Gyre

Gyre was a Comet in the universe that a group of humans crash landed on. These humans then started to become Primitive only thinking that the comet was planet Earth. The comet was also going to collide with an Asteroid and if that happened then everyone in the galaxy nearby would have been whiped out, including the Sittun. The Doctor and Amy did visit the coment and found all this out they aided the Sittun with their plan to destroy it as it was the kindest way for the humans to go. (NSA: Night of the Humans)