Pluto

From its discovery in 1930 until 2006, Pluto was classified as a planet. In the late 1970s, following the discovery of minor planet 2060 Chiron in the outer Solar System and the recognition of Pluto's relatively low mass, its status as a major planet began to be questioned. In the late 20th and early 21st centuries, many objects similar to Pluto were discovered in the outer Solar System, notably the scattered disc object Eris in 2005, which is 27% more massive than Pluto. On August 24, 2006, the International Astronomical Union (IAU) defined what it means to be a "planet" within the Solar System. This definition excluded Pluto as a planet and added it as a member of the new category "dwarf planet" along with Eris and Ceres.After the reclassification, Pluto was added to the list of minor planets and given the number 134340. A number of scientists hold that Pluto should continue to be classified as a planet, and that other dwarf planets should be added to the roster of planets along with Pluto.

The choice of name was partly inspired by the fact that the first two letters of Pluto are the initials of Percival Lowell, and Pluto's astronomical symbol (♇, unicode ♇) is a monogram constructed from the letters 'PL'. Pluto's astrological symbol resembles that of Neptune, but has a circle in place of the middle prong of the trident.