Eris. Computer artwork of the dwarf planet Eris, formerly known as 2003 UB313 or Xena. Eris is the largest dwarf planet. 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. Eris fails because it is one of many Kuiper Belt objects in the outer solar system. Pluto is composed of rock and ice.