Why is 2003 UB313 a planet?
Recent Hubble Space Telescope images have resolved the size of 2003 UB313 showing it to be as large as, or larger than Pluto. Any object having this size, and any reasonable estimate of density, is understood to have sufficient mass that its own gravity will pull it into a nearly spherical shape determined by hydrostatic equilibrium. Therefore, 2003 UB313 is a planet because it satisfies the IAU definition of “planet.” [Published reference: M. Brown et al. (2006). Astrophysical Journal643, L61-L63. Dr.
