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 (CH4) production is limited, even though CH4 biosignatures in anoxic atmospheres may be readily detectable with the James Webb Space Telescope. We assess environmental context for CH4 biosignatures and conclude that abundant atmospheric CH4 coexisting with CO2, and CO:CH4 ≪ 1 is suggestive of biological production, although precise thresholds are dependent on stellar context and sparsely characterized abiotic CH4 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.
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The data outputs from Supplementary Video 1 are available at https://doi.org/10.5281/zenodo.5719456.
The Python code for our atmosphere evolution model is open source and available at https://doi.org/10.5281/zenodo.4539040.
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J.K.-T. is a NASA Hubble Fellow and was supported by the NASA Sagan Fellowship and through NASA Hubble Fellowship grant number HF2-51437 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. for NASA under contract number NAS5-26555. We acknowledge use of the lux supercomputer at UC Santa Cruz, funded by NSF MRI grant number AST 1828315.
The authors declare no competing interests.
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Supplementary Video 1
Video version of Fig. 2. Time evolution of atmospheric oxygen (left), carbon dioxide (middle) and water vapour (right) as a function of planet–star separation for a sample of simulated, lifeless planets. The colour scale shows the mean surface temperature, and the black dashed line shows the runaway greenhouse limit for an Earth-like albedo, which evolves with stellar luminosity (a G star is assumed). Squares denote non-zero surface liquid water inventories whereas circles show model runs with uninhabitable surface conditions. The simulated planet population has a wide range of initial volatile inventories and parameter values that govern atmosphere–interior exchange of volatiles. Models such as this can be used to predict trends in non-biological oxygen accumulation alongside their contextual clues. The grey shaded regions denote numerical limits; lower abundances may be realized but fluxes cut off here for numerical efficiency.
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Krissansen-Totton, J., Thompson, M., Galloway, M.L. et al. Understanding planetary context to enable life detection on exoplanets and test the Copernican principle. Nat Astron 6, 189–198 (2022). https://doi.org/10.1038/s41550-021-01579-7