If we were so inclined, could we smash planets into other planets? While it would take more kick to get them moving, it would be satisfying to reject your opponent's attempt to take a planet by crushing it with a bigger planet.
People have asked about this dozens of times already. So far, Uber hasn't no. I don't think they've gone as far as saying yes, of course (someone please correct me if I'm wrong) yet though. While it would be cool to have, I think this should remain something the modding community can whack together.
I think they have said that the engines for asteroids sinply don't have the thrust needed to move anything larger (Requiring the entire side of an asteroid to have enough to move it).
Perhaps they'll make an engine that requires massive amounts of energy which are capable of pushing a planet but there's really no point to it for the vanilla game, I'd rather them make it to where you can just edit different parts of the engine and mod it in as hitting something with a planet seems more like a 'cool factor' than really accomplishing much.
I really like this idea, picture if you will...a battle on a planet that's being propelled at another world....your goal is to capture or destroy the engines in time to actually prevent the impact...probably capture, since destroying them would also remove your ability to stop the devastation. I think things like this would be epic for large scale multiplayer, with temporary alliances forming between different factions in a desperate bid to prevent Planetary Annihilation (natch!) This being not a story element, but something an insane trolling player decided to do (or is doing to eliminate competition while he hangs out on an asteroid). Could be awesome.