Lets think about it, One tile away from a planet is roughly 5 or so earth-masses away from the planet, Missiles cant go that far, Laser beams get dissapated in the atmosphere and mass drivers sure as hell cant make it that far.
That takes a one sector defensive radius for planets out of the question.
But anyways, For sanity's sake use your imagination, Just because during the planetary invasion screen you see a fairly average battlefield, Let your brain fill in the details, Imagine civilians who are good at playing missile command conscripted to staff Silo's, Imagine Drengin Shock Troops running the streets in tight, efficient packs, Use your imagination!