Grand Theft Moons. Formation of habitable moons around giant planets

Abstract

Of the few thousand discovered exoplanets, a significant number orbit in the habitable zone of their star. Many of them are gas giants lacking a rocky surface and solid water reservoirs necessary for life as we know it. The search for habitable environments may extend to the moons of these giant planets. No confirmed exomoon discoveries have been made as of today, but promising candidates are known. Theories suggest that moon formation is a natural process in planetary systems. We aim to study moon formation around giant planets in a phase similar to the final assembly of planet formation. We search for conditions for forming the largest moons with the highest possibility in circumplanetary disks, and investigate whether the resulting moons can be habitable. We determined the fraction of the circumplanetary disk's mass converted into moons using numerical N-body simulations where moon embryos grow via embryo-satellitesimal collisions, investigated in disks around giant planets consisting of 100 fully interacting embryos and 1000 satellitesimals. In fiducial simulations, a 10 Jupiter-mass planet orbited a solar analog star at distances of 1-5 au. To determine the habitability of the synthetic moons, we calculated the stellar irradiation and tidal heating flux on these moons based on their orbital and physical parameters. The individual moon mass is found to be higher when the host planet orbits at a smaller stellar distance. However, moons leave the circumplanetary disk due to the stellar thief effect, which is stronger closer to the star. We find that 32% of synthetic moons can be habitable in the circumstellar habitable zone. Due to the intense tidal heating, the incidence rate of moon habitability is similar at 2 au, and decreasing to 1% at larger distances (<5 au). We conclude that the circumstellar habitable zone can be extended to moons around giant planets.