[Will]

To protect planets, go beyond the planets themselves.

The best defense in my experience is an agressive offense and several layers of defensive measures. Make a few large attack fleets (10-20 ships) and raid enemy targets. Put mines on all the warps to your systems, including between your own systems. Station fighter Groups/satellite Groups over warps along with mines. If enemy ships aren't destroyed by the mines, they probably will be by satellites/fighters. Have fighter Groups on patrol as well, along with a few capital ships. Have a "sensor buoy" satellite in every system (Large sat., 1-comp,1-tachyon_sensor,1-LR_scanner,1-shield,1-ecm). Have satellites in orbit of planets armed with missiles and Point-Defense. Use weapon platforms that are all shields (damage absorbers) and with weapons. Have some fighters on the planet as well, and it would be a good idea to keep some troops too.

Now, with all this, it would take a while to put up all defenses, but they make it a lot more difficult to penetrate into your space. Basically you will want to use standard fleets for defense of outer areas and "automated" defenses for your inner colonies/shipyards.