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Does the extrasolar planet K2-18b host life?

Recently, planetary astronomers and astrobiologists have been discussing the possibility of introducing a gradated "life detection scale", running from 0 to 10, with the goal of telegraphing to the public how much confidence scientists have in any particular intimation that exoplanetary life (or a biosignature) has been detected on a given planet. The scale would be similar in spirit to the Torino Scale for asteroid threats or the San Marino Scale for determining risks associated with deliberate transmissions to possible extraterrestrial intelligent life.

The purpose of this question (and succeeding questions to form a question series) is to explore the feasibility of using Metaculus to determining a probability consensus that can be mapped onto a numerical score.

So on to the specific question itself.

Water vapor has recently been detected (with an independent detection described here) in the atmosphere of the extrasolar planet K2-18b, which has roughly three times Earth’s radius, nine times Earth’s mass, and receives a similar radiative flux from its parent star as Earth receives from the Sun. This has led to speculation about whether K2-18b might host life, particularly in the press, e.g. here and here. Given the level of popular interest, and given the rather startling range of opinions that were evident after the announcement, it seems useful for both domain experts and experts in prediction (here's looking at you, Metaculus users!) to have a forum for providing feedback on this issue.

We thus ask:

Will a definitive biosignature be detected on K2-18b?

Resolution is by a measurement and an independent confirming measurement published in the peer-reviewed literature. Positive resolutions are provided by (1) O2 detected in the atmosphere with false positives ruled out, or (2) observation of a significant chemical disequilibrium between CH4 and CO2 in an anoxic atmosphere. Ideally, however, we don't want to limit the question to just these conditions, and moreover, knowledge in the field is advancing quickly. Discussion leading to an improved set of resolution criteria is thus requested. We will consider a time horizon covering the next decade (ending January 1, 2030). There appears to be limited benefit to waiting longer, since K2-18b is close to its star, so it is unlikely to be resolvable by future direct imaging instruments.


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