Some scientists at Caltech think there might be nine planets (ten if you count poor Pluto). (www.gizmag.com/solar-system-nine-planets-caltech/41423/) Velikovsky spent most of his adult life insisting that there was a tenth planet and that the inhabitants of it had uplifted humankind from apes in order to mine gold for them back in ancient Babylon. Just watch Ancient Astronauts if you want to know more about that scenario.
UPDATE: As my friends Chuck Puckett and Shelley Wilmoth pointed out on Facebook, I misremembered who it was that was hypothesizing the Ancient Aliens. It was Zecharia Sitchin that wrote about Nibiru, the twelfth planet. Immanuel Velikovsky wrote Worlds In Collision. That will teach me to do my research before I rip off a blog post.