Can Moons Have Moons?

TL;DR

This paper explores the potential for moons to have their own moons, called submoons, analyzing their stability and the conditions under which they could exist in our Solar System and beyond.

Contribution

It provides the first detailed analysis of the tidal-dynamical stability of submoons around large moons and exoplanetary candidates, highlighting which moons could host long-lived submoons.

Findings

Submoons can survive around large, widely separated moons.

Tidal forces destabilize submoons around small or closely orbiting moons.

Certain Solar System moons and exomoon candidates could host stable submoons.

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 ${\it submoons}$). 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.