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 the Solar System and in other planetary systems.
Possible natural instances
The possible detection 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, 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.
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.
Many spacecraft have orbited the Moon, including manned craft of the Apollo program. As of 2018[update], none have orbited any other moons. In 1988 the Soviet Union unsuccessfully attempted to put two unmanned probes on quasi-orbits around the Martian moon Phobos.
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Media related to Extrasolar moons at Wikimedia Commons
- Shadow Moons: The Unknown Sub-Worlds that Might Harbor Life
- Likely First Photo of Planet Beyond the Solar System
- Working Group on Extrasolar Planets – Definition of a "Planet" Position statement on the definition of a planet. (IAU)
- The Hunt for Exomoons with Kepler (HEK): I. Description of a New Observational Project