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, worlds beyond Pluto's orbit were discovered after The Sun Makers was produced, including Haumea, Makemake and Eris. While in the past these 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. In the context of the Doctor Who Universe, the fact K9, a robot from the year 5000, refers to Pluto as a planet suggests that at some future time before 5000 Pluto's planetary status will be restored. As for Cassius, it's possible that the other bodies found beyond Pluto (Eris, etc) will either never qualify for planetary status, or one of them will be renamed. In 2011 a number of scientists predicted that a new NASA space telescope may be able to detect an undeniably planet-sized object dubbed Tyche.