Hi,
I support the gist of this thread. The AI is not a challenging adversary, especially when it comes to defense, so any arguments that your defenses are currently effective are dubious.
Just think about how long it takes you to conquer an AI home planet with 16 or so gauss platforms clumped together. Do they even come close to destroying your battleships? They don't for me.
Many people have argued that it should be required to throw ships at a planet to defend it. This is a valid point. Assuming your defensive stations aren't totally useless, it is logical that you can defend a planet with a fleet somewhat weaker than the attacking fleet.
I would warn you guys up front that this developer has historically been resistant to wondrous ideas of new things. Chances are: it isn't going to happen. Not that I don't think it would be truly awesome if we had planets shooting antimatter missiles up from the surface, but the developer probably won't do it.
In order to maximize the chance of offering a suggestion that might matter, try to stick to concepts that are already present in the game.
I like BinaryAlgorithm's suggestion to add specialized defense platforms, like LRM platforms and flak platforms. This seems like a glaring omission to the game balance. Someone can send one Arcova to your system, find out where your defenses are, and send Perchiriens in at your weakest jump point. Then the Perchiriens just sit there and bomb your defenses unretaliated. They could also bring several fighters to kill your bombers, if you have hangar defenses. This is a balance issue.
LRM platforms would help defend against a load of Krosov Siege Frigates (maybe with a Kodiak or two thrown in for extra fodder). It's pretty easy to position, say, 20 Krosovs in a way that they aren't getting destroyed very fast, and by the time your fleets respond, you've lost the planet. Even if they aren't able to colonize it (they COULD with a well-positioned Protev, though), they'd wipe out your planetary improvements. This is an annoying hit and run tactic which exploits the weaknesses of the defense system.
Unless we have more effective static defenses, we'll need to resort to defending planets with ships. Due to the agility of having many, fast, agile frigates running around defending, this will mean the decline of the battleship in the game for anyone who wants to hang on to their planets. If you build battleships, there just aren't enough of them, and they aren't fast enough, to respond to attacks. It would be unfortunate to see the highly-competitive, top-of-the-ladder multiplayer battles being a delicate frigate-defense war (mostly Kodiaks and Perchiriens), with battleships hardly ever used.