A planet was the home planet of Plex after his original home planet had been destroyed.
The Ninth Doctor visited Plex to deliver the news that it had been destroyed in a freak singularity but discovered that Plex had created hundreds of clones of himself to populate the empty planet where he now resided. In light of Plex having recently become the last of his species, the Doctor realised that Plex's plan made sense and gave him a modified Chameleon Arch biodata module to "copy and paste" his memories into the clones. A statue depicting the Doctor with the body of Plex's species was mounted over the entrance of Plex's tomb. (COMIC: The Promise)