The categorization of Ceres have changed more than once and have been the subject of some disagreement . Johann Elert Bode believed Ceres to be the " missing planet " he had proposed to exist between Mars and Jupiter , at a distance of 419 million km ( 2 @.@ 8 AU ) from the Sun . Ceres was assigned a planetary symbol , and remained listed as a planet in astronomy books and tables ( along with 2 Pallas , 3 Juno , and 4 Vesta ) for half a century .