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.''