Read that a few days ago. The likely existence of a 10th planet brings to mind Zecharia Sitchin's
The 12th Planet which talks about a 10th planet with a 3600 year orbit - the existence of which was known to the Sumerians. According to Sitchin, the planet was captured by the sun early in the solar system's history and was responsible for the destruction of a planet between Earth and Mars (the asteriod belt and our moon are all that's left of this former planet).