Planets next to AI homeworlds are known as 'Core' planets, and they work a bit differently than regular planets. They have two warp gates, and are able to reinforce at these warp gates.
As an aside, leaving warp gates on conquered planets is a viable strategy, but not recommended. This is especially so on difficulty 7 and up. If you're yet to play on 7 or above, I would suggest doing so, because waves are much more dangerous on higher difficulties. Difficulty 7 is considered the standard difficulty, and is what all AI War players should consider aiming for to get the full AI War experience. I jumped from 5 to 7 a while back and the change in wave threat is extremely pronounced.