Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment

Source-supplied abstract

Here we review how environmental context can be used to interpret whether O₂ is a biosignature in extrasolar planetary observations. This paper builds on the overview of current biosignature research discussed in Schwieterman et al. (2017), and provides an in-depth, interdisciplinary example of biosignature identification and observation that serves as a basis for the development of the general framework for biosignature assessment described in Catling et al., (2017). O₂ is a potentially strong biosignature that was originally thought to be an unambiguous indicator for life at high-abundance. In exploring O₂ as a biosignature, we describe the coevolution of life with the early Earth's environment, and how the interplay of sources and sinks in the planetary environment may have resulted in suppression of O₂ release into the atmosphere for several billion years, a false negative for biologically generated O₂. False positives may also be possible, with recent research showing potential mechanisms in exoplanet environments that may generate relatively high abundances of atmospheric O₂ without a biosphere being present. These studies suggest that planetary characteristics that may enhance false negatives should be considered when selecting targets for biosignature searches. Similarly our ability to interpret O₂ observed in an exoplanetary atmosphere is also crucially dependent on environmental context to rule out false positive mechanisms. We describe future photometric, spectroscopic and time-dependent observations of O₂ and the planetary environment that could increase our confidence that any observed O₂ is a biosignature, and help discriminate it from potential false positives. The rich, interdisciplinary study of O₂ illustrates how a synthesis of our understanding of life's evolution and the early Earth, scientific computer modeling of star-planet interactions and predictive observations can enhance our understanding of biosignatures and guide and inform the development of next-generation planet detection and characterization missions. By observing and understanding O₂ in its planetary context we can increase our confidence in the remote detection of life, and provide a model for biosignature development for other proposed biosignatures.

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