A distant body discovered in 2005, Eris has a diameter comparable with that of Pluto, but is rather more massive (its mass approaches a third of that of the Earth), making it the most massive of the dwarf planets currently known.
Eris follows a highly eccentric orbit taking it far outside the plane of the Solar System. It is thought to have originated in the inner parts of the Kuiper Belt, and been thrown into this extreme orbit by interactions with Neptune early in its history. At its farthest extent, Eris' orbit takes it nearly 100 AU from the Sun, making it the most distant significant object in the Solar System (it is presently close to this aphelion distance). At other points in its 557-year orbit, it can approach the Sun much more closely, sometimes coming nearer than Pluto.
The distant orbit of Eris carries it through the cold outer depths of the Solar System, and its surface is thought to the be covered with frozen methane, at least while at its current distance. As it approaches the Sun every few centuries, its warming surface may cause this methane covering to sublimate, or turn to gas; the details of this process are unclear.
Eris has a single known small moon named Dysnomia. When first discovered, Eris and its moon (which was identified some months after the dwarf planet itself) were provisionally named Xena and Gabrielle, after characters in the TV series Xena: Warrior Princess. The more formal names Eris and Dysnomia were later established to reflect the standard naming conventions for planets of this type.