Subsatellite

A subsatellite, also known as a submoon, moonlet or informally a moonmoon, is a "moon of a moon" or a hypothetical natural satellite that orbits the moon of a planet. It is inferred from the empirical study of natural satellites in the Solar System that subsatellites may be rare, albeit possible, elements of planetary systems. In the Solar System, the giant planets have large collections of natural satellites. The majority of detected exoplanets are giant planets; at least one, Kepler-1625b, may have a very large exomoon, named Kepler-1625b I, which could theoretically host a subsatellite. Nonetheless, aside from human-launched satellites in temporary lunar orbit, no subsatellite is known in the Solar System or beyond. In most cases, the tidal effects of the planet would make such a system unstable on an astronomical timescale.