The plan for the 40 player maps is that you start in groups of layers, each on you own planet. There's a unit cap divided over all players. As players are killed the unit cap for the remaining players increases.
The planet maps are actually fairly rectangular due to some optical trick. As for impacts, I think they said that almost every decent sized impact will result in a planetary catasthope. The difference is how fast it happens. I'm not sure of this though.
Also, the devs have said they ain't going to focus on space, so probably not much detailed movement. I think they might just allow us to move things from one orbit to another, but I hope they allow us to modify the trajectories, so we can avoid enemy fire or do an attack run or something.
Seem to me its only rendering object close to your camera and depending on the size. I.E.: while you are zoomed in on your little asteroid placing rockets, you only see the planet in front of you but not whats going on. So if the system is very large with 40ish player on it, you might not know wtf is going on on the other side of the system unless you zoom in and see an asteroid moving toward a planet, you see the asteroid but not whats on it and so on. Each game will be like a mini-mmo server i dont know but its very doable.
The entire 40 player match is confirmed to happen on one single server. These will probably be compagny supported servers and special events, since it will be fairly resource intensive. I'm pretty sure they are going to do a fog of war thing though, so that you can't see what happens on the other planets.