Water ice lines and the formation of giant moons around super-Jovian planets

TL;DR

This study simulates accretion disks around super-Jovian planets, showing that Mars-sized moons can form and suggesting water ice lines act as migration traps, with implications for moon detectability around exoplanets.

Contribution

It presents new simulations of moon formation around super-Jovian planets, highlighting the role of water ice lines as migration traps and contrasting planet and moon formation processes.

Findings

Mars-sized moons can form around super-Jovian planets.

Water ice lines may act as migration traps for moons.

Icy moons larger than Earth's smallest exoplanet are detectable by current telescopes.

Abstract

Most of the exoplanets with known masses at Earth-like distances to Sun-like stars are heavier than Jupiter, which raises the question of whether such planets are accompanied by detectable, possibly habitable moons. Here we simulate the accretion disks around super-Jovian planets and find that giant moons with masses similar to Mars can form. Our results suggest that the Galilean moons formed during the final stages of accretion onto Jupiter, when the circumjovian disk was sufficiently cool. But in contrast to other studies, with our assumptions, we show that Jupiter was still feeding from the circumsolar disk and that its principal moons cannot have formed after the complete photoevaporation of the circumsolar nebula. To counteract the steady loss of moons into the planet due to type I migration, we propose that the water ice line around Jupiter and super-Jovian exoplanets acted as a migration trap for moons. Heat transitions, however, cross the disk during the gap opening within 10^4 yr, which makes them inefficient as moon traps. This indicates a fundamental difference between planet and moon formation. We find that icy moons larger than the smallest known exoplanet can form at about 15 - 30 Jupiter radii around super-Jovian planets. Their size implies detectability by the Kepler and PLATO space telescopes as well as by the European Extremely Large Telescope.