I was hoping for some sort of planetary or orbital based anti-ship defenses like you have in MOO
End of TheTim's quote
They have never been in the GC franchise. Which isn't to say they never will be in GC4 (it's always a possibility, right?), but don't count on it. If you want to defend your planets, you need a navy. Even the military starbases' primary purpose is to support naval combat within its zone of influence.
Luckily, it's easier to defend a small area (ie your planets) with a fleet, than it is to send a fleet across the map and successfully attack you. There are a few mechanics to this - but the defender does have the edge here. Plus, if you build enough warships, no one will attack you in the first place, this is probably the easiest defense - no one gets mugged when they're holding a gun (except for those two guys in the movie 'Snatch')...
Hope this helps,
-tid242