Can Moons Have Moons?

Abstract

Each of the giant planets within the Solar System has large moons but none of these moons have their own moons (which we call $submoons$). By analogy with studies of moons around short-period exoplanets, we investigate the tidal-dynamical stability of submoons. We find that 10 km-scale submoons can only survive around large (1000 km-scale) moons on wide-separation orbits. Tidal dissipation destabilizes the orbits of submoons around moons that are small or too close to their host planet; this is the case for most of the Solar System's moons. A handful of known moons are, however, capable of hosting long-lived submoons: Saturn's moons Titan and Iapetus, Jupiter's moon Callisto, and Earth's Moon. Based on its inferred mass and orbital separation, the newly-discovered exomoon candidate Kepler-1625b-I can in principle host a large submoon, although its stability depends on a number of unknown parameters. We discuss the possible habitability of submoons and the potential for subsubmoons. The existence, or lack thereof, of submoons, may yield important constraints on satellite formation and evolution in planetary systems.

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• On the orbital evolution of meteoroid 2020 CD3, a temporarily captured orbiter of the Earth-Moon system
• Cronomoons: origin, dynamics, and light-curve features of ringed exomoons
• On the Stability of Additional Moons Orbiting Kepler-1625 b
• On the motion of satellite around the natural moons of planets using the concept of ER3BP with variable eccentricity
• The Dynamics of Co-orbital Giant Exomoons -- Applications for the Kepler-1625 b and Kepler-1708 b Satellite Systems
• The missing rings around Solar System moons
• Kilometre-scale Jovian moon characterized for a potential JUICE flyby