The name of the tenth planet is Eris. It is the largest dwarf planet that is known and is more massive than Pluto. It is currently the most distant known natural object in the solar system along with its moon Dysnomia.
1 Additional Answer
I hate to be the one to tell you this but there are only eight planets. There where nine but recently Pluto has been degraded to a planetoid. Beyond Pluto is the Ort Cloud where comets and frozen debris reside.