Pluto

American astronomer Clyde Tombaugh discovered Pluto at Lowell Observatory on 18 February 1930. The name Pluto, after the Roman god of the underworld, was suggested by Venetia Burney, an 11-year-old schoolgirl in Oxford, and selected by the observatory. Pluto was initially regarded as the ninth planet. In 2006, the International Astronomical Union (IAU) defined a planet in the Solar System as a body that orbits the Sun, is nearly round under its own gravity, and has cleared its orbital neighbourhood. Pluto meets the first two criteria but shares its region with other bodies, so under the IAU definition it is classified as a dwarf planet. This is a classification, not a change in Pluto’s orbit or physical nature.

Pluto lies in the Kuiper Belt, a distant region beyond Neptune that contains many small icy bodies left over from the Solar System’s formation. Its orbit is more elongated and more tilted relative to the plane of the planets than the major planets’ orbits. Pluto’s distance from the Sun varies substantially during its roughly 248-Earth-year orbit; it can travel inside Neptune’s average orbital distance without approaching Neptune closely, because the two bodies’ orbits are arranged in a stable resonance. The Sun is so distant that its light takes more than five hours to reach Pluto.

Images from New Horizons revealed a varied surface of mountains, valleys, plains, craters and glaciers. Much of the landscape is shaped by volatile ices, including nitrogen and methane, rather than by liquid water at the surface. Pluto has a thin atmosphere made mainly of nitrogen, with methane and carbon monoxide also detected. As Pluto moves around the Sun, surface ices can turn to gas and replenish the atmosphere; as it recedes, some atmospheric gases can freeze back onto the surface. These seasonal changes occur in an exceptionally cold environment, where the average surface temperature is about −230 °C.

Pluto has five known moons: Charon, Styx, Nix, Kerberos and Hydra. Charon is by far the largest, at about half Pluto’s size; the pair are sometimes described as a double system. Before spacecraft observations, Pluto’s small size and great distance made detailed study of its surface difficult. NASA’s New Horizons mission flew past Pluto on 14 July 2015, returning the first close images and measurements of the dwarf planet and its moons. The encounter showed that Pluto is geologically varied, rather than a featureless frozen point of light.