"A black hole as massive as Earth would be about the size of a Ping-Pong ball and would be exceptionally hard to see."

"No such primordial black holes have been detected yet. But neither has their existence been ruled out. Dr. Scholtz and Dr. Unwin pointed out that an experiment called OGLE, for Optical Gravitational Lensing Experiment, based at the University of Warsaw in Poland, had detected the presence of a half-dozen dark objects in the direction of the center of our Milky Way galaxy. Their gravitational fields had acted as lenses, briefly amplifying the light from distant stars that they drifted in front of. Those objects could be free-floating planets, the authors said, with masses ranging from half to about 20 times that of Earth. But they could as easily be primordial black holes floating around the galaxy, the astronomers proposed. If that were the case, the putative Planet Nine could well be a black hole, too, in a distant orbit around the sun.... The prospect of finding a black hole in our own solar system 'is as startling as finding evidence that someone might be living in the shed in your backyard,' [wrote Avi Loeb, chair of the astronomy department at Harvard]. 'If so, who is it, and how did it get there?'"

From "Is There a Black Hole in Our Backyard?/Astrophysicists have recently begun hatching plans to find out just how weird Planet Nine might be" (NYT).