A subsatellite, also known as a submoon or moonmoon, is a natural or artificial satellite that orbits a natural satellite, i.e. a "moon of a moon". 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, with speculations of a subsatellite. Nonetheless, aside from human-launched satellites in temporary lunar orbit, no notable "moon of a moon" or subsatellite is known in the Solar System or beyond. In most cases, the tidal effects of the planet would make such a system unstable.