planet has been poisoned. How is this remedied?

My planet earth was poisoned by one of the minor races. It went from about a 18 to a 10. I was told to exterminate the race to get rid of the problem and did so. I am curious as to how to get my planet back up to 18 as it is still at 10. Perhaps it heals over time?
The game is great by the way..it sure has sucked me in!
Thanks in advance..I'm still learning a lot!