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D.C. Catling (SETI Institute/NASA Ames Research Center), C.P. McKay (NASA Ames Research Center)
The existence of a high partial pressure of oxygen in planetary atmospheres elsewhere in the universe is a likely prerequisite for the existence of animal-like life and hence the existence of intelligent life. Whether a planet develops such an atmosphere is influenced by planetary parameters including the composition and size of the planet. The transition to a high oxygen atmosphere would depend on the ability of plate tectonics to provide for organic carbon burial that prohibits back-reaction with photosynthesized atmospheric oxygen. On Earth, plate tectonics accompanied by the oxygen production of photosynthetic bacteria in the Precambrian oceans led to the growth in the level of atmospheric oxygen detectable around 2.0 Gyr ago. Thus, there is a minimum size for an extrasolar planet that develops a high oxygen atmosphere based on the requirement to sustain plate tectonics. On the other hand, if the planet were too big, the loss of early hydrogen may be too slow on the timescale of solar evolution. The timing of the accumulation of atmospheric oxygen, which on Earth resulted in the biological evolution of eukaryotes (eventually including oxygen-utilizing metazoans such as ourselves), would thus appear to depend on geophysics. This may imply a relationship between planetary size and possible degree of biological complexity through time on terrestrial-type planets elsewhere in the Universe.