Blowing up a planet is hard. Really, really, really, really hard. In fact, if you had one "really" in that sentence for every Joule of energy it would take to make the Earth all explodey, you'd need more than 2 x 1032 of them. That's a lot of "really"s.
I actually calculated that number using some basic physics and math, and then decided to write an entire article around it, which is now up on Blastr. It doesn't matter how big a supervillain you are, blowing up a planet is next to impossible, despite the non-existence of Ceti Alpha 6.
There are ways of tearing a planet apart, actually, but I didn't want the article to go on too long, and I figure exploding one versus ripping it apart are different things. Maybe I'll do a follow up article. And really, why blow it up at all? If you want to kill everything on it, just set up a massive ad campaign for hair spray, sell the inhabitants a billion cans of the stuff, and then sit back and wait for them to destroy their ozone layer. Done and done.
[P.S. Today marks the 12th anniversary, ironically, of the Moon being blasted out of Earth orbit. Happy Breakaway day!]