Pluto

Pluto is a dwarf planet in the outer Solar System, orbiting the Sun in the Kuiper Belt beyond Neptune. It was previously considered the ninth planet, but was reclassified in 2006 by the International Astronomical Union.

Discovered in 1930 by Clyde Tombaugh at Lowell Observatory during a search for a planet beyond Neptune, Pluto was named after the Roman god of the underworld. The name Pluto was suggested by an 11-year-old girl and was officially adopted that year.

Pluto's diameter is about 2,377 kilometers, and its mass is roughly 1.303 × 10^22 kilograms. It orbits the Sun at an average distance of about 39.5 astronomical units (AU) with a period of about 248 Earth years. Its orbit is highly inclined and eccentric, bringing it closer to and farther from the Sun over the course of centuries. The Pluto–Charon system is sometimes described as binary because the system’s barycenter lies near or outside Pluto itself.

The surface is dominated by ices of nitrogen, methane and carbon monoxide. The northern-to-southern hemispheres include a prominent heart-shaped region known as Tombaugh Regio, which contains Sputnik Planitia, a large plain of nitrogen ice. The terrain displays a mix of flat plains, polygonal ice regions, and rugged mountains thought to be made of water ice.

Pluto has five known moons: Charon, Nix, Hydra, Kerberos, and Styx. Charon is relatively large, and the Pluto–Charon system form a gravitational pair with a barycenter that lies outside Pluto.

In 2015, NASA's New Horizons spacecraft performed a flyby, providing the first close-up images and measurements of Pluto, revealing a complex and varied landscape and an atmosphere of nitrogen with minor components.