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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.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. Therefore, it is reasonable to assume that subsatellites may exist in...