History of water loss and atmospheric O2 buildup on rocky exoplanets near M dwarfs

It is recently proposed that early stellar luminosity evolution of M dwarfs leads to severe water loss and the buildup of massive O2 atmospheres on rocky exoplanets in the habitable zone of these stars if interactions of such O2 atmospheres with planetary surfaces are inefficient. Here we show that even without considering atmosphere–surface interactions, the existence of a massive O2 atmosphere on such exoplanets is not an unavoidable consequence around M0–M3 stars and depends on stellar XUV properties, the mass of the exoplanets, and most importantly the initial planetary water inventories. In the case of inefficient atmosphere–surface interactions, the distribution of atmospheric O2 contents on these exoplanets should be bi-modal and such a distribution could be verified by future surveys of rocky exoplanets. •A semi-analytical method is derived to calculate the loss of water and possible buildup of oxygen in hydrodynamic escape process.•Massive oxygen atmospheres is not unavoidable.•There is a possible bi-modal distribution for exoplanets having massive oxygen atmospheres.