They may not be black or holes. Some astronomers believe there is a massive planet, far beyond the orbit of Neptune, orbiting the sun — but after years of searching, scientists have not found this theoretical world, which they've dubbed "Planet Nine." This has spurred theorists to consider a radical hypothesis: Perhaps Planet Nine is not a planet but rather a small black hole that might be detectable from the theoretical radiation emitted from its edge, so-called Hawking radiation. For centuries, astronomers have used variations in planetary orbits to predict the existence of new planets. When a planet's orbit doesn't...