If you've been following the Prattle for a while, you will know that tomorrow, or maybe a week tomorrow, or perhaps at some unspecified time in the future, we will be mostly wiped out by a planet we cannot see yet.
Right.Phil Plait, author of Bad Astronomy, has put together a few pages on The Planet X Saga
Doomsayers seem to pop up every few years. Last time, it was the alignment of the planets that somehow managed not to destroy the Earth in May 2000. Now, we have Planet X. A few people are claiming that a heretofore unknown planet in our solar system is on a very long, elliptical orbit. In May 2003, it will pass close enough to the Earth to affect it in some way, causing it to flip over (what many call apole shift) and spur many other huge disasters. The end result will be the deaths of many billions of people. There are a large number of web pages, chat rooms and books about Planet X and its horrible effects on the Earth. So the question is, does this planet exist, and will it come by in May 2003 and cause all this horror?
No, and no.
The site goes into a lot of detail and includes links to sources, including primary ones. A simpler summary providing an introduction the the various claims and brief explanations of why they're bollocks can be found on the page The Planet X Saga: The Scientific Arguments in a Nutshell.