Planet (Dark Water)

At some point in history, whilst travelling alone, The Twelfth Doctor landed his TARDIS on this rocky, purple-skied planet.

Astronomical data
The planet had a bright purple sky and its surface appeared rocky and barren, littered with strange monolithic structures and walkways connecting to what appeared to be a cave system built into a large cliff face. The entrances to such caves were covered in large, glowing red egg-sac type growths, possibly an alien hatchery, suggesting the planet contained some native lifeforms.

Following the death of Danny Pink, Clara Oswald tried to call the Doctor, who was at the time visiting the planet. It took some time for him to pick up the phone because he apparently was "busy". (TV: Dark Water)