Planet 14

Planet Fourteen was the location of an as yet unseen encounter between The Doctor and the Cybermen. The cyber-coordinator told Tobias Vaughn that the Cybermen had seen The Doctor on Planet Fourteen prior to their invasion of earth circa 1979. (DW: The Invasion)

It is believed that Planet Fourteen is or was in the Sol star system. The name refers to a time when there were at least fourteen planets in the solar system. This count of planets is likely to include Mondas, The Moon, the Fifth Planet, Eris and Pluto as planets. (Doctor Who: Cybermen)

Significance
A faction of Mondassians are believed to have left Mondas, whilst it was drifting out of the solar system, to settle on Planet Fourteen and pursue their goal of total Cyber-conversion (Doctor Who: Cybermen). The existence of such a faction explains the survival of the Cyber race beyond the destruction of Mondas.