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Pluto is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. Discovered by Clyde Tombaugh in 1930, Pluto was originally considered to be the ninth planet from the Sun. Following the discovery of several objects of similar size in the Kuiper belt, its status as a planet was questioned, and in 2006 the International Astronomical Union (IAU) gave a definition of the term "planet" that excluded Pluto. The largest and second-most-massive known dwarf planet in the Solar System, Pluto is primarily made of ice and rock. It is relatively small, with a moderately eccentric and inclined orbit.

This photograph of Pluto is a composite of four near-true color images taken by the New Horizons spacecraft in 2015. The most prominent feature in the image, the bright, youthful, nitrogen ice plains of Sputnik Planitia, the left lobe of heart-shaped Tombaugh Regio, is at right center. This contrasts with the darker, more cratered terrain of Cthulhu Macula at lower left.Photograph: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute