Understanding planetary context to enable life detection on exoplanets and test the Copernican principle

Abstract

The search for life on exoplanets is motivated by the universal ways in which life could modify its planetary environment. Atmospheric gases such as oxygen and methane are promising candidates for such environmental modification due to the evolutionary benefits their production would confer. However, confirming that these gases are produced by life, rather than by geochemical or astrophysical processes, will require a thorough understanding of planetary context, including the expected counterfactual atmospheric evolution for lifeless planets. Here, we evaluate current understanding of planetary context for several candidate biosignatures and their upcoming observability. We review the contextual framework for oxygen and describe how conjectured abiotic oxygen scenarios may be testable. In contrast to oxygen, current understanding of how planetary context controls non-biological methane (CH$_4$) production is limited, even though CH$_4$ biosignatures in anoxic atmospheres may be readily detectable with the James Webb Space Telescope. We assess environmental context for CH$_4$ biosignatures and conclude that abundant atmospheric CH$_4$ coexisting with CO$_2$, and CO:CH$_4$ << 1 is suggestive of biological production, although precise thresholds are dependent on stellar context and sparsely characterized abiotic CH$_4$ scenarios. A planetary context framework is also considered for alternative or agnostic biosignatures. Whatever the distribution of life in the Universe, observations of terrestrial exoplanets in coming decades will provide a quantitative understanding of the atmospheric evolution of lifeless worlds. This knowledge will inform future instrument requirements to either corroborate the presence of life elsewhere or confirm its apparent absence.