Abiotic Oxygen-dominated Atmospheres on Terrestrial Habitable Zone Planets

Detection of life on other planets requires identification of biosignatures, i.e., observable planetary properties that robustly indicate the presence of a biosphere. One of the most widely accepted biosignatures for an Earth-like planet is an atmosphere where oxygen is a major constituent. Here we show that lifeless habitable zone terrestrial planets around any star type may develop oxygen-dominated atmospheres as a result of water photolysis, because the cold trap mechanism that protects H₂O on Earth is ineffective when the atmospheric inventory of non-condensing gases (e.g., N₂, Ar) is low. Hence the spectral features of O₂ and O₃ alone cannot be regarded as robust signs of extraterrestrial life.