Simplify the following text for a Grade 18 native English-speaking person.
Original: Still others strike a middle ground , such as Christopher McKay , who argues that terraforming is ethically sound only once we have completely assured that an alien planet does not harbor life of its own ; but that if it does , while we should not try to reshape the planet to our own use , we should engineer the planet 's environment to artificially nurture the alien life and help it thrive and co-evolve , or even co-exist with humans .
Simplified: Others say that terraforming sounds ethically sound once we know that the planet we are terraforming has no other life of its own , but if it does , while we should not try to reshape the planet to our own use , we should engineer the planet 's environment to artificially nurture the alien life and help it thrive and co-evolve , or even co-exist with humans .