Could Planet Nine Be a Black Hole?

A peculiar alignment of orbits among some small bodies in the outer Solar System has astronomers hunting for a planet lurking well past Neptune. But what if this alleged Planet Nine—likely several times as massive as Earth and hundreds of times farther from the Sun—is not a planet but, instead, a diminutive black hole? This scenario is not only plausible, say Jakub Scholtz of Durham University, UK, and James Unwin of the University of Illinois at Chicago, but it may also help explain hints of small, dark, enigmatic objects flitting about our Galaxy.