  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.
