Artist's concept of exomoon Kepler-1625b I orbiting exoplanet Kepler-1625b. Kepler-1625b I is thought to possibly have a moonmoon itself.[1][2]

A subsatellite is a natural or artificial satellite that orbits a natural satellite, i.e. a "moon of a moon", also known as a moonmoon, submoon, or grandmoon.[3][4]

It is inferred from the empirical study of natural satellites in the Solar System that subsatellites may be 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.[1][2][5][6] Therefore, it is reasonable to assume that subsatellites may exist in the Solar System and in other planetary systems.

Nonetheless, no "moon of a moon" or subsatellite is known as of 2018 in the Solar System or beyond. In most cases, the tidal effects of the planet would make such a system unstable.[3]

Possible natural instances


Artist's concept of rings around Rhea, a moon of Saturn

The possible detection[7] of a ring system around Saturn's natural satellite Rhea led to calculations that indicated that satellites orbiting Rhea would have stable orbits. Furthermore, the suspected rings are thought to be narrow,[8] a phenomenon normally associated with shepherd moons. However, targeted images taken by the Cassini spacecraft failed to detect any subsatellites or rings associated with Rhea.[9]


It has also been proposed that Saturn's satellite Iapetus possessed a subsatellite in the past; this is one of several hypotheses that have been put forward to account for its unusual equatorial ridge.[10]

Other Languages
Afrikaans: Submaan
Ελληνικά: Υποδορυφόρος
español: Subsatélite
français: Sous-satellite
日本語: 孫衛星
svenska: Submåne