Avatar, also, proves that you don't need Earth to hang in the balance, or much of Earth at all, to make a successful science fiction film.
That a scifi story needs Earth to be successful is bullshit talk anyway. The success of a film depends on the execution and not on a list of checkpoints of stuff to put into. Oh, we need a scene on Earth at 8 minutes, a hot babe at 12 minutes, an explosion at 15 minutes, and the car chase at 24 minutes needs to be with BMW cars because market research shows that people associate BMW cars with aggression and action. All that is bullshit.
Sad thing is that some producers and writers actually think like that.