I refer you back to said SE modding community.
Seriously though, the planetary defense penalty is good in the sense that I'm sure I'm not the only one who watched in horror as one of their ships closed in on an enemy planet, fired all it's weapons... and missed completely. Nor, I'm betting, am I the only one who cried "You missed a planet?! How can you miss a planet?!!" But I do think there should be some representation of the fact that while it is very easy to hit a planet, it's a fair bit trickier to hit a specific target on a planet. I can't think of how that might work though. Maybe planets should have a large amount of leaky armor, thus making it so that while you may be guaranteed hitting the planet, hitting a weapon's platform or population in general would become considerably harder.