In 2006, the International Astronomical Union decided that Pluto was not a planet in the Earth's solar system. Pluto was named a dwarf planet after a discovery of a similar-sized object deeper in the Kuiper belt, which was later named Eris.
One reason Pluto was declassified as a planet was that its orbit around the sun is shaped like an oval, and it does not clear other objects out of its path. This is because it is too small to have a large enough gravitational pull to push them out of the way. Other dwarf planets include Haumea, Makemake and Ceres.