Colliding black holes aren't always as evenly matched as scientists expected, according to a cosmic chirp astronomers have puzzled over for a year.

On April 12, 2019, gravitational wave detectors picked up a signal of space-time ripples caused by colliding black holes — which in and of itself has gone from groundbreaking to nearly mundane over the past five years. But as scientists studied the detection more closely, they realized that it didn't match the signals they have seen so far.

Instead of two evenly matched black holes, the new detection seemed to be triggered by a lopsided merger in which one black hole was three or four times more massive than the other. Scientists affiliated with the Laser Interferometer Gravitational-wave Observatory (LIGO) announced the discovery April 18 at an online meeting of the American Physical Society.

"It's an unlikely observation," Maya Fishbach, a doctoral candidate at the University of Chicago who presented the new discovery, said during her talk. "It's an exceptional event because we just wouldn't have expected it based on those first 10 binary black holes."

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An artist's depiction of mismatched black holes colliding. (Image credit: N. Fischer, H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme Spacetimes project)

Scientists studied those 10 mergers during LIGO's first two observing runs, conducted between 2015 and 2017. Each time, no matter how big the collision, the two black holes involved were about the same size. Then, just weeks into LIGO's third observing run in 2019, the newly reported signal appeared and turned that trend on its head.

“We had detected several binary black hole mergers before, but never one where the bigger black hole is nearly four times more massive than its companion," Frank Ohme, a LIGO scientist at the Max Planck Institute for Gravitational Physics in Germany, said in a statement . "It’s clear we are just beginning to understand the diversity of black hole binaries out there, and I am excited to decipher the universe's secrets every day a little more."

The newly announced discovery involved objects about 2.4 billion light-years away, Fishbach said, with one black hole about eight times the mass of our sun and the other about 30 times the mass of our sun.

"This is roughly equal to the ratio of filling in a regular Oreo to [that] in a Mega Stuf Oreo," Christopher Berry, a gravitational-wave scientist at Northwestern University, wrote in a blog post about the detection. (Don't get too excited: "Investigations of connections between Oreos and black hole formation are ongoing," he added.)

The detection gives scientists a better understanding of how black holes pair up. "We are learning that systems of this kind exist and how rare they are," Giancarlo Cella, researcher at Istituto Nazionale di Fisica Nucleare in Italy and the data analysis coordinator for LIGO's European counterpart Virgo, said in a statement . "This will allow us to deduce how they formed."

LIGO's third observing run, which was cut short by the spreading coronavirus pandemic, netted a treasure trove of more than 50 detections, Fishbach said. Scientists are still analyzing those observations, so other unbalanced mergers could be hiding in that data. But even just the one asymmetric merger dramatically reshapes the range of black hole pairs scientists are now prepared to expect.

"This one event represents a big step forward in our understanding of the population," Fishbach said.

The new research is described in a paper posted on April 18 by the LIGO collaboration but not yet published or peer-reviewed.