Well most of the time the planet has enough energy for a few hundred attacks, which I don't care to carry out. Novas are a good option later in the game, but are impractical early. 10B shields gets you about 260, which is 280 ship tech, and requires about 295 to break at least. And I'm really not of the opinion that novas should always be necessary.
When it comes down to it, there's just about no planet that cannot be taken with novas, and there are few planets (assuming the builder has invested much in them) that don't require novas. I'd just like to see the game move away from them. I don't have much of a suggestion to change this in a balanced way, so I can't expect you to do anything, but I'm just pointing this out anyway.
Oh, and the planet would have to shield a HUGE area. It should cost way more for the same shields per square meter for example, since they're so much more dissipated. As it is, it doesn't make sense, but it is balanced.