Eris

Eris is a dwarf planet in our solar system that is located beyond the orbit of Pluto (its distance from the sun is 96.6 AU, three times that of Pluto). With its discovery in 2005, astronomers had to either include it as a 10th planet or redefine the definition of a planet (this ended up removing Pluto from planet status). It is unknown whether Eris is actually larger than Pluto, but Eris does has a moon, Dysnomia. Both Pluto and Eris are now considered dwarf planets in our solar system.