Terra Scores do not decrease unless you overuse a terraforming tool. They normally stay where you put them, assuming that you first terraform it with the tools, then stabilize the results with the insertion of plants and animals....something you have to do in order for the terraforming to be of any USE!
When I place a colony on a new planet and go to build/add buldings, houses or whatever it says the Terra Scroe is to low and the buttons are disabled I have tried placing animals and plants and the dam animals die within a few seconds(lol) so am I missing something?
I know your suppose to try and get the little dot-thing to the center by useing the tools,lower atmospher or raise the tempature but it NEVER works for me and never goes in the center so what are you suupose to do to make it "perfect"