Subsatellite

Artist's concept of exomoon Kepler-1625b I orbiting exoplanet Kepler-1625b[1][2]

A subsatellite is a natural satellite (or an 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 planetary systems beyond the Solar System.

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

Possible natural instances

Rhea

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]

Iapetus

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
svenska: Submåne