It’s a big, dark presence at the farthest reaches of our solar system, a mysterious force powerful enough to skew the paths of planets in orbit and yet so subtle that it slips undetected past even the most powerful telescopes on Earth. For centuries, it has eluded some of the most brilliant minds in astronomy — some say it even destroyed one. It’s the subject of endless calculations and rampant speculation, crackpot theories and countless hours spent gazing, fruitlessly, at the night sky. It’s known as Planet X. And on Tuesday, a group of astronomers said they’d found not just...