Gliese 581d

Gliese 581d was the destination of a colony ship that the Fourth Doctor, Romana II and K9 Mark II encountered at some point after the 40th century. K9 indicated the planet was known as Earth 2.7 at the time of the ship's departure. (PROSE: All Snug in Their Beds)

Behind the scenes

 * In real life, Gliese 581 D is a possible extrasolar planet that could potentially be habitable.
 * During the time of the 2017 episode Smile, Gliese 581d is given as the name of the planet being visited in the story, on BBC Doctor Who website,  and is named as such by the interviewer of the story's writer Frank Cottrell-Boyce in DWM 512 - though not by Cottrell-Boyce himself.