 Ceres ( / <unk> / ; minor @-@ planet designation : 1 Ceres ) is the largest object in the asteroid belt that lies between the orbits of Mars and Jupiter . Its diameter is approximately <unk> kilometers ( <unk> miles ) , making it the largest of the minor planets within the orbit of Neptune . The thirty @-@ third @-@ largest known body in the Solar System , it is the only one identified orbiting entirely within the orbit of Neptune that is a dwarf planet . <unk> of rock and ice , Ceres is estimated to comprise approximately one third of the mass of the entire asteroid belt . Ceres is the only object in the asteroid belt known to be rounded by its own gravity . From Earth , the apparent magnitude of Ceres ranges from 6 @.@ 7 to 9 @.@ 3 , and hence even at its brightest , it is too dim to be seen with the naked eye , except under extremely dark skies . 
