Cassius

The Doctor mentioned that until the discovery of Cassius, Human astronomers regarded Pluto as the farthest-out in the Sol system.


 * Quite possibly one of the planets further out than Pluto orginally had another name and got the name of Cassius later.

Behind the Scenes
After the making of The Sun Makers, astronomers in the real world did find a tenth planet in the Solar system, but it received a different name: Eris.  Another plutoid known as Makemake was officially named a dwarf planet and is closer ot the Sun than Eris, so it may also be a real-life "Cassius".