Pluto (Ancient Greek: Πλούτων), designation 134340 Pluto, is a dwarf planet in the Kuiper belt. It was formerly considered the 9th and smallest planet, but has been classified as a dwarf planet since 2006. It is the largest dwarf planet and the 2nd most massive, behind Eris.
Name and symbol
The name Pluto comes from the name of the Roman god of the underworld. Orcus is named after one of the other gods of the underworld, referencing that it could be considered a subordinate of Pluto.
Pluto has 2 commonly used symbols: ♇ and ⯓.
Characteristics
Pluto's surface is made mostly of ice and rock. The ice is mostly nitrogen ice, with small amounts of water, methane, and carbon monoxide ice. Pluto has a distinctive pale heart shape on its surface. In terms of internal structure, Pluto probably has a rocky core surrounded by mantle of water ice.
Pluto's atmospheric pressure is estimated to be 1 Pa.
Pluto's orbit is eccentric and inclined, ranging from 30 to 49 AU (4.5 to 7.3 billion km). Sunlight takes 5.5 hours to reach Pluto. 1 year on Pluto is equal to 247.94 Earth years. Pluto has a highly stable 2:3 orbital resonance with Neptune: for every 2 orbits Pluto makes, Neptune would've made 3 orbits.
Moons
Pluto has 5 moons, including the largest, Charon, and 4 much smaller moons, Nix, Kerberos, Styx, and Hydra.
Discovery
Pluto was discovered on 18 February 1930 by Clyde Tombaugh from photographic plates captured on 21, 23, and 29 January. The news of the discovery were telegraphed to the Harvard College Observatory on 13 March.