Planet X was the most boring planet in an alternate Doctor's universe. It was named Planet X because it was too boring to give the planet a name.
It was a human colony. During the Great War, the planet's government decided to make their colony so boring that nobody in the conflict would be interested in going to the planet. The plan worked and Planet X was the only planet to be unaffected by the Great War.
It's inhabitants were killed if they showed any signs of wanting to change the planet. (AUDIO: Planet X)