Predicted diversity in water contents of Earth-like rocky exoplanets orbiting M dwarfs

Abstract A majority of stars in the solar neighbourhood are M dwarfs with subsolar mass and luminosity. Recent exoplanet surveys around M dwarfs have detected a growing number of exoplanets with Earth-like insolation. Those exoplanets are expected to include habitable planets with temperate climates. Climatology, however, indicates that planetary habitability requires not only moderate stellar insolation but also an appropriate amount of seawater. Previous models of planetary population synthesis predict that terrestrial planets orbiting in the habitable zone around M dwarfs have no water or too much water, leading to a discouraging conclusion that habitable planets are pretty rare around M dwarfs. Here we develop a new planetary population synthesis model, introducing updated theories for several components of the planet formation process and the effects of water vapour enrichment in the primordial atmosphere of nebular origin; such enrichment can occur through water production by oxidation of the atmospheric hydrogen with rocky materials from incoming planetesimals and the magma ocean. Our state-of-the-art model of planetary population synthesis yields rocky planets with a wide range of water content. In particular, the water production in the primordial atmosphere is found to significantly impact the occurrence of Earth-like aqua planets. Our results suggest that aqua planets with appropriate amounts of seawater for temperate climates will be normally found around M dwarfs, encouraging efforts of M dwarf habitable planet search, in contrast to the prediction of the previous models.