Pluto is a dwarf planet located in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the largest known trans-Neptunian object by volume, but is slightly less massive than Eris. Pluto is made primarily of ice and rock and is much smaller than the inner planets. It has mountains, valleys, plains, craters, and maybe glaciers. Pluto is named after the Roman god of the underworld and is the only planet named by an 11-year-old girl.

Pluto was discovered on February 18, 1930, at the Lowell Observatory in Flagstaff, Arizona, by astronomer Clyde W. Tombaugh, with contributions from William H. Pickering. Pluto used to be considered the ninth and most distant planet from the sun, but in August 2006, the International Astronomical Union (IAU) downgraded its status to that of a "dwarf planet". This means that from now on only the rocky worlds of the inner Solar System and the gas giants of the outer system will be designated as planets. The IAU defines a "dwarf planet" as a celestial body in direct orbit of the Sun that is massive enough that its shape is controlled by gravity, but has not cleared its orbit of other debris. Pluto satisfies the first two criteria, but not the third. Even one of its own moons, Charon, is about half of Plutos size.

Since Pluto is so far from Earth, little was known about the dwarf planets size or surface conditions until 2015, when NASAs New Horizons space probe made a close flyby of Pluto. New Horizons showed that Pluto has a diameter of 1,473 miles (2,370 km), less than one-fifth the diameter of Earth, and only about two-thirds as wide.