The only real defense for your planets is your own fleet.

Mines can be a useful defense in the early game, and specialist warp-point defenses can help (satelites, bases, ship-capture and ram-ships). But in the end mobility (both strategic and tactical) is key.

All you can do, is keep the other guy honest. Make sure that you have enough mines and weapons platforms (or whatever) so that taking or glassing each planet requires a serious effort. What you'll never want to see is a handful of ships raiding "behind the lines" and blowing up loads of planets before you catch them.

Mark