The Kuiper Belt, the region immediately beyond Neptune, harbors TNOs of many sizes. The largest is Pluto, which was discovered more than 60 years before any of the others. Some TNOs are "detached objects," which orbit so far from the sun that they're not appreciably affected by the gravity of Neptune or any other known planet. Perhaps the most famous of these is Sedna, which takes 11,400 years to make a single orbit and never comes closer to the sun than 20 times farther out than Pluto. In 2016, astronomers Konstantin Batygin and Mike Brown announced that a distant undiscovered...