Grand theft moons

Formation of habitable moons around giant planets

¹ Gothard Astrophysical Observatory, HUN-REN – ELTE Exoplanet Research Group, Szent Imre herceg u. 112, 9700 Szombathely, Hungary
² HUN-REN Konkoly Observatory, Research Centre for Astronomy and Earth Science, Konkoly-Thege Miklós út 15–17, 1121 Budapest, Hungary
³ CSFK, MTA Centre of Excellence, Budapest, Konkoly Thege Miklós út 15–17, 1121 Budapest, Hungary
⁴ Kapteyn Astronomical Institute, University of Groningen, 9747 AD, Landleven 12, Groningen, The Netherlands

^★ Corresponding author: zdencs@gothard.hu

Received: 18 February 2025
Accepted: 23 May 2025

Abstract

Context. 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 the solid water reservoirs necessary for life as we know it. The search for habitable environments can be extended to the moons of these giant planets. No confirmed exomoon discoveries have been made as of today, but promising candidates are known, and theories suggest that moon formation is a natural process in planetary systems.

Aims. 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 we investigate whether the resulting moons can be habitable.

Methods. 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, which we 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.

Results. We find the individual moon mass 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 intense tidal heating, the incidence rate of moon habitability is similar at 2 au and decreases to 1% at larger distances (<5 au).

Conclusions. We conclude that the circumstellar habitable zone can be extended to moons around giant planets.