Ceres. Hubble Space Telescope (HST) image of the dwarf planet Ceres, the largest object in the asteroid belt, with a diameter of 950 kilometres. Bright regions on the surface are thought to have been caused by impacts. A dwarf planet is defined as an object large enough to be spherical due to its gravity, and with an independent orbit around the Sun (ie. not a satellite of a planet). A true planet has to dominate its orbit around the Sun. Ceres fails because it is one of many objects in the asteroid belt. Image taken by Hubble's Advanced Camera for Surveys.