Pluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System (after Eris) and the tenth-largest body observed directly orbiting the Sun. Originally classified as a planet, Pluto is now considered the largest member of a distinct population called the Kuiper belt.
Like other members of the Kuiper belt, Pluto is composed primarily of rock and ice and is relatively small: approximately a fifth the mass of the Earth's Moon and a third its volume. It has an eccentric and highly inclined orbit that takes it from 30 to 49 AU (4.4–7.4 billion km) from the Sun. This causes Pluto to periodically come closer to the Sun than Neptune.
From its discovery in 1930 until 2006, Pluto was considered the Solar System's ninth planet. In the late 1970s, following the discovery of minor planet 2060 Chiron in the outer Solar System and the recognition of Pluto's very low mass, its status as a major planet began to be questioned. Later, in the early 21st century, many objects similar to Pluto were discovered in the outer Solar System, notably the scattered disc object Eris, which is 27% more massive than Pluto. On August 24, 2006, the International Astronomical Union (IAU) defined the term "planet" for the first time. 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 continue to hold that Pluto should be classified as a planet.
Pluto and its largest moon, Charon, are sometimes treated together as a binary system because the barycentre of their orbits does not lie within either body. The IAU has yet to formalise a definition for binary dwarf planets, and until it passes such a ruling, they classify Charon as a moon of Pluto. Pluto has two known smaller moons, Nix and Hydra, discovered in 2005.
- Ian Ridpath (December 1978). "Pluto—Planet or Impostor?". Astronomy: 6–11.
- Astronomers Measure Mass of Largest Dwarf Planet. hubblesite (2007). Retrieved on 2007-11-03.
- A. Akwagyiram (2005-08-02). Farewell Pluto?. BBC News. Retrieved on 2006-03-05.
- T. B. Spahr (2006-09-07). MPEC 2006-R19 : EDITORIAL NOTICE. Minor Planet Center. Retrieved on 2006-09-07.
- D. Shiga (2006-09-07). Pluto added to official "minor planet" list. NewScientist. Retrieved on 2006-09-08.
- Richard Gray (2008-08-10). Pluto should get back planet status, say astronomers. The Telegraph. Retrieved on 2008-08-09.
- C.B. Olkin, L.H. Wasserman, O.G. Franz (2003). "The mass ratio of Charon to Pluto from Hubble Space Telescope astrometry with the fine guidance sensors" (PDF). Icarus 164: 254–259. doi:10.1016/S0019-1035(03)00136-2. Retrieved on 2007-03-13.
- O. Gingerich (2006). The Path to Defining Planets (PDF). Harvard-Smithsonian Center for Astrophysics and IAU EC Planet Definition Committee chair. Retrieved on 2007-03-13.
- B. Sicardy, W. Beisker et al. (2006). Observing Two Pluto Stellar Approaches In 2006: Results On Pluto's Atmosphere And Detection Of Hydra. Retrieved on 2007-03-13.