 The categorization of Ceres has changed more than once and has been the subject of some disagreement . Johann <unk> Bode believed Ceres to be the " missing planet " he had proposed to exist between Mars and Jupiter , at a distance of <unk> 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 . 
