Pluto was discovered on February 18, 1930, 90 years ago and was a planet until 2006. However, when the International Astronomical Union (IAU) defined the term planet, Pluto was reclassified as a dwarf planet.
Pluto is an icy dwarf planet in the Kuiper belt which is a ring of bodies beyond the orbit of Neptune. It was the first Kuiper belt object to be discovered and is the largest known dwarf planet.
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Pluto was discovered by Clyde Tombaugh in 1930 as the ninth planet from the Sun. After 1992, its status as a planet was questioned after the discovery of several objects of similar size in the Kuiper belt. In 2005, Eris, a dwarf planet in the scattered disc which is 27% more massive than Pluto, was discovered.
This led the International Astronomical Union (IAU) to define the term “planet” formally in 2006, during their 26th General Assembly. That definition excluded Pluto and reclassified it as a dwarf planet.
Pluto has five known moons: Charon (the largest, with a diameter just over half that of Pluto), Styx, Nix, Kerberos, and Hydra. Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body.
Despite Pluto’s orbit appearing to cross that of Neptune when viewed from directly above, the two objects’ orbits are aligned so that they can never collide or even approach closely.