Pluto
Dwarf planets
(134340) Pluto
134340 Pluto
A dwarf planet in the solar system which until 2006 was known as the 9th major planet. Pluto revolves around the Sun in a highly elliptical orbit at a mean distance of 39.5 astronomical units once every about 248 years. The orbit eccentricity is 0.25 (compare with the Earth's 0.02) yielding a perihelion distance of 29.66 astronomical units and an aphelion distance of 48.87 AU. Its orbital inclination is 17 degrees, which is much higher than those of the other planets. Pluto's mass is 1.308 x 10^22 kg, that is 0.00218 Earth mass (0.177 Moon mass), its equatorial radius is 0.19 Earth radius, and its rotation period is equal to 6.39 Earth days. It has five known satellites, in order of distance from Pluto: Charon, Styx, Nix, Kerberos, and Hydra. Pluto's radius is estimated to be about 1150 km (0.18 Earths). Pluto is smaller than seven of the solar system's satellites (the Moon, Io, Europa, Ganymede, Callisto, Titan, and Triton). Pluto's surface has an estimated temperature of 37.5 K and is composed of more than 98% nitrogen ice, with traces of methane and carbon monoxide.