Right now it is possible to spam 20+ siege frigates and ninja take out any planet (irrelevent of the defenses) before any possible response can be achieved. Frankly, this lame tactic is used by the AI regularly.
Either lower the planetary damage of siege frigates or implement some kind of system where you must eliminate defenses first. It is IMPOSSIBLE to defend a planet with structures from all 6 sides (or anything short of a huge fleet when you get spammed by 20+ siege frigs). The power of siege frigs have turned this game in to musical chairs (who can knock out who's planets most frequently).