One of the distinguishing features of black holes is that they are hard to see. Astronomers looking for them sometimes have luck, but at the cost a star: when a ravenous black hole tears off and devours stellar material from an orbiting star, the resulting accretion disk emits radiation that can be measured. Almost all known black holes have been discovered this way.
But it seems logical that those aren’t the only ones out there. Black holes are formed when heavy stars die. And many of these giant stars die alone, without a companion that a resulting black hole could nibble on. Thus, presumably the known black holes represent just a selection – a selection that might not be representative in all aspects.
Chinese astronomers have now succeeded in finding a black hole the way planets are discovered: by its influence on the motion of a visible star. Here, the researchers bumped into a surprise that doesn’t fit nicely into current theories of star formation.