Planet Nine might not be a planet at all
A ninth planet is one form the solar system’s mysterious mass could take. (Caltech/R. Hurt (IPAC)/)After centuries of watching the skies, the map of our local solar system has grown quite detailed. We live on one of the rocky inner planets. Next there’s a belt of asteroids, two gas giants, two ice giants, and then a second belt of many smaller icy bodies. In the impenetrable outer solar system, however, lies an unseen dragon, researchers have come to suspect. Scanning the darkness, astronomers have managed to catch a few glimmers of what might fill the nether regions far beyond Neptune. And what they see doesn’t make sense. Where researchers predicted chaos—strewn flotsam leftover from the solar system’s tumultuous formation—they see unexpected order. Orbits of distant objects cluster together. Their points of closest approach stop short of a certain line for no apparent reason. In a handful of such patterns, many...