In the 2010s, astronomers noticed something remarkable: supermassive black holes at the centers of galaxies, despite vast differences in size and brightness, fall onto a single straight line when you plot their millimeter emission against X-ray emission and mass. This line was dubbed the "millimeter fundamental plane of black hole accretion."

Now, a team of researchers has tested whether stellar-mass black holes, those born from the collapse of massive stars, follow the same rule. They took data from five X-ray binary systems, where a black hole pulls matter from a companion star, and found that these objects also land on the same plane. It's as if all black holes in the universe, from the tiniest to the most gigantic, obey one universal law.

The scientists also tested theoretical models describing matter falling onto a black hole. Both leading models (one with a hot, rarefied flow and the other with narrow plasma jets) predict the existence of such a plane for black holes of any mass. However, the accuracy of mass prediction from this plane differs between the models, opening the door for further research.