With the increasing number of small exoplanets being discovered, there is growing interest in the abundance of exoplanets with a water content comparable to that of the Earth. Conventional theoretical predictions are based on the assumption that water-rich planetesimals beyond the snow line in the protoplanetary disk are the source of water. It is also predicted that planets in the habitable zone around M dwarfs, where most of the recent exoplanet observation has been carried out, do not obtain much water. On the other hand, since planets are generally formed in protoplanetary disks, they acquire disk gas to form primordial atmospheres. In this study, we focused on the production of water by the reaction of hydrogen in the primordial atmosphere with oxides in the magma ocean as another process of water capture. When this reaction occurs efficiently, it is known that even a planet of less than the mass of the Earth can obtain more water than the Earth ocean mass(Kimura & Ikoma 2020). However, when planetary growth, migration, giant impacts, and dissipation of protoplanetary disk gas occur simultaneously, it is still unknown to what extent this water production process affects the final planetary water abundance distribution. Therefore, we developed a planetary population synthesis model including the effects of water production in the primordial atmosphere and predicted the water content distribution of exoplanets theoretically. The results show that, in contrast to the results of previous studies, the terrestrial planets in the habitable zone of M dwarfs have a diverse water content.
In particular, the water production process in the primordial atmosphere is found to have a great impact on the occurrence of Earth-like aqua planets. This result suggests that planets with appropriate water content for a temperate climate will be normally found around M dwarfs.