I think that the destruction of a planet would be a significant event that may be seperate from a space battle itself. I think that perhaps orbit of the planet must be fought for with the death star before one can destroy it. But its just a thought.
So in my eyes, the planets destruction would be the end of that battle.
i dont know about the DS but i notice if u retreat in a space battle, any enemy units near my space station would get damaged or destroyed.poor suckers