Can moons have moons?

Not one of the solar system’s many moons has its own natural satellite, or submoon. The absence of submoons could be a statistical quirk. Maybe moons in other systems harbor them. Or maybe planets form in a way that makes it difficult for moons to acquire and keep submoons. Evaluating the two possibilities awaits, respectively, better observations of exoplanets and deeper understanding of planet formation. But a third possibility—that submoons are too dynamically unstable to survive even when they do form—can be investigated now.

That’s what Juna Kollmeier of the Carnegie Observatories in Pasadena, California, and Sean Raymond of the University of Bordeaux in France have done. Their starting point was a 2002 analysis by Jason Barnes and David O’Brien of the dynamical stability of exomoons around close-in exoplanets. The study’s main ingredient consisted of tidal forces, from both the star and the host planet, which can conspire to cause an exomoon to crash into its exoplanet. Another ingredient in Kollmeier and Raymond’s analysis: A moon has to be sufficiently massive and sufficiently distant relative to the host planet that the predominant gravitational attraction that a submoon feels is to the moon, not the planet.

Kollmeier and Raymond derived a stability criterion and then applied it to the solar system’s largest moons and to the exomoon candidate Kepler-1625b-I . Neptune’s moons were the least hospitable. Only the largest, Triton, could host a submoon, and only then if its radius was less than 5 km. Surprisingly, Jupiter’s Callisto, Saturn’s Titan and Iapetus (shown here), and Earth’s Moon could all host submoons of a radius of 20 km or larger. The most accommodating moon of all turned out to be Kepler-1625b-I.

The finding that four solar-system moons could once have had stable submoons invites explanations for how the moons lost their submoons. It also means that formation theories that yield submoons cannot be ruled out solely on the basis that a stable submoon has never been found. (J. Kollmeier, S. N. Raymond, Mon. Not. R. Astron. Soc. Lett. 483, L80, 2019 .)