Conditions for water ice lines and Mars-mass exomoons around accreting super-Jovian planets at 1−20 AU from Sun-like stars

Abstract

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 (H 2 O) 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 H 2 O 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 H 2 O 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 H 2 O 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 H 2 O 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.

On the Radio Detection of Multiple-Exomoon Systems Due to Plasma Torus Sharing

“…The dependence of the emission power on the exomoon's crosssectional area (pR S 2 ) leads to the question of how large exomoons can be. Formation models suggest that moons can form around Jovian planets with masses up to 10 −4 times the mass of their host planet (Canup & Ward 2006;Heller & Pudritz 2015). Therefore, large exoplanets of M 3 J or larger could form a Mars-sized exomoon, and M 10 J exoplanets could have several Mars-sized exomoons or larger (Heller & Pudritz 2015) (where M 1 J is one Jupiter mass).…”

“…Formation models suggest that moons can form around Jovian planets with masses up to 10 −4 times the mass of their host planet (Canup & Ward 2006;Heller & Pudritz 2015). Therefore, large exoplanets of M 3 J or larger could form a Mars-sized exomoon, and M 10 J exoplanets could have several Mars-sized exomoons or larger (Heller & Pudritz 2015) (where M 1 J is one Jupiter mass). In contrast, smaller Jovian exoplanets are unlikely to harbor large exomoons, unless those moons are captured (Heller & Pudritz 2015).…”

“…Therefore, large exoplanets of M 3 J or larger could form a Mars-sized exomoon, and M 10 J exoplanets could have several Mars-sized exomoons or larger (Heller & Pudritz 2015) (where M 1 J is one Jupiter mass). In contrast, smaller Jovian exoplanets are unlikely to harbor large exomoons, unless those moons are captured (Heller & Pudritz 2015). Hence, large Jovian exoplanets are of primary importance to the existence of large exomoons, and consequently to the mechanism described here.…”

Formation of exomoons: a solar system perspective

“…These studies explore disk conditions at super-jovian planets, at a variety of distances away from their parent star, and find that super-jovian planets that form more than 5 AU from a sun-like star can harbor moons up to Mars' mass (about 4× the mass of Ganymede) [31,32]. Varying the stellar distance has the effect of changing the relative heating rates from insolation and gravitational contraction of the planet, as well as the composition of material pulled into the circumplanetary disk [31,32].…”

“…In one of the more successful formulations of the coaccretion theory, the satellite system-to-total mass ratio shared by all four planet/satellite systems is thought to arise from a balance between satellite loss by Type I migration and satellite growth from sweep-up of solids in the mid-plane of the circumplanetary disk [27,30]. The value of M S /M pl is relatively insensitive to planetary mass [30], suggesting that primordial satellite systems around extrasolar planets could have M S /M pl ∼ 10 −4 as well [31,32].…”