Pluto was discovered in 1930 and was originally considered the ninth planet from the Sun. Its status as a planet fell into question following further study of it and the outer Solar System over the following 75 years. Starting in 1977 with the discovery of the minor planet Chiron, numerous icy objects with eccentric orbits were found.[15] The scattered disc object Eris, discovered in 2005, is 27% more massive than Pluto.[16] The knowledge that Pluto is only one of several large icy bodies in the outer Solar System prompted the International Astronomical Union (IAU) to formally define the term "planet" in 2006, which excluded Pluto and reclassified it as a member of the new "dwarf planet" category (and specifically as a plutoid).[17] Astronomers who oppose the exclusion assert that Pluto should remain classified as a planet and other dwarf planets, and even moons, should be added to the list of planets.[18][19][20]
Pluto has five known moons: Charon (the largest, with a diameter just over half that of Pluto), Styx, Nix, Kerberos, and Hydra.[21] Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body.[22] The IAU has not formalized a definition for binary dwarf planets, and Charon is officially classified as a moon of Pluto.[23]