What could (plausibly) destroy a planet?

I'm writing a Science Fiction book and I have two questions I can't seem to find an answer to. First, what is a plausible way to destroy a planet? I don't want to fall back on the old "Super planet destroying laser" or "Unexplained planet destroying bomb" conventions that most of Sci-Fi uses. I was thinking of drilling a hole to a planet's core and planting one (or several) nuke(s). Is that plausible? Second question, what would happen if a planet in our solar system, specifically Mars, were destroyed in this manner? Would there be adverse effects on Earth? Would the pieces form a new asteroid belt? Any science related answers would be much appreciated. Thanks in advance!