Novel oxygen false positives on habitable zone planets

Abstract:

Oxygen is a promising exoplanet biosignature due to the evolutionary advantage conferred from harnessing starlight for photosynthesis, and the low likelihood of producing and maintaining oxygen-rich atmospheres without life. Several hypothetical scenarios have been proposed for non-biological oxygen accumulation on planets around late M-dwarfs, where the UV stellar spectrum and extended pre-main sequence may favor abiotic O₂ accumulation. However, for planets around F, G, and K-type stars, few plausible mechanisms for generating abiotic oxygen have been suggested. The relative robustness of oxygen biosignatures around sun-like stars has motivated the development of next-generation telescopes capable of directly imaging habitable zone planets around F/G/K stars and searching for oxygen biosignatures.

Here, we develop a coupled thermal-geochemical-climate model of terrestrial planet evolution to illustrate several scenarios whereby significant abiotic oxygen can accumulate around sun-like stars. For Earth-mass planets we find abiotic oxygen can accumulate to modern levels if the CO₂:H₂O ratio of the initial volatile inventory is high, or if the total volatile inventory is dramatically different to that of the Earth. Fortunately, these abiotic oxygen scenarios could probably be distinguished from biological oxygen with appropriate contextual information. This highlights the need for broadly capable next-generation telescopes that are equipped to constrain surface conditions via time-resolved photometry and search for temporal biosignatures or disequilibrium combination biosignatures to assess whether oxygen is biogenic.