Eris

In January 2005, Eris was discovered, sparking a debate among scientist about what the definition of a planet really was. Eris was originally deemed the 10th planet, but it and other Kuiper and Asteroid Belt objects were given the new class of Dwarf Planet. Eris is beyond the orbit of Pluto, and is roughly the same size, although is was originally thought to be larger than Pluto. Eris has one known moon, named Dysnomia. Eris and Dysnomia are the most distant known natural objects in the Solar System.