What do planetary defenses do?

I believe it gives a bonus to you're defending soldiers in an invasion.

I thought the game explained that they give a bonus to ships defending in the system.

thats what i thought aswell

Well, there's a bit of a problem here -- the tech description talks about the ships defending the system, but the planetary improvements talk about defending against invasions (if you click the ? button that shows up in the middle when you select Planetary Defenses from the build list).

This question has been popping up on the board almost every other day since the release -- any chance someone from Stardock could settle this once and for all?

And also answer whether or not the defenses stack between multiple planets at the same star. Going by the descriptions i would expect Star System Defense to stack but not Planetary Defense, however as a programmer i suspect that it's all or nothing

well if they stacked there would be a benefit to colonizing multiple barely haitable planets with 100 mil civs lets say and building those defensive structures. However I wouldnt like that makes it too complex, they should amke it average of all bonuses divided by number of colonies in the system :/ Makes the most sense since you arent inviading palnets that dont have colonies and you are invading all colonies at once.