Pluto
Dwarf planet
Facts
- Diameter
- 2377 km, about two-thirds of the Moon's
- Mean distance from the Sun
- 5.9 billion km
- Greatest distance from the Sun
- 7.4 billion km
- Orbital period
- 248 years
- Rotation
- 6.4 days, backwards
- Mean temperature
- −225 °C
Pluto is a dwarf planet in the Kuiper belt, beyond Neptune. For decades it was considered the ninth planet; in 2006 the International Astronomical Union reclassified it as a dwarf planet.
It was discovered by Clyde Tombaugh, an assistant at the Lowell Observatory in Arizona, on photographic plates taken in February 1930. The name was suggested by Venetia Burney, an 11-year-old girl from Oxford: that of the Roman god of the underworld.
NASA's New Horizons probe flew past it on 14 July 2015, the first and so far only mission to explore it. Its images show a large bright region shaped like a heart. Pluto also has plains of frozen nitrogen with no craters.
It has a thin atmosphere, mostly nitrogen, which expands as it nears the Sun, like a comet's. Its largest moon, Charon, is about half the size of Pluto and always shows it the same face.