 The 2006 debate surrounding Pluto and what constitutes a planet led to Ceres being considered for <unk> as a planet . A proposal before the International Astronomical Union for the definition of a planet would have defined a planet as " a celestial body that ( a ) has sufficient mass for its self @-@ gravity to overcome rigid @-@ body forces so that it assumes a hydrostatic equilibrium ( nearly round ) shape , and ( b ) is in orbit around a star , and is neither a star nor a satellite of a planet " . Had this resolution been adopted , it would have made Ceres the fifth planet in order from the Sun . This never happened , however , and on 24 August 2006 a modified definition was adopted , carrying the additional requirement that a planet must have " cleared the neighborhood around its orbit " . By this definition , Ceres is not a planet because it does not dominate its orbit , sharing it as it does with the thousands of other asteroids in the asteroid belt and constituting only about a third of the mass of the belt . <unk> that met the first proposed definition but not the second , such as Ceres , were instead classified as dwarf planets . 
