A subsatellite (or submoon or moonmoon or grandmoon, as per "grandchild") is a satellite of a moon, i.e., a body orbiting a body that orbits a planet. The term may be used for artificial satellites as well as natural ones and space probes have orbited the Moon, some still doing so. However, no natural subsatellites are known within the solar system. Extra-solar planets showing signs of substantial moons (e.g., through transit timing variations) are candidates for having them. It is still thought there is a real possibility of instances within the solar system.