Why Earth has an oxygen-rich atmosphere is not a solved problem, although the crucial importance of O2 to life on Earth, and its generation by life on Earth, are unquestioned. The factors that promote or frustrate the generation of free oxygen are central to what we mean by habitability, because it is O2 that makes a world fit for creatures like us attending conferences like these. The astronomical mission to identify and characterize an inhabited planet remains justifiably focused on the quest to detect O2 (or its byproduct O3) because free oxygen remains the bronze standard of habitation as we know it. We can expect that eventually, perhaps within 100 years, we will have accumulated a database of such exoplanets and we will begin to be able to evaluate basic hypotheses regarding the origin of oxygen (if not the origin of life).
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