Could be a New Planet in our Solar System, well beyond Pluto's orbit

There have been inquiries into a 'Planet X' (where X would mean a 10th planet) for centuries, sometimes by those who had folly-type claims to others being serious astronomers, but now there is calculational evidence there should be something pretty large, about the size of Neptune (an order of magnitude more massive than earth) that orbits the Sun once every 15,000 years! Check out this article from Science. There is, as of now, no direct observational evidence for the planet, but calculations show how the gravity of something this size, at that distance and with that period, has perturbed other objects in the outer solar system. Cool example of how theory and simulations can lead to exciting, possible new discoveries - of course, we still need to actually see the thing before we should completely accept it!