I recently discovered that if you destroy a colony using the Destroy Colony button under planet details, it reduces the Planet Quality to 0, making uncolonizable from that point onward. I first discovered this in beta 4 of TA, but it is still true in v1.80g Beta 4a. Is this meant to be the case? It wasn't like that in Dread Lords (I just tried it out). I hadn't caught this until now because I am not in the habit of abandoning colonies.