io.html">producing large electrical currents</a> and bright ultraviolet (UV) aurora.</p>
<p>On Saturn, the strongest auroras are in the UV and infrared bands of the colour spectrum and so would not be visible to the human eye. But weaker (and rarer) pink and purple auroras <a href="http://www.sciencedirect.com/science/article/pii/S0019103515002328">have also been spotted</a>.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/101162/original/image-20151108-16255-11oqoqm.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/101162/original/image-20151108-16255-11oqoqm.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=591&fit=crop&dpr=1 600w, https://images.theconversation.com/files/101162/original/image-20151108-16255-11oqoqm.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=591&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/101162/original/image-20151108-16255-11oqoqm.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=591&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/101162/original/image-20151108-16255-11oqoqm.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=743&fit=crop&dpr=1 754w, https://images.theconversation.com/files/101162/original/image-20151108-16255-11oqoqm.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=743&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/101162/original/image-20151108-16255-11oqoqm.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=743&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Hubble Space Telescope captures Saturn’s aurora.</span>
<span class="attribution"><span class="source">NASA/ESA/Hubble</span></span>
</figcaption>
</figure>
<p>Mercury also has a magnetosphere and so we might expect aurora there too. Unfortunately, Mercury is too small and too close to the sun for it to retain an atmosphere, meaning the planet doesn’t have any molecules for the solar wind to excite and that means no auroras.</p>
<h2>The unexpected auroras</h2>
<p>On Venus and Mars, the story is different. While neither of these planets has a large-scale magnetic field, both have an atmosphere. As the solar wind interacts with the Venusian ionosphere (the layer of the atmosphere with the most charged particles), it actually creates or induces a magnetic field. Using data from the <a href="http://www.esa.int/Our_Activities/Space_Science/Venus_Express">Venus Express</a> spacecraft, <a href="http://www.sciencemag.org/content/336/6081/567">scientists found</a> that this magnetic field stretches out away from the sun to form a “magnetotail” that redirects accelerated particles into the atmosphere and forms an aurora.</p>
<p><a href="https://theconversation.com/how-did-mars-lose-its-habitable-climate-the-answer-is-blowing-in-the-solar-wind-50258">Mars’s atmosphere is too thin</a> for a similar process to occur there, but it still has aurora created by localised magnetic fields embedded in the planet’s crust. These are the remnants of a much larger, global magnetic field that disappeared as the planet’s core cooled. Interaction between the solar wind and the Martian atmosphere generates “discrete” auroras that are confined to the regions of crustal field. </p>
<p>A [recent discovery]([(https://theconversation.com/how-did-mars-lose-its-habitable-climate-the-answer-is-blowing-in-the-solar-wind-50258) by the <a href="https://www.nasa.gov/mission_pages/maven/main/index.html">MAVEN mission</a> showed that Mars also has much larger auroras spread across the northern hemisphere, and probably the whole planet too. This “diffuse” aurora is the result of solar energetic particles raining into the Martian atmosphere, rather than particles from the solar wind interacting with a magnetic field.</p>
<p>If an astronaut were to stand on the surface of Mars, they might still see an aurora but it would likely be rather faint and blue, and, unlike on Earth, not be necessarily near the planet’s poles.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/101272/original/image-20151109-29333-u6cfas.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/101272/original/image-20151109-29333-u6cfas.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/101272/original/image-20151109-29333-u6cfas.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/101272/original/image-20151109-29333-u6cfas.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/101272/original/image-20151109-29333-u6cfas.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/101272/original/image-20151109-29333-u6cfas.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/101272/original/image-20151109-29333-u6cfas.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">Brown dwarf with red aurora.</span>
<span class="attribution"><span class="source">Chuck Carter and Gregg Hallinan/Caltech</span></span>
</figcaption>
</figure>
<p>Most planets outside our solar system are too dim compared to their parent star for us to see if they have auroras. But scientists <a href="http://www.nature.com/nature/journal/v523/n7562/full/nature14619.html">recently discovered</a> a brown dwarf (an object bigger than a planet but not big enough to burn like a star) 18 light years from Earth that is believed to have a bright red aurora. This raises the possibility of discovering other exoplanets with atmospheres and magnetic fields that have their own auroras.</p>
