From: Vermont, USA
Are you aware that troops that land on planets that still have orbital defenses take serious losses on the way down? We decided that it was equally unrealistic, given that any landing most likely consists of many troop landing pods, to disallow any landing at all without complete space superiority. Instead, we designed a system that would cause increasingly severe losses to any invasion forces based on the strength of each layer of defense.
If your fleet and bases truly have superiority, destroying a troop transport before it can drop its landing pods is pretty easy. Bases will prioritize troop transports over other ship types if they are getting close to a planet. However, if your defenses are overwhelmed by a massive fleet battle taking place at the same time as the attempted landings, then it's reasonable to assume that some troops could in fact get through and land. In that case, the level of remaining defense determines the condition of those forces when they reach the surface (along with any surface planetary defense units).
Space control while the ground battle is underway then gives you a substantial bonus, but the only sure way to stop an invasion is to have good defenses at each level - a system fleet, good defensive bases, good planetary defenses and a mix of defending ground troops and installations.