Eris is a dwarf planet beyond Pluto, discovered in 2005. It is the most massive and second-largest known dwarf planet in the Solar System. It is also the ninth-most-massive body known to orbit the Sun directly. Eris is estimated to be 2, 326 kilometers in diameter and is composed of rock and ice. It is believed to have a highly reflective surface, which is why it was discovered by astronomers.