Okay. Blowing up planets doesn't work that way. The "size" doesn't mean much--you have to work out the gravitational binding energy. The formula for this is:mysticboy wrote:Feel free to correct me.
3GM² / 5r
Where G is the universal gravitational constant, M is the mass of the planet/moon, and r is its radius. In other words, a planet is more difficult to blow up when it's a) bigger (as in, a larger radius) and b) heavier (as in, it has more mass, and thus more gravity holding it together). Using the formula, we can work out that the Earth is almost 1800 times harder to blow up than the moon. If Piccolo could blow up the moon with a battle power of 400, a battle power of 720,000 would be needed for the Earth, assuming it all scales linearly. Of course, Kame-sennin blew up the moon with a smaller battle power than that; assuming it was 180 (the highest number attributed to him in any guide), 324,000 would be needed for the Earth.
Interestingly, when you consider how large and dense Planet Vegeta must be to have ten times Earth's gravity, the above means that a battle power of over five billion (edit: whoops, it's actually more like eleven billion) would probably be needed to blow it up...!
This is all pretty silly, though. Toriyama obviously never put this much thought into it. But hey, I always find it fun!
PS: The thing about "destroying the planet's core so it blows up" also makes no sense. Realistically speaking, you can't destroy a planet more easily by somehow targeting the core. Not that I'm saying Dragon Ball is supposed to be realistic.