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Testing Earth-like Atmospheric Evolution on Exo-Earths

Published onJun 01, 2020
Testing Earth-like Atmospheric Evolution on Exo-Earths

The coming decades could offer the exciting opportunity to search for spectroscopic biosignatures in the atmospheres of dozens or even hundreds of potentially habitable exoplanets. Such a sample would allow us to identify statistical patterns in the properties of these planets and test hypotheses regarding their geological — and potential biological — evolution. As our only example of an inhabited planet, Earth serves as the logical starting point in the study of how habitable worlds evolve. One of the defining traits of Earth’s atmospheric evolution is a dramatic increase in oxygen levels over time, most noticeably at the end of the Archean era. This suggests a starting hypothesis that might be testable in the future: if other habitable planets evolve in a manner similar to Earth, it could lead to an observable relationship between their ages and their atmospheric oxygen levels (Fig. 1). We estimate the potential of existing space mission concepts to test this hypothesis, and thus determine whether the atmospheres of other habitable planets evolve like Earth’s or in qualitatively different ways. We show that their ability to do so hinges critically on the age distribution of targeted host stars. Finally, we discuss how this proposed “biosignature pattern” could be interpreted as independent evidence for life even if oxygen by itself is not a completely reliable biosignature.

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