Well. Here's one way to do it. In Civilization II: Test of Time, there were two mods that had 'levels'. In the Fantasy mod, there was 4. In the Science Fiction mod, there were 2. These two were space and the planet surface.
So, ya could do somethin similar. Make it so space and planets are two separate levels. Then, on a planet, you can never leave the planet w/o spacecraft.
Jus an idea. Idk. I could elaborate if you wish.