A Mysterious Mars-Sized Object Could Be Hiding at the Edge of Our Solar System

It's been about 11 years since Pluto was demoted to dwarf planet status, leaving a 1,473 mile (2,370 kilometers)-size void in our hearts. Since then, the hunt for Planet X--aptly renamed Planet 9--has grown into an international movement to find such an object in the Kuiper Belt beyond Neptune's orbit. Now, scientists ...