Eris, optical image. Formerly known as 2003 UB313 or Xena, the dwarf planet Eris was discovered in 2003. Eris is the largest dwarf planet, and has at least one moon, Dysnomia (just right of centre). 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. A true planet has to dominate its orbit around the Sun. Eris fails because it is one of many Kuiper Belt objects in the outer solar system. Photographed at the W. M. Keck Observatory, Mauna Kea, Hawaii.