Why is Pluto not a planet anymore?

Pluto was discovered in 1930 and counted as the ninth planet for 76 years. In the 1990s and 2000s astronomers found many similar icy bodies in the Kuiper Belt, including Eris, which was initially thought to be larger than Pluto. To settle the question, the International Astronomical Union voted in August 2006 on a formal definition of a planet. A planet must orbit the Sun, be massive enough to become round and have cleared its orbit of other debris. Pluto meets the first two criteria but shares its orbit with many Kuiper Belt objects, so it was classed as a dwarf planet.