<p>Such discoveries are exciting and beautiful, but they are also scientifically useful. Investigating auroras gives scientists tantalising clues about a planet’s magnetic and particle environment and could further our understanding of how charged particles and magnetic fields interact. This could even unlock the answers to other physics problems, <a href="http://news.mit.edu/2010/fusion-ldx-0125">such as nuclear fusion</a>.</p><img src="https://counter.theconversation.com/content/50341/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Nathan Case receives funding from the Science and Technology Facilities Council. Nathan is a member of the AuroraWatch UK team at Lancaster University which issues alerts of potential aurora visibility from the UK.</span></em></p>Recent Martian findings are just the latest discoveries of aurora on other planets, both in and out of our solar system.Nathan Case, Senior Research Associate in Space and Planetary Physics, Lancaster UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/258562014-05-02T02:21:57Z2014-05-02T02:21:57ZSay hello to our new close – but cold – neighbour in space<figure><img src="https://images.theconversation.com/files/47585/original/zkchfkvc-1398984667.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">An artist's conception of WISE J0855-0714.</span> <span class="attribution"><a class="source" href="http://photojournal.jpl.nasa.gov/jpeg/PIA18001.jpg">NASA, JPL-Caltech and Penn State University</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>Author Douglas Adams famously had his Hitchhiker’s Guide to the Galaxy remark that “space is really big”. But to my mind the sheer vastness of space is better encapsulated in the recent announcement of a <a href="http://lanl.arxiv.org/pdf/1404.6501v1.pdf">new brown dwarf</a> just 7.2 <a href="https://theconversation.com/explainer-light-years-and-units-for-the-stars-16995">light years</a> away from the sun.</p>
<p>The new inhabitant of the solar neighbourhood is WISE 0855-0714 – a cool <a href="http://www.nasa.gov/vision/universe/starsgalaxies/brown_dwarf_detectives.html">brown dwarf</a> somewhere between three and 10 times more massive than the planet Jupiter.</p>
<p>Given that both astronomers and the public are used to regular announcements of the detection of new objects lying tens of billions of light years away at the furthest detectable reaches of the universe, you might wonder why is finding something so nearby so interesting?</p>
<h2>A cold discovery</h2>
<p>This discovery represents a significant breakthrough because the brown dwarf has an estimated effective temperature of about 250K (on the absolute temperature scale of <a href="http://lamar.colostate.edu/%7Ehillger/temps.htm">Kelvin</a>), or -23C. This makes it far and away the coldest, compact object detected outside our solar system yet.</p>
<p>At a temperature of around 250K, WISE 0855-0714 sits about halfway in temperature between the previous record holders (a group of cold brown dwarfs – also discovered by the Wide-field Infrared Survey Explorer <a href="http://www.jpl.nasa.gov/wise/index.cfm">WISE</a> – known as “Y dwarfs” with temperatures of around 350K or 77C, about the same temperature as a cup of tea) and the giant planets of our solar system (such as Jupiter) with temperatures of 130K (-143C).</p>
<p>This is critical because although astronomers have discovered more than a thousand <a href="https://theconversation.com/topics/exoplanets">exoplanets</a> orbiting other stars over the past 20 years, almost none can be directly observed.</p>
<p>We discover these exoplanets via the impact they have on their host star (either because they transit across their star, or because they make their star wobble), rather than seeing light from them.</p>
<p>WISE 0855-0714’s discovery is exciting because it opens a door to detailed observations that will explore <em>directly</em> the properties of an object with a temperature similar to that of gas giant planets.</p>
<h2>New neighbours next door</h2>
<p>WISE 0855-0714 lies just 2.2 parsecs (7.2 light years) from our sun, which makes it the <a href="http://www.jpl.nasa.gov/spaceimages/details.php?id=pia18003">fourth closest system</a> now known to us. The even closer systems are the <a href="http://heasarc.gsfc.nasa.gov/docs/cosmic/nearest_star_info.html">Alpha Centauri AB</a> and <a href="http://www.nasa.gov/content/goddard/hubbles-new-shot-of-proxima-centauri-our-nearest-neighbor/">Proxima Centauri</a> system at 1.3 parsecs, Barnard’s star at 1.8 parsecs and the <a href="http://lanl.arxiv.org/pdf/1303.2401.pdf">WISE 1049-5319</a> system at 2 parsecs. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/47519/original/4th46fd8-1398919966.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/47519/original/4th46fd8-1398919966.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/47519/original/4th46fd8-1398919966.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/47519/original/4th46fd8-1398919966.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/47519/original/4th46fd8-1398919966.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/47519/original/4th46fd8-1398919966.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/47519/original/4th46fd8-1398919966.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/47519/original/4th46fd8-1398919966.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The locations of the star systems closest to the sun. The year when the distance to each system was determined is listed after the system’s name.</span>
