You might want to check that out i’m sure it’d be pretty useful to you. Also for planets this could be replicated in a few different ways actually. I remember a very long time ago working with the OGRE3D Engine and an old friend had created a ‘rounded’ planet with proper camera controls. So yes, it is more then possible to create this with Unreal Engine 4.
So basically to answer your question; yeah that’s all more then possible. You might want to take a look at Space Engine here http://en.spaceengine.org/ and look at the problems that they have encountered. I can promise you one thing; a proper space game is going to take ALOT of memory to run.