Thus far, Rein notes, scientists haven’t been able to agree upon a single chemical—oxygen, for example—that could be a conclusive sign of extraterrestrial life. But researchers generally agree that certain mixes of two or more chemicals in an exoplanet atmosphere could be a strong sign of life, Rein explains. Here’s the idea: A mixture of gases that would normally react until one is completely gone simply can’t exist over the long term, unless one or both of the gases is being constantly replenished—possibly by the biological activity of life forms. One easy-to-understand example, Rein says, is a mix of methane and oxygen. Left to themselves, those two substances react to form carbon dioxide. And if life on Earth weren’t continuously producing those two gases, chemical reactions between the two would eventually scrub the less prevalent one from the atmosphere, leaving only the other.
Now, Rein and his colleagues propose a scenario that could easily lead researchers looking for extraterrestrial life astray. What if, they say, a distant exoplanet has a moon with an atmosphere of its own? And furthermore, what if that “exomoon’s” atmosphere contains large amounts of a gas that would typically react with one from the exoplanet’s atmosphere if given a chance? Even if neither body hosted life, the combined light from the two objects might easily be mistaken as having passed through a single atmosphere of a body that hosts life, the researchers report online today in the Proceedings of the National Academy of Sciences.
See also: The Science Fictions series at your fingertips (cosmology) for why many people just need to go on believing it anyway.
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