Class zero planets are a waste to me, you can't do anything with them. Here's a couple of ideas.
1. mining or something like that.
2. Extreme planets could be just more expensive, and all planets are either extreme, or terran.
3. Same as above; except, that extreme planets are not more expensive. Class zero now becomes an extreme planet that requires you to research all advance options of extreme planets first. You could make class zero extreme planets more expensive. if you wanted to there would be no more advance option for class zero extreme planets.