Pluto (dwarf planet)
For other uses of the name, see Pluto (disambiguation).
From its discovery in 1930 by American astronomer Clyde Tombaugh until 2006, Pluto was considered equal to the eight other planets of the solar system - the furthest one from the sun, taking 248 Earth years to complete one orbit. Like most other worlds in the region, it hosts moons - Charon was identified in 1978, with Nix and Hydra discovered in 2005. However, astronomers came to realise that Pluto was actually the largest body of a much bigger collection at the fringes of the solar system - rocky, icy objects remaining from its formation, that together make up the Kuiper belt. Therefore, in 2006 Pluto's status was reassigned by the International Astronomical Union to 'dwarf planet' - something with enough mass to become spherical, but which has not through its own gravity 'cleared the neighbourhood' of other objects around it.[1]
Pluto has a highly elliptical orbit that is outside the plane of the other eight planets in an orbit of 3.282 x 10E10 km [2] and an orbital velocity of 4,749.0 m/s.[3] Its nearest approach to the sun brings it closer than the planet Neptune.Its average distrance from the sun of 5,906,380,000 km with a perihelion of 4.43682 x 10E9 km (29.658 A.U. at its nearest approach) and an aphelion of 7.37593 x 10E9 km (49.305 A.U. at the time it is furthest from the sun).
It has an equatorial circumfrence of 7,232 km, a volume of 6.390 X10E9 km3, and a calculated mass of 13 X 10E21 kg. With its significantly lower mass its gravitational force with an escape velocity of 1,270 m/s (2,840 mph) is much lower than that of earth which has an escape velocity of 11.18 m/s (25,022 mph).
Pluto's day (sideral rotation period) is about 6.387 Earth days or 153.3 hours. However, its rotational direction is retrograde, the opposite that of earth.