Cassius

Cassius was a planet located beyond the orbit of Pluto. (TV: The Sun Makers) The Battle of Cassius was fought against the Daleks near the planet. (PROSE: Love and War)

Behind the scenes

 * In the real world, worlds beyond Pluto's orbit were discovered after The Sun Makers was produced, including, and . While in the past these might have qualified for "tenth planet" status (NASA did indeed at one point declare the as-yet-unnamed Eris as the tenth planet soon after its discovery), a decision by the International Astronomical Union (IAU) in 2006 stripped these worlds, along with Pluto and  (which was once considered a planet and subsequently an asteroid), of their status as planets, creating a new category of dwarf planet. A controversial decision, the IAU ruling is expected to be officially challenged at future conferences. In the context of the Doctor Who Universe, the fact that 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 is 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 (or Cassius exists closer to the Sun than Eris or the others). In 2011 a number of scientists predicted that a new NASA space telescope may be able to detect an undeniably planet-sized object provisionally dubbed Tyche. However, pending a future change in IAU policy, even if Tyche is confirmed, it would only bring the solar system back up to having nine planets.