The head of the team that made the discovery said that: "Black holes of such mass should not even exist in our Galaxy, according to most of the current models of stellar evolution".
But researchers believed that typical stars in the Milky Way shed most of their gas through stellar winds, preventing the emergence of a black hole the size of LB-1, Liu said.
"In contrast, the LB-1 black hole seems impossible to explain as stars massive enough to form a 68-solar-mass black hole monster should be totally destroyed by powerful pair-instability supernova explosions that leave only scattered gas and dust behind, and not black holes". The black hole called LB-1 is a whopping 70 times as massive as our Sun, forcing scientists to rethink their assumptions.
The newfound black hole is part of LB-1, a star-black-hole binary system located some 13,800 light-years away in the constellation of Gemini.
LB-1 is not the biggest black hole ever discovered - but it may be the largest of its kind.
The direct sighting of LB-1, however, proves that this population of "over-massive" stellar black holes exist in our very own Milky Way, said LIGO Director David Reitze, a physicist from the University of Florida.
But this method has limited usefulness because only a small number of black hole systems where the companion star orbits very close to the black hole would emit detectable X-rays, Liu said at a press conference.
Instead, the team that discovered LB-1 tracked the movements of "huge numbers of stars over a long period of time", before identifying LB-1 based on the motion of its companion star, Liu said.
The Laser Interferometer Gravitational-Wave Observatory at Caltech, overseen by Reitze, had previously detected ripples in spacetime that suggested the possibility of black holes in distant galaxies that were much bigger than what was thought possible.
However, most of these bodies "are not engaged in a cosmic banquet", so only about two dozen galactic stellar black holes have been accurately identified and measured, the new study reported. "Now theorists will have to take up the challenge of explaining its formation", he said. Stellar black holes are often isolated, rarely pulling in large objects, and that makes them hard to detect and study.
Stellar black holes are believed to be commonly scattered across the universe, but they are hard to detect because they do not normally emit X-rays - only doing so when they gobble up gas from a star that has ventured close enough.
The researchers at the Chinese Academy of Sciences tried a different approach.
Their efforts paid off - they soon spotted a giant star eight times heavier than the sun, orbiting around what turned out to be LB-1.
"This discovery forces us to re-examine our models of how stellar-mass black holes form", Reitze added.
There have been several other discoveries over the past year that have added to this renaissance.