It is a natural celestial object with a recognized orbit around a planet of the Solar System, some as small as a kilometre across. In the Solar System, there are six terrestrial satellite systems covering 185 known natural satellites. Four IAU-Mentioned dwarf planets are also known to have natural satellites: Pluto, Haumea, Makemake, and Eris. As of September 2018, there are 334 other small planets known to have moons.The Earth-Moon structure is unique in that the ratio of the mass of the Moon to the frame of Earth is much greater than that of any other natural-satellite–planet proportion in the Solar System (although there are minor-planet systems with even greater ratios, notably the Pluto–Charon system).