Eris (aka 2003 UB313) is the second largest dwarf planet in the Solar System, next to Pluto, and almost three times farther from the Sun.
This is the best image we have so far using the adaptive optics of the 10-meter Keck Telescope. This photo also revealed that Eris has a moon, named Dysnomia, visible just to the right of Eris.
This animation shows the movement of Eris on the photographs that led to its discovery by Mike Brown's team a few months later. Originally thought to be a planet its discovery was the impetus for the IAU to finally come up with an official definition of a "planet" which ultimately lead to Pluto's demotion to a dwarf planet.