Pluto was reclassified from a planet to a dwarf planet in 2006.
The change followed a decision by the International Astronomical Union, or IAU, which adopted a formal definition of a planet. Under that definition, an object must orbit the Sun, be massive enough for gravity to make it nearly round, and have cleared most other objects from its orbital neighborhood. Pluto satisfies the first two conditions but not the third.
Pluto was discovered in 1930 by Clyde Tombaugh at Lowell Observatory and was widely taught as the Solar System’s ninth planet for more than 75 years. The discovery of other distant icy bodies, including Eris, forced astronomers to reconsider whether Pluto should remain in the same category as the eight larger planets.
Pluto is now the best-known member of the dwarf-planet population beyond Neptune. It has a complex system of moons, including Charon, which is unusually large compared with Pluto itself. NASA’s New Horizons spacecraft flew past Pluto in July 2015, revealing mountains of water ice, nitrogen glaciers, and a heart-shaped region called Tombaugh Regio.