It was first seen in 2005 and is around 67 astronomical units away from the Sun, that's 67 times further away from the Sun then we are on planet Earth. It is the biggest of all of the dwarf planets we currently know of - and because it's bigger then Pluto its discovery caused a debate amongst astronomers as to whether the new object would be a planet. The discussion eventually led to Pluto being de-classified as a proper planet in 2006.
Although we cannot yet be sure, we think that Eris is an icy, rocky object that is about three-quarters the size of our Moon. Like other dwarf planets, Eris's orbit is tilted by 44° to the orbits of the main planets, and takes 558 years to complete one journey around the Sun, in a very elliptical orbit.
Eris has one known moon of it's own, called Dysnomia, which is around 150km across and orbits the dwarf planet every 15.8 days.