There is (I think) still several NPC homeworlds that are not setup yet. I would suspect that the various planets made for factions have had cities moved to them, but I wouldn't risk it. Not until a Sim News has been made stating the fact.
That or you noticed that a planet was controlled, and it has become uncontrolled in the last few days or so (ie there are slabs already there). Then there is a chance its just the usual incompetence that has caused it to lose control.