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In the search for life on Earth-like planets around other stars, the first(and likely only) information will come from the spectroscopic characterizationof the planet's atmosphere. Of the countless number of chemical speciesterrestrial life produces, only a few have the distinct spectral features andthe necessary atmospheric abundance to be detectable. The easiest of thesespecies to observe in Earth's atmosphere is O$_{2}$ (and its photochemicalbyproduct, O$_{3}$). But O$_{2}$ can also be produced abiotically by photolysisof CO$_{2}$, followed by recombination of O atoms with each other. CO isproduced in stoichiometric proportions. Whether O$_{2}$ and CO can accumulateto appreciable concentrations depends on the ratio of far-UV to near-UVradiation coming from the planet's parent star and on what happens to thesegases when they dissolve in a planet's oceans. Using a one-dimensionalphotochemical model, we demonstrate that O$_{2}$ derived from CO$_{2}$photolysis should not accumulate to measurable concentrations on planets aroundF- and G-type stars. K-star, and especially M-star planets, however, may buildup O$_{2}$ because of the low near-UV flux from their parent stars, inagreement with some previous studies. On such planets, a 'false positive' forlife is possible if recombination of dissolved CO and O$_{2}$ in the oceans isslow and if other O$_{2}$ sinks (e.g., reduced volcanic gases or dissolvedferrous iron) are small. O$_{3}$, on the other hand, could be detectable at UVwavelengths ($\lambda$ < 300 nm) for a much broader range of boundaryconditions and stellar types.

Abiotic O$_{2}$ Levels on Planets around F, G, K, and M Stars: Possible False Positives for Life?