An international team of astronomers has discovered an incredible object inside the Milky Way. A black hole roughly 70 times the mass of our Sun. This might not seem impressive given that we know of much bigger black holes, but the curious thing is that it couldn’t have formed where it is, according to current star formation theories. The discovery is reported in Nature.

Black holes have been divided into three classes: Supermassive ones, which inhabit the core of galaxies; intermediate-mass ones, which are still very mysterious and found in very peculiar environments; and stellar-mass black holes. The latter are formed when certain massive stars go supernova and should weigh a few tens of times the mass of our Sun at most.

The biggest stellar-mass black holes come from the most ancient stars. These are very poor in heavy elements and for this reason can be extremely large, producing an equally large black hole. The newly discovered black hole, dubbed LB-1, is not just peculiar because of its mass; it should not have formed in the rich stellar environment of our galaxy.

“Black holes of such mass should not even exist in our galaxy, according to most of the current models of stellar evolution,” Professor Liu Jifeng, from the National Astronomical Observatory of China, said in a statement. “We thought that very massive stars with the chemical composition typical of our galaxy must shed most of their gas in powerful stellar winds, as they approach the end of their life. Therefore, they should not leave behind such a massive remnant. LB-1 is twice as massive as what we thought possible. Now theorists will have to take up the challenge of explaining its formation.”

LB-1 was discovered in a very unusual way. There are an expected 100 million stellar-mass black holes in our galaxy but we can't see most of them. They are black, after all. The team looked for stars that seem to orbit nothing. Using China's Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), they identified a star eight times heavier than the Sun following a weird orbit.

Follow-up observations from Spain's Gran Telescopio Canarias and the Keck I telescope in the United States allowed them to precisely characterize the system. The star orbits the black hole every 79 days.

A similar-sized black hole was the end product of the first detected emission of gravitational waves. While the mass of LB-1 might be explained away with a (maybe multiple) merger scenario, the presence of the star certainly complicates the explanation.

“This discovery forces us to re-examine our models of how stellar-mass black holes form,” said LIGO Director Professor David Reitze from the University of Florida. “This remarkable result along with the LIGO-Virgo detections of binary black hole collisions during the past four years really points towards a renaissance in our understanding of black hole astrophysics.”

Unexpected black hole LB-1 and its star are located 15,000 light-years away.