Reflections on O₂ as a Biosignature in Exoplanetary Atmospheres

Oxygenic photosynthesis is Earth's dominant metabolism, having evolved to harvest the largest expected energy source at the surface of most terrestrial habitable zone planets. Using CO₂ and H₂O-molecules that are expected to be abundant and widespread on habitable terrestrial planets-oxygenic photosynthesis is plausible as a significant planetary process with a global impact. Photosynthetic O₂ has long been considered particularly robust as a sign of life on a habitable exoplanet, due to the lack of known "false positives"-geological or photochemical processes that could also produce large quantities of stable O₂. O₂ has other advantages as a biosignature, including its high abundance and uniform distribution throughout the atmospheric column and its distinct, strong absorption in the visible and near-infrared. However, recent modeling work has shown that false positives for abundant oxygen or ozone could be produced by abiotic mechanisms, including photochemistry and atmospheric escape. Environmental factors for abiotic O₂ have been identified and will improve our ability to choose optimal targets and measurements to guard against false positives. Most of these false-positive mechanisms are dependent on properties of the host star and are often strongest for planets orbiting M dwarfs. In particular, selecting planets found within the conservative habitable zone and those orbiting host stars more massive than 0.4 M_⊙ (M3V and earlier) may help avoid planets with abundant abiotic O₂ generated by water loss. Searching for O₄ or CO in the planetary spectrum, or the lack of H₂O or CH₄, could help discriminate between abiotic and biological sources of O₂ or O₃. In advance of the next generation of telescopes, thorough evaluation of potential biosignatures-including likely environmental context and factors that could produce false positives-ultimately works to increase our confidence in life detection. Key Words: Biosignatures-Exoplanets-Oxygen-Photosynthesis-Planetary spectra. Astrobiology 17, 1022-1052.