The discovery of the dwarf planet Eris was announced on July 29, 2005. It is the ninth largest body orbiting the Sun (yep, bigger than Pluto.) It’s roughly one-quarter more massive than Pluto and one-quarter as massive as the Earth. Eris appears grey and has methane ice on the surface. It has a small moon called Dysnomia.
It’s hard to believe something in our solar system bigger than Pluto was discovered so recently, but it is very far away from the Sun and is not on the plane where we expect to find planets. It’s about three times farther away from the Sun than Pluto right now. It has a very eccentric orbit and the distance can vary from 37.9 AU to 97.5 AU.
When it was first discovered, it was called the tenth planet. This led to the redefinition of the term planet and the demotion of Pluto to dwarf planet status. It was known informally first as Planet X and then Xena until it received the formal name Eris, after the Greek goddess of strife and discord. The moon Dysnomia is named for the Greek goddess of lawlessness (note the Xena connection–astronomers do enjoy their jokes.)