Over the past few years, researchers have noticed an odd clustering in the orbits of multiple trans-Neptunian objects (TNOs), which dwell in the dark depths of the far outer solar system. Some scientists have hypothesized that the TNOs' paths have been sculpted by the gravitational pull of a big object way out there, something five to 10 times more massive than Earth (though others think the TNOs may just be tugging on each other). This big "perturber," if it exists, may be a planet — the so-called "Planet Nine," or "Planet X" or "Planet Next" for those who will always...