Is the presence of H2O and O3 in an exoplanet a reliable signature of a biological activity?

We revisit the validity of the presence of H2O and O3 in the atmosphere of an extra-solar planet as a criterion for the presence of biological activity in progress. We consider different objections to the criterion and specify the conditions for it to apply. Noll et al. (1997) have recently suggested a new process: the synthesis of O2 by particle bombardment of pre-cometary icy grains in the protoplanetary nebula, subsequent input of the species into the planet’s atmosphere by comet impacts and UV synthesis of O3. We estimate the amount of O2 produced by this process in comets within the framework of recent disk models and find: [O2]/[H2O] < 1.3 10−7, a value significantly less than that suggested by Noll et al. Our result is in agreement with the absence of O2 detection in Halley’s comet by the Giotto HIS mass spectrometer (upper limit: [O2]/[H2O] < 0.5% ). Further, the geological record from the Young Earth shows that the heavy cometary bombardment prevailing at that time had not been able to build a O2 rich atmosphere on our planet. We conclude that Noll et al.’s objection fails. The presence of abundant H2O and O3 does demonstrate the presence of biological activity on an extra-solar planet located in the Continuously Habitable Zone of its star (0.95-1.15 AU for a Sun-like star).