Reflections on O(2) 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 CO(2) and H(2)O—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 O(2) 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 O(2). O(2) 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 O(2) 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 O(2) generated by water loss. Searching for O(4) or CO in the planetary spectrum, or the lack of H(2)O or CH(4), could help discriminate between abiotic and biological sources of O(2) or O(3). 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.