Conditions for water ice lines and Mars-mass exomoons around accreting super-Jovian planets at 1

Context. The first detection of a moon around an extrasolar planet (an “exomoon”) might be feasible with NASA’s Kepler or ESA’s upcoming PLATO space telescopes or with the future ground-based European Extremely Large Telescope. To guide observers and to use observational resources most efficiently, we need to know where the largest, most easily detected moons can form. Aims. We explore the possibility of large exomoons by following the movement of water (H2O) ice lines in the accretion disks around young super-Jovian planets. We want to know how the different heating sources in those disks affect the location of the H2O ice lines as a function of stellar and planetary distance. Methods. We simulate 2D rotationally symmetric accretion disks in hydrostatic equilibrium around super-Jovian exoplanets. The energy terms in our semi-analytical framework – (1) viscous heating; (2) planetary illumination; (3) accretional heating of the disk; and (4) stellar illumination – are fed by precomputed planet evolution models. We consider accreting planets with final masses between 1 and 12 Jupiter masses at distances between 1 and 20 AU to a solar type star. Results. Accretion disks around Jupiter-mass planets closer than about 4.5 AU to Sun-like stars do not feature H2O ice lines, whereas the most massive super-Jovians can form icy satellites as close as 3 AU to Sun-like stars. We derive an empirical formula for the total moon mass as a function of planetary mass and stellar distance and predict that super-Jovian planets forming beyond about 5 AU can host Mars-mass moons. Planetary illumination is the major heat source in the final stages of accretion around Jupiter-mass planets, whereas disks around the most massive super-Jovians are similarly heated by planetary illumination and viscous heating. This indicates a transition towards circumstellar accretion disks, where viscous heating dominates in the stellar vicinity. We also study a broad range of circumplanetary disk parameters for planets at 5.2 AU and find that the H2O ice lines are universally between about 15 and 30 Jupiter radii in the final stages of accretion when the last generation of moons is supposed to form. Conclusions. If the abundant population of super-Jovian planets around 1 AU formed in situ, then these planets should lack the previously predicted population of giant icy moons, because those planets’ disks did not host H2O ice in the final stages of accretion. But in the more likely case that these planets migrated to their current locations from beyond about 3 to 4.5 AU they might be orbited by large, water-rich moons. In this case, Mars-mass ocean moons might be common in the stellar habitable zones. Future exomoon detections and non-detections can provide powerful constraints on the formation and migration history of giant exoplanets.

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