Retreating to a 300 size moon

Enemy colonize a planet with the intent to use it as a missile
You colonize another planet, he now has a choice of what planet to hit with his one planet rocket.

And if there is only two planets available, then he will either launch his commander onto your planet, or you can secure your planet with orbital fighters, and when the planet rocket comes, just lift your commander into orbit.

I do think there should be a way to get to another planet faster, unit cannons will solve this i hope.