In astronomy, the smallest of the major planets, usually ninth from the sun. Pluto was discovered in 1930 and is named for the Roman god of the underworld. (See 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.