Pluto

Pluto has an diameter of about 1,477 miles (2,377 kilometers). Pluto is about 1/5th the width of Earth. Pluto is about two-thirds the diameter of Earth's Moon and probably has a rocky core surrounded by a mantle of water ice. Interesting ices like methane and nitrogen frost coat the surface. Due to its lower density, Pluto's mass is about one-sixth that of Earth's Moon. Pluto's orbit around the Sun is unusual compared to the planets: it's both elliptical and tilted. Pluto's 248-year-long, oval-shaped orbit can take it as far as 49.3 astronomical units (AU) from the Sun, and as close as 30 AU. From 1979 to 1999, Pluto was near perihelion, when it is closest to the Sun. During this time, Pluto was actually closer to the Sun than Neptune. One day on Pluto takes about 153 hours. Its axis of rotation is tilted 57 degrees with respect to the plane of its orbit around the Sun, so it spins almost on its side. Pluto also exhibits a retrograde rotation; spinning from east to west like Venus and Uranus. Pluto has five known moons: Charon, Nix, Hydra, Kerberos, and Styx. This moon system might have formed by a collision between Pluto and another similar-sized body early in the history of the solar system. The temperature on Pluto can be as cold as -375 to -400°F (-226 to -240°C). Discovered in 1930, Pluto was long considered our solar system's ninth planet. But after the discovery of similar intriguing worlds deeper in the Kuiper Belt, tiny Pluto was reclassified as a dwarf planet in 2006 by the International Astronomical Union. Pluto was named by an 11-year-old girl. In 1930, Venetia Burney of Oxford, England, suggested to her grandfather that the new discovery be named for the Roman god of the underworld. He forwarded the name to the Lowell Observatory and it was selected. Pluto is the name of the Roman god of the underworld, equivalent to the Greek Hades.