How do i counter a 'Planet Killer' Swarm

Is planetary shields possible? If the AI sends only sieges, then these can reduce the damage your planet takes greatly.

Planet bombing mitigation for 1st level of Planetary Shields is 30%, then 50%, then 80% at max research.

Large hanger compliments with a small defense fleet set to engage locally inside a mass of repair platforms and turrets will do a number on those siege frigate fleets.  Siege frigates get weighted very small in the firepower matchup, so it's easy to present the ai with a losing situation they will run from, even when they can kill the planet.

Simple. Go for his core planets, take out his ship producing facilities, and lose a couple of your outer planets in exchange. Build hangar defences and Phase Inhibitors so there is no escape from your bomber spam. Planetary shields and suchlike are also good.

In this situation your goal is not to protect the planet but simply hold his units there while you crush his frigate factories. Alternatively, instead of building capital ships, build light, fast frigates to counter his siege frigate spam. Cobalt comes to mind. They're fast and do plenty of damage. When your opp sends 90 siege frigates at you, use a Phase Inhibitor, Planetary shields,and 90 Cobalts. He won't be coming back for more.

dont spam bombers vs siege frigs,

always keep armor types in mind, fighters do alot more damage vs lrf's, siege frigs and bombers

whilst bombers are meant to take out most of the remainers

Use Fighter + Phase inhibitor + Planetary Shiled.

You can also use LV6 Marza to kill the siege quickly.

And if they build 90 siege frigate, they will run out of limit very soon.

90 Siege Frigates must be goddamn'd expensive, in both credits, resources, and fleet supply.

They get torn apart by Fighters though. Fighter squadrons deal 19-22 DPS(depending on faction) each to siege boats, and carrier cruisers and hangars can pack 2 or 3 squadrons. That's serious damage raining down on those flimsy pricy trinkets that have no self-defense capability.

Phase Jump Inhibitors will screw 'em if they try to run, and you can milk experience points from the dying siege boats to level up your capital ships present in the system.

About the best option. Hell, I'd park all my new caps in that system and level them like crazy off the work of that one Marza. Easier then pirates !

But 90 siege frigates ? They can't have much of a fleet if they're building that many... 

Did I say 90 Siege??  It was more like ...120!  No, it must have been...150 Siege!  Yeah...it was 150.  What are you gonna do then, huh?  Huh?

1.  I don't think you got attacked by 90 siege.  Unless it was from about 5 AI's at one time.  Or you are adding up the siege from multiple attacks.

2.  Build more defenses

3.  Split your God Fleet up a bit and leave a little on defense.

4.  Go destroy the enemies worlds with your superior fleet, an eye for an eye.

thanx all, the strategies were helpful. Came up with a rather interesting sollution as well. I kept the bulk of the fleet (escort craft and the like, with a good 3/4 of the support craft) at the key border systems, and had my Capitol ships with a minimum escort and some support craft (repair frigates and such) head to the enemy worlds. 16 Capitol ships seems to do the trick rather nicely.

If your wondering about composition.

8x Kol Battle ships

1x Akkan Battle Cruiser

2x Sova Carrier

2x Dunov

3x Marza

15x Repair Frigs

20x Heavy Cruisers

20x Flacks

10x Light Carriers

I cant even count the defence fleet.

Late game i tend to get 20 or so siege crafts , thats a rough  number .

Thats too many caps. I find 6 to be the upper limit of usefulness.

Guys, he's playing the AI.  A long game against them without cornering their fleet gets you one hell of a siege frigate mass.  90 is not an unreasonable number for him to be running into.

Kole, you didn't list command cruisers.  They are quite useful, get them in equivalent numbers to the repair cruisers.

Cataclysm, TEC can play the AI with just capital ships easy, Advent too.  The ai is too braindead to get past the Dunov shield restore tactics, although he has way too many Kols and way too few Dunov's to make it stick.  He can still butcher less than game ending fleets just fine.  The fleet is near perpetual with half that many capital ships too, if you hate reinforcing, it's the only way to fly.

Too many capship and too few HC and carrier.

You'll die if you face the hard or unfair AI or human opponents.

Ok...sorry...I have just never seen this many siege....ever.  But I admit that I stopped playing the AI in epic multi day games many months ago.  If the AI is building these types of fleets, you can try making packs of carriers with fighters to do hit and run attacks on them.  Siege isn't real fast, and you should be able to inflict a lot of damage on it with some raiding.  Just use a scout first to locate teh siege pack and make sure it is safe for carriers, and run the carriers if help comes.  Better to fight the siege in some other system than wait for it to come to yours.