In the Dr. Who episode "Image of the Fendahl", it was revealed that the Timelords had destroyed the 5th planet of our solar system and time-locked it to keep anyone from time-traveling to the pre-destruction period because of the sheer noxiousness of the race inhabiting it. (Parasitic, predatory mind-controlling creatures that assimilated less mentally-powerful races--namely humans--into group minds controlled by the Fendahl leader, IIRC).