What is the name of the 10th planet?

The name of the 10th planet is 'Eris.' The planet was discovered by Dr. Mike Brown of the California Institute of Technology, David Rabinowitz of Yale University and Chad Trujillo of the Gemini Observatory in Mauna Kea, Hawaii in July 2005 and was officially named the following year.

At nine billion miles from the sun, Eris is the farthest known object in the solar system, resting in the Kuiper Belt, the dark disc of icy bodies beyond the planet Neptune. It was first photographed in 2003, but was so far away that its size, at roughly 70 miles wider than Pluto, and motion were not yet known.