|the second-largest dwarf planet in the solar system, located in the Kuiper belt; discovered in 1930 by Clyde Tombaugh (1906--97); classified as a planet until 2006, when it was reclassified as a dwarf planet. It has a diameter of 2390 km|
|[Latin, from Greek Ploutōn, literally: the rich one]|
|Pluto (pl'tō) Pronunciation Key
The ninth and usually farthest planet from the Sun as well as the smallest in size, with a diameter about one-sixth that of Earth. Pluto was not discovered until 1930, when Clyde Tombaugh noticed it while searching for an unknown planet thought to influence Uranus's orbit. Pluto's surface is covered with frozen methane and other ices, and its extremely thin atmosphere consists primarily of methane and nitrogen. Between 1979 and 1999 Pluto crossed inside Neptune's orbit and became, temporarily, the eighth planet in distance from the Sun. Because of its small size (it is smaller than Earth's moon) and the unusual eccentricity and inclination of its orbit, many astronomers have questioned whether it should be regarded as a planet at all, suggesting that Pluto and its moon, Charon, are actually large Kuiper belt objects. See Table at solar system.
Note: Astronomers in the late nineteenth century, thinking they saw disturbances in the orbits of Uranus and Neptune, suspected that there was a ninth planet, not yet discovered, exerting gravitation on the other two. In the early twentieth century, astronomers searched for that planet and found Pluto. Ironically, Pluto is much too small to be the planet they sought.
Note: Pluto's orbit is a stretched ellipse, unlike the orbits of the other major planets, which are nearly circular. As a result, for a period ending in 1999, Pluto was actually closer to the sun than Neptune.
Note: There is some debate among astronomers as to whether Pluto should really be classified as a planet or should instead be considered a large asteroid-like body.