Can you remember anything else that happened in the game with regard to this particular planet?
I may've renamed it
before I found out there were aliens living there, but I'm not sure. I don't think anything else happened. They don't seem to have any ships around, and the colony may be tiny, considering the Thermal and EM signature: 55.
EDIT: I think I've come across another bug, this time much more serious. Every now and then, the game would hang when processing long turns (1+ days), often around the time NPRs fought elsewhere in the galaxy. But just now, the game has begun to refuse to process turns longer than 20 minutes. The game just stops responding indefinitely. I'll keep trying to advance time, hoping this problem goes away.
EDIT2: The problem's gone now, but it should be looked into, if at all possible. Maybe there's something wrong with the automatic sub-pulse selection, or the NPR-controlled time-skipping.