This is very nitpicky, but I was browsing the trivia archives and came upon what Admiral Tolwyn says the output of Behemoth's main gun is: 500 million gigawatts (or 500 petawatts). According to this: http://en.wikipedia.org/wiki/Megaton one megaton of explosive power equals 4.2 petajoules. This would mean that one second of continuous firing from the Behemoth's main gun equals a nuclear bomb with explosive power of roughly 120 megatons. I don't remember accurately how long the Behemoth fired during the test run, but I'd say it was five seconds at most. A 600 megaton blast would leave a freaking big crater and destroy everything for miles and miles around it and burn up a significant amount of the atmosphere around the point of impact, but I doubt it would come even close to blowing an Earth-sized planet into pieces.
Please, somebody prove me wrong.
Please, somebody prove me wrong.