Would it be possible for a group of players to just. leave their session taking their planets and battlefield and go to another one. and that session just stays as is without them? Like if the first group (the one that leaves their session) has a time player but not a space player and they join a group that has a space player but not a time player?

I don’t see why not. As long as they have someone that can navigate the endless abyss that is the Furthest Ring.