Saw this article on Discover.com earlier this month and thought it was really interesting: The Solar System’s Lost Planet.

Nesvorny, who runs computer simulations to study how the solar system evolved over time, kept encountering the same problem: The four giant gas planets, whose orbits are comfortably far apart from each other today, kept violently jostling with each other in his models of the early solar system. Jupiter would end up tugging on Uranus or Neptune and casting one of them out into interstellar space. Obviously, that never happened. So Nesvorny came up with a clever explanation: He proposed that a fifth gas giant emerged from the planet-birthing cloud 4.5 billion years ago. Suddenly his simulations started matching reality. The outer planets still jockeyed for position, but this time Jupiter spared Uranus and Neptune and ejected the extra planet instead.

Not that we’d ever be able to know if this is correct (probably), but it certainly sounds logical. I just hope the Planet X/Nibiru nuts don’t jump all over this as proof of pending doom.