Planet 14

Planet 14, or Sol XIV, was the fourteenth planet from Sol, and inhabited by Cybermen, who had emigrated to the planet from Mondas. (PROSE:  Iceberg) The Cyber-Planner once told Tobias Vaughn that both the Second Doctor and Jamie McCrimmon had been on Planet 14 prior to Vaughn's first encounter with the duo. (TV: The Invasion)

History
The Cyberman species emigrated to Planet 14 from Mondas whilst it was drifting through space, where they evolved separately from the Mondans who remained on Mondas. Whilst on Planet 14 they pursued their goal of total Cyber-conversion. (PROSE: Iceberg) They were driven away when the planet's own power was used against them, resulting in a catastrophic explosion. (PROSE: Scientific Adviser) This may have been an unrecorded encounter between the Doctor and the Cybermen, as the Cyber-Planner told Tobias Vaughn that the Cybermen had seen the Doctor and Jamie McCrimmon on Planet 14 prior to their invasion of Earth circa the 1970s. (TV: The Invasion)

Behind the scenes

 * One account of the Cybermen's creation shows the Voord of the planet Marinus using stolen time technology to "quick-evolve" themselves into the earliest Cybermen. (COMIC: The World Shapers) Marinus is explicitly called Planet 14 before it is "quick-evolved" into Mondas. This is a contradiction, though, because the Faction cannot move to Planet 14 from Mondas after Planet 14 became Mondas. If this is to be reconciled, Marinus can be called "Planet 14" but cannot be the 14th planet of the Sol system.
 * Accoring to REF: Doctor Who: Cybermen, a non-narrative source, Planet 14 may have once been a part of the Solar System.