Dwarf planets. Computer artwork of three of the dwarf planets of the solar system, as defined in August 2006. From left: Eris (formerly 2003 UB313), Pluto and Ceres. Pluto, which used to be a planet, was demoted to the status of dwarf planet, reducing the solar system's number of planets to eight. The International Astronomical Union (IAU), in a vote on 24 August 2006, accepted the definition of a dwarf planet as an object large enough to be spherical due to gravity. But a true planet has to dominate its orbit around the Sun. Pluto and Eris fail because they are two of many objects in the Kuiper Belt of the outer solar system. Ceres is the largest body in the asteroid belt.