Why is Ceres now being called a “planet”?
An object in orbit around a star is now being called a “planet” if it is large enough (or more technically, massive enough) for its own gravity to pull it into a nearly spherical shape. Recent Hubble Space Telescope images that resolve the size and shape of Ceres show it to be nearly spherical. More technically, Ceres is found to have a shape that is in a state of hydrostatic equilibrium under self-gravity. Therefore Ceres is a planet because it satisfies the IAU definition of “planet.” [Published reference for shape of Ceres: P. Thomas et al. (2005), Nature 437, 224-227. Dr. Peter Thomas is at Cornell University.] Q: Is Ceres a dwarf planet? A: If one describes any planet smaller than Mercury as a “dwarf planet”, then Ceres could be called a dwarf planet. Note that “dwarf planet” is simply a descriptive category like “terrestrial planet” and “giant planet.” None of these descriptive categories are formally defined by the IAU. Q: Is Ceres a “pluton”? A: No. Q: What is 2003 UB313? A: “2
