Eris

Eris is a dwarf planet located in the outer solar system, and was considered the 10th planet after Pluto and Neptune before the IAU decided that planets and dwarf planets should be decoupled in 2005. It is barely smaller than Pluto. Eris has a seasonal atmosphere, and is in the outer Solar System.

Discovery
Eris was discovered in 2003 by Mike Brown and his team. It had a provisional designation of 2003 UB313.