why can't she be possessed off planet? Maybe im missing something here.
Well, we know that the Phoenix is on a collision course with Earth. To the best of my knowledge, that's all we know. Cap's presuming that it's coming because of Hope because they share the same energy source.
Taking a step back, there's two options here:
1) Get Hope off-world and hope that the Phoenix follows.
2) Keep Hope on world and hope that she can control it.
If they choose option one and the Phoenix doesn't follow, Earth goes boom. If they choose option two and they're wrong, they stab the shit out of Hope and put the Phoenix down (just like what happened the last time it came to Earth).
Unless there's some sort of evidence that the Phoenix is definitely going to follow Hope, I wouldn't move her off world. I can understand Cap wanting to control the situation for Option Two (especially as he has an Avenger brought onto the team expressly for killing things) but Option One seems as needlessly risky as doing nothing and trusting Cyke that everything's going to be alright.