Astronomers have been searching for the mysterious Planet X for hundreds of years. It was the search for a theoretical planet beyond Uranus that turned up Neptune, and then again for Pluto. And even now there are some astronomers who think there's a more distant planet out there. Oh, and there are a bunch of pseudoscience cranks trying to freak people out about the end of the world. Don't worry, we'll make time for them too, but first let's start with some real science.