Constraints on formation of the Moon and exomoons

Abstract:

More than four thousands exoplanets have been confirmed to date. Finding a second Earth that has the potential to host life has been one of the ultimate goals of astronomy and planetary science. One of the key aspects of Earth is that it has a fractionally large Moon, which may contribute to stabilize the Earth’s spin axis and its stable climate, potentially offering a better environment for life. The Moon is thought to have formed from a moon-forming disk generated by a large impact approximately 4.5 billion years ago, even though the details are actively debated. Given that planetary impacts are common during the planet accretion phase, exomoons around exoplanets must be common as well, but no exomoon has been confirmed to date. This is primarily due to technical challenges, but it is also possible that not all the planets can form impact-induced moons. In this study, we propose that an initially vapor-rich moon-forming disk is not capable of forming a large impact-induced moon because growing moons would experience strong gas drag from the vapor and fall towards the planet in a very short time scale. This effect diminishes as the disk cools, but a significant portion of the disk mass is lost to the planet as time passes, failing to form a large moon. We find that terrestrial planets larger than 5 Earth masses (∼1.5 R⊕) and icy planets larger than 1 Earth mass (∼1.2R⊕) would produce entirely vapor disks and therefore are not able to form a large moon. This indicates the following conclusions; (1) our model supports the Moon-formation models that produce low vapor mass fractions, and (2) rocky and icy exoplanets whose radii are smaller than 1.5R⊕ are good candidates for hosting exomoons. Our hypothesis is consistent with solar system moons that are thought to have formed by impact (the Moon, Martian moons, and Pluto-Charon), given that the vapor mass fractions of the disks are small.