(C) Daily Kos This story was originally published by Daily Kos and is unaltered. . . . . . . . . . . Top Comments: How to Infer the Presence of Life on Exoplanets [1] ['This Content Is Not Subject To Review Daily Kos Staff Prior To Publication.'] Date: 2023-10-08 Here at Top Comments we strive to nourish community by rounding up some of the site's best, funniest, most mojo'd & most informative commentary, and we depend on your help!! If you see a comment by another Kossack that deserves wider recognition, please send it either to topcomments at gmail or to the Top Comments group mailbox by 9:30pm Eastern. Please please please include a few words about why you sent it in as well as your user name (even if you think we know it already :-)), so we can credit you with the find! To date, more than 5,000 exoplanets (that is planets that orbit stars in the galaxy that are not our Sun) have been discovered. None of these planets have been directly observed—they’re too far away for optical astronomy to obtain an actual picture of any of them. Their presence is inferred by indirect ways, either by observing the dimming of the light of the central star as the planet passes before it, or through the motion of the star that indicates there are massive bodies orbiting it. As such, it may be surprising that, if we can’t even see what these planets look like, that we might be able to identify signs of life on them. However, the powerful method of spectroscopy makes it possible to identify particular molecules present in the atmospheres of these planets. Spectroscopy involves the interaction of light and matter. Ordinary matter, in the form of atoms and molecules, absorbs light at characteristic wavelengths such that observing light absorption at particular characteristic wavelengths enable the definitive identification of a particular molecule. Chemists use this technique all the time for that purpose. Many of these characteristic absorption features occur in the infrared portion of the electromagnetic spectrum (wavelengths longer than visible red light); molecular absorption of infrared light is usually associated with molecular vibrations. So how is it possible to obtain an infrared absorption spectrum for the atmosphere of an exoplanet? As an exoplanet passes in front of its star, the light from the star passes through its atmosphere, where any molecules present can absorb some of that light. So scientists on Earth then look at the spectrum to find wavelengths where light intensity is reduced compared to when there is no planet between the star and the spectrometer. ADAPTED FROM NASA, ESA, CSA, STSCI, JOSEPH OLMSTED (STSCI) It is not enough to observe just one molecule in the atmosphere and then be able to declare it as evidence of life because most molecules associated with life can be produced by non-biological processes. A signature for life would be a combination of such molecules, with additional evidence of fluctuation in the quantities of these molecules over time. For example, at one time, the presence of elemental oxygen by itself was considered as evidence for life: on Earth, it is produced almost exclusively by plants undergoing photosynthesis, and, in its elemental form, it’s highly reactive and so wouldn’t last very long in any environment. However, there are ways of producing measurable levels of oxygen that don’t involve life. Similarly, we know that life on Earth involves molecules such as water, carbon dioxide or methane, and while detecting these molecules on another planet might indicate that life (as we know it) might be possible there, it’s not evidence of life. On the other hand, if you detect oxygen, water, carbon dioxide and methane all in the same atmosphere at the same time, that is a much stronger indication that something dynamic and possibly life-like is happening. In particular, oxygen and methane are not simultaneously stable, so if these two molecules are constantly present in an atmosphere, some processes must be occurring to continuously produce them, which might be life. Just last month, it was announced that an exoplanet called K2-18 b, which orbits a red dwarf star in the constellation Leo, contains water vapor, carbon dioxide, and methane, which is a hopeful sign. On the other hand, this point of view is chauvinistic. Scientists from Earth look for water, oxygen, carbon dioxide and a variety of organic molecules as signs of life because they are associated with life on Earth. We don’t yet know if it’s possible for life to evolve in a way that doesn’t use these molecules, and if we don’t broaden our perspective, we might miss evidence of life hiding in plain sight. A signature of life that is not dependent on the actual molecular composition of the atmosphere is signs that the composition changes over time. Scientists in the field call this disequilibrium. Observable changes indicate that something is working to modify atmospheric composition, though of course there are plenty of ways that can happen that don’t involve life. For example, on Earth, the atmospheric composition changes with the seasons, and periodic volcanic activity ejects uncommon substances into the atmosphere. So it is necessary to be able to distinguish between changes due to climate or vulcanism from changes due to potential life. On the other hand, if the composition of the atmosphere on a planet never changes, it’s unlikely there’s any life there. One other approach is to look for something weird present in the atmosphere. The idea here is that if there’s extraterrestrial life, it’s rare, and given how little we know about possible ways life can arise, we must expect the unexpected. Looking for life elsewhere in the universe is a quest to find out if we are truly alone in the universe. How special are we? To a large extent, the history of science has recorded the dethroning of humanity from perceived special status. For example, the Copernican model of the Solar System removed the Earth from the center of the universe, and Darwin showed that humans are not a special creation, but just another species that has evolved beside all the rest of life on the planet. If we do find life elsewhere in the universe, Earth will be dethroned once again, and I look forward to it. Comments are below the fold. From squarewheel: In the front-page story on how an NBC reporter was able to embarrass a GOP congressman on the subject of impeaching Joe Biden — SocioSam has an absolutely hilarious comment and — like The Onion — will eventually become true, I think. From inkstainedwretch: In the diary by CubanoInLA about pushing-back on Tommy Tuberville — there are many great comments, but these two (one from TheCriticalMind, and the other from Cal Worthington) just hit me one after the other. 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