Hmmm... the Nemesis Theory... basically:
There's a 10th body that orbits the sun beyond Pluto. This body could be a large planet, or even a small black hole. This body, dubbed "Nemesis," is on a highly ecliptic orbit, much like a comet, and it never even comes close enough to be observed - even if it did, the last time would have been millions of years ago. Now there's a spherical cloud of ice, rock, and interstellar debris far beyond Pluto called the "Oort Cloud." Nemesis's orbit takes it through the Oort Cloud, where Nemesis's gravity begins to fling chunks of rock and ice in all directions. A few of those chunks end up entering the inner solar system - this is the source of Halley's Comet, the Hale-Bopp Comet, and possibly the comet that struck Earth sixty million years ago and wiped out the dinosaurs and formed the Gulf of Mexico.

Of course, this is all just a theory.