<span class="attribution"><span class="source">NASA/Penn State University</span></span>
</figcaption>
</figure>
<p>Of these four systems, two (Alpha/Proxima Centauri and Barnard’s star) have been known for more than a hundred years, while the other two (WISE 0855-0714 and WISE 1049-5319) have been discovered in just the past few years.</p>
<p>Why is this? How can we be still be uncovering new solar neighbours in an age when discoveries at the edge of the observable universe are the stuff of every day news? Surely objects at seven light years should be a doddle to find, compared to things at tens of billions of light years? </p>
<p>But you’d be wrong. </p>
<p>And the reason is that objects such as WISE 0855-0714 (and indeed gas giant planets such as <a href="http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jupiter">Jupiter</a>) are incredibly faint. We are used to thinking of Jupiter as a bright and easily visible object in our night skies.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/47592/original/v3kbcynv-1398990799.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/47592/original/v3kbcynv-1398990799.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/47592/original/v3kbcynv-1398990799.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/47592/original/v3kbcynv-1398990799.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/47592/original/v3kbcynv-1398990799.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/47592/original/v3kbcynv-1398990799.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/47592/original/v3kbcynv-1398990799.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/47592/original/v3kbcynv-1398990799.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Jupiter is the largest object in our solar system and easy to see from Earth.</span>
<span class="attribution"><a class="source" href="http://www.nasa.gov/centers/goddard/multimedia/largest/Jupiter_sphere.jpg.html">NASA</a></span>
</figcaption>
</figure>
<p>But that is only so because we see light from our sun reflected from Jupiter. Without that reflected sunlight, Jupiter would be invisible at optical wavelengths and only detectable in the infrared at wavelengths beyond 10um (micrometres). </p>
<p>Even then Jupiter would only be detectable because it is so close. It sits a distance of about 778 million kilometres from the sun. This means Jupiter is 88,000 times closer to us than WISE 0855-0714, or 7.7 billion times brighter.</p>
<p>It’s this combination that has made the detection of WISE 0855-0714 such a challenge – it is very faint and only detectable at infrared wavelengths longer than 4um.</p>
<h2>More than a WISE discovery</h2>
<p>Its discovery has required the development and launch of a US$208 million satellite – NASA’s <a href="http://www.jpl.nasa.gov/wise/index.cfm">Wide-field Infrared Survey Explorer</a>.</p>
<p>It took a detailed examination of a database containing more than 700 million objects from WISE’s survey of the entire celestial sphere to find the one needle in the 700 million blade haystack that is WISE 0855-0714.</p>
<p>This was followed by observations using the world’s largest ground-based telescopes, combined with additional data from NASA’s US$2.2 billion <a href="http://www.spitzer.caltech.edu/">Spitzer</a> infrared space observatory. </p>
<p>And even then, WISE 0855-0714 remains ridiculously hard to observe. On a recent observing run using the 6.5m <a href="http://www.lco.cl/observer-information/telescopes-information/magellan/">Magellan telescope</a> in Chile, I spent over an hour obtaining an image of the field where this object lies.</p>
<p>But even then, we couldn’t see WISE 0855-071 at a depth of 24th magnitude in the near-infrared. It is so very cool that Magellan – and every other ground-based telescope in the world – has so far failed to be able to detect it.</p>
<p>Which brings me back to why space is so very big – its not just because the distances are ridiculously large (and they are). But also because sometimes the most interesting things to look for are very, very faint.</p>
<p>It’s a combination that means the things we most want to find in the universe are still sitting out there in the depths of space, waiting to be discovered.</p><img src="https://counter.theconversation.com/content/25856/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Chris Tinney receives funding from the Australian Research Council.</span></em></p>Author Douglas Adams famously had his Hitchhiker’s Guide to the Galaxy remark that “space is really big”. But to my mind the sheer vastness of space is better encapsulated in the recent announcement of…Chris Tinney, Professor and Associate Dean (Research), UNSW SydneyLicensed as Creative Commons – attribution, no derivatives.