It's a tough call. In MY games, the minimum number of separate Keeper cards you must use to meet a Goal is equal to the number of Keepers required for that goal, but it remains to be seen what Andy has to say about this. I expect we'll have a FAQ entry soon. (Perhaps he'll go ahead and post one for your Earth question, too.)
Thanks for your response. I'm starting to suspect that it wasn't Andy at all; maybe I read somebody's house rule somewhere. If that's so, it would be worthwhile getting an official answer to that one, too.