Eris is a Dwarf Planet in our Solar System, located three times as far away as Pluto. It is inside the Kuiper belt. Eris is slightly more massive than Pluto and is sometimes called the tenth planet though it isn't clasified as a Planet.
Eris is further from the Sun than any other known dwarf planet, because Eris is so far away from the sun it's cold there at -231°C. Eris takes 560.90 Earth Years to orbit the sun.  That means when Eris was last in the same position relative to the sun where it is now people still believed the Geocentric theory and Copernicus hadn't written his important book showing the Heliocentric theory (that the earth goes round the sun) makes sense. We found out a great deal about Science during that one revolution of Eris.
Other dwarf planets may be discovered even further from the sun, we don't know. Eris has a moon called Dysnomia.