[Aris_Sung]

Oh, one last suggestion before I forget.

If you put mines around your planet, it should decrease the amount of resources that gets exported from that planet.
Think about it. The more you mine a planet for its defense, you're basically cluttering up the space lanes that resource transports need to use to come and go. So with reduced lanes, there would only be so many transports that can come and go at a time.
This idea is just for balance, so that a defender doesn't mine all his planets with no down-side.