Cassius

Cassius was a planet located beyond the orbit of Pluto. (DW: The Sun Makers) The Battle of Cassius against the Daleks occurred in space near the planet. (NA: Love and War)

Behind the Scenes
In the real world, other worlds beyond Pluto's orbit were discovered after The Sun Makers was produced, including Haumea, Makemake and Eris. While in the past these worlds might have qualified for "tenth planet" status, a decision by the International Astronomical Union (IAU) in 2006 stripped these worlds, along with Pluto and Ceres (which was once considered a planet and subsequently an asteroid), of their eligibility to be considered planets. A controversial decision, the IAU ruling is expected to be officially challenged at its future conferences.