On January 5, 2005 the dwarf planet and trans-Neptunian object Eris was discovered. It was named after the Greek goddess of Strife and Discord – quite appropriately. Eris brought on the debate about the definition of a planet and subsequently the demotion of Pluto to a dwarf planet.
Dwarf planet Eris and its moon Dysnomia. Image by NASA, ESA & M. Brown.