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, REF: Doctor Who: Cybermen) 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, thus these "earliest Cybermen" may be the Cybermen of Planet 14, and Marinus may be an alias of Planet 14.
 * Accoring to REF: Doctor Who: Cybermen, a non-narrative source, Planet 14 may have once been a part of the Solar System.