Our solar system may have a hidden planet beyond Neptune – no, not that one

There are eight recognized planets in our solar system, four inner rocky planets and four outer gas giants. But beyond the orbit of Neptune, dozens of dwarf planets the size of Pluto or smaller populate a region known as the Kuiper Belt, and new computer models show there may be something even bigger lurking out there – or at least there might have been in the past. In a paper in the Annual Review of Astronomy and Astrophysics this month, Brett Gladman of the University of British Columbia and Kathryn Volk of the University of Arizona argue that new models indicate that the likelihood that a Mars-sized planet orbiting in the Kuiper Belt region is at least 50%, though they also claim that it was ejected from the solar system entirely at some point in the past. This would be a different planet then the theoretical one currently being called Planet Nine, which is believed to be a Neptune-sized gas giant far out beyond the Kuiper Belt. To get a better sense of how the solar system formed, researchers like Gladman and Volk typically use powerful computers to run simulations with different variables to see how changes in any one variable affects the kind of solar system that would result.