He ' s still one of my favourite cards up to now . <| sep |> You do still have size constraints because the shield has to be large enough to cover the power generating facilities , and whatever else of value exists on the planet . Further , sieges are actually practical . I ' d be willing to assume that a massive planetary shield would require some exotic materials and substantive production facilities . A sustained barrage would eventually stress and degrade the system enough to render it useless because it could not be meaningfully resupplied , even if the planet were selfsustaining for food . The idea that you could shield an entire planet is also a little bit silly as a matter of scale . It is not cheaper , because rather than shielding a few square kilometers of a battleship you ' re looking at millions of kilometers of shielding being necessary Sol III is 510 million km 2 and that goes up exponentially as you shield the atmosphere . Rather than casting a shield a few meters out from the surface you ' re protecting , you ' re casting a shield kilometers into the atmosphere otherwise I would simply set your atmosphere on fire , virus bomb you , or diffuse dust and freeze you . To protect one planet , you are looking at expending as much on shields as for a fleet of millions of ships . Not practical . It ' s also just tactically dumb . Even if you perfectly defend your planets from attack , they ' re useless little rocks floating in space completely surrounded and cut off from any activity . Holding planets doesn ' t make sense as a matter of war strategy . Initiative and mobility is all that matters .