At our current level of knowledge, many exoplanet findings take us by surprise. The only atmospheric chemistry we can see with clarity is Earth\u2019s, and we still have many unanswered questions about how our planet and its atmosphere developed. With Earth as our primary reference point, many things about exoplanet atmospheres seem puzzling in comparison and generate excitement and deeper questions. <\/p>\n
That\u2019s what\u2019s happened with GJ-3470 b, a Neptune-like exoplanet about 96 light-years away. <\/p>\n
<\/p>\n Astronomers discovered the planet during a 2012 High Accuracy Radial Velocity Planet Searcher (HARPS) campaign. The campaign was searching for short-period planets orbiting M-dwarfs (red dwarfs). When it was discovered, it was called a hot Uranus. It doesn\u2019t take an astrophysicist to figure out why that term has fallen out of favour, and now it\u2019s called a sub-Neptune planet. <\/p>\n GJ-3470 b is about 14 times more massive than Earth, takes 3.3 days to complete one orbit, and is about 0.0355 AU from its star. <\/p>\n New research presented at the 244th meeting of the American Astronomical Society and soon to be published in Astrophysical Journal Letters shows that the planet\u2019s atmosphere contains more sulphur dioxide than expected. The lead researcher is Thomas Beatty, Professor of Astronomy at the University of Wisconsin, Madison. <\/p>\n \u201cWe didn\u2019t think we\u2019d see sulphur dioxide on planets this small, and it\u2019s exciting to see this new molecule in a place we didn\u2019t expect since it gives us a new way to figure out how these planets formed.\u201d<\/p>\n Thomas Beatty, University of Wisconsin, Madison<\/cite><\/p><\/blockquote>\n<\/figure>\n GJ-3470 b\u2019s atmosphere is well characterized among exoplanets. The JWST has aimed its powerful spectroscopic eyes at the planet and revealed more detail than ever. Spectroscopy examines the light from its star as it passes through the planet\u2019s atmosphere, revealing its chemical constituents. <\/p>\n Sub-Neptunes like GJ-3470 b are the most common type of exoplanet detected. Astronomers have detected carbon and oxygen in two of them, TOI-270d and K2-18b, which are important scientific results. But in GJ-3470 b\u2019s atmosphere, astronomers also detected water, methane, and, more significantly, sulphur dioxide (SO2). <\/p>\n \u201cThe thing is, everybody looks at these planets, and often everybody sees flat lines,\u201d said Beatty. \u201cBut when we looked at this planet, we really didn\u2019t get a flat line.\u201d <\/p>\n This is the coldest and lightest exoplanet with sulphur dioxide in its atmosphere. The finding is significant in the effort to understand the different ways that planets form and evolve. The sulphur dioxide probably comes from chemical reactions in the atmosphere, as radiation from the nearby star tears hydrogen sulphide molecules apart, freeing the sulphur, which then bonds to oxygen, forming sulphur dioxide.<\/p>\n The amount of sulphur dioxide is also surprising. There\u2019s about one million times more SO2 than expected. <\/p>\n \u201cWe didn\u2019t think we\u2019d see sulphur dioxide on planets this small, and it\u2019s exciting to see this new molecule in a place we didn\u2019t expect since it gives us a new way to figure out how these planets formed,\u201d said Beatty, who worked as an instrument scientist on the James Webb Space Telescope before joining the UW\u2013Madison faculty. \u201cAnd small planets are especially interesting because their compositions are really dependent on how the planet-formation process happened.\u201d<\/p>\n Astronomers found sulphur dioxide in WASP-39b, a hot Jupiter. But it\u2019s 100 times more massive and two times hotter than GJ-3470 b. It forms the same way on both planets. <\/p>\n \u201cOn both planets, SO2 is produced through photochemistry on the planetary daysides: light from the star hits the top of the atmosphere and breaks apart sulfur-bearing molecules, and then the sulfur-atom wreckage from those photon\/molecule collisions recombines with other molecules in the atmosphere and forms into SO2,\u201d Beatty told Universe Today. <\/p>\n Beatty and his co-researchers tried to identify the pathways that could create SO2 through recombination. But the planet\u2019s coolness led to dead ends. <\/p>\n \u201cIdentifying the correct recombination pathways was an important part of understanding SO2 on WASP-39b \u2013 but these predicted effectively zero SO2 on a planet as cool as GJ 3470b,\u201d Beatty told Universe Today. It turns out that the atmospheric metallicity allows it to happen.<\/p>\n \u201cAs a part of these observations, we determined that the high metallicity of GJ 3470b\u2019s atmosphere (it\u2019s about 100x more metal-rich than WASP-39b) can drive SO2-producing reactions at much lower temperatures,\u201d Beatty explained in an email exchange. \u201cPut another way, we realized that all of the ambient water and carbon dioxide in GJ-3470 b\u2019s atmosphere make the recombination process to form sulphur dioxide much more efficient than on larger giant exoplanets like WASP-39b.\u201d<\/p>\n Astronomers can\u2019t piece together a planet\u2019s formation history without a complete account of its atmospheric constituents. With a complete list, they can start to tell the story of its formation. \u201cDiscovering sulfur dioxide in a planet as small as GJ 3470 b gives us one more important item on the planet formation ingredient list,\u201d said Beatty.<\/p>\n But there\u2019s more to the planet\u2019s story than the SO2 and other atmospheric chemicals. It follows a polar orbit, which is a strong clue that the planet has been bullied out of its original orbit. It\u2019s also extremely close to its star and has likely lost much of its atmosphere, blown away into space by the star\u2019s powerful stellar wind. It may have lost 40% of its atmosphere. <\/p>\n \u201cThat migration history that led to this polar orbit and the loss of all this mass \u2014 those are things we don\u2019t typically know about other exoplanet targets we\u2019re looking at,\u201d Beatty said. \u201cThose are important steps in the recipe that created this particular planet and can help us understand how planets like it are made.\u201d<\/p>\n\n
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