Discovery of two black holes that are in record proximity to Earth

Both also belong to a family of black holes that is all new to us.

Astronomers using the Gaia mission have revealed two previously unknown black holes. The black holes, called Gaia BH1 and Gaia BH2, are both about ten times more massive than our sun and are located 1560 and 3800 light years away respectively. This makes them the two closest black holes discovered to date.

new family
The fact that the two new black holes are close to Earth at an all time high is of course very special. But what makes the discovery even more spectacular is that Gaia BH1 and Gaia BH2 also belong to a family of black holes that is all new to us, the researchers write in their study. “What sets this new group apart is its large distance from companion stars,” said Kareem El-Badry, lead author of the study. The distance of stars to black holes and their orbits around them are much larger than in other known binary systems. In fact, Gaia BH1 and Gaia BH2 appear to have the most widely separated orbits of any known black hole. “This suggests that these black holes probably formed in a very different way,” El-Badry said.

You might be wondering how it is possible that the existence of these nearby black holes, which are practically in our cosmic backyard, has gone unnoticed for so long. This is because black holes are extreme objects from which nothing, not even light, can escape. Therefore, we cannot observe them directly.

Not completely invisible
Yet most black holes often betray themselves. It has to do with the fact that they are not completely invisible. When material falls on them, they can emit radio and x-ray light. Especially when a star and a black hole orbit each other in close orbits, so a lot of material falls towards the black hole, the black hole is easier to find. So far, all known black holes have been discovered through this emission of light. New black holes, on the other hand, are truly black due to their large distance from their companion star, making them virtually invisible.

Mission Gaia
How do researchers know they exist? That’s thanks to the Gaia Space Telescope, which has been studying the night sky for years to map the position, distance and movement of millions of stars. The motion of stars can provide vital clues about objects that gravitationally affect those stars. These objects can be other stars or exoplanets, but also, as in this case, black holes.

Stir
When astronomers studied the orbits of stars, they found that some appeared to “wobble” in the night sky, as if influenced by the gravitational pull of a massive object. Using various telescopes, researchers scoured the skies for these objects. But the search proved futile. Since no light was found, only one possibility remained: black holes. “The accuracy of Gaia’s data was critical to this discovery,” says astronomer Timo Prusti. “We were able to reveal the black holes thanks to the small oscillation of their companion stars. No other instrument is capable of such measurements.

Not a big eater
In short, new black holes can only be recognized by their gravitational effects. Researchers continued to search for x-rays and x-rays, but nothing came of it. “The fact that we didn’t detect anything was still incredibly valuable information,” says researcher Yvette Cendes. “This tells us more about the environment of the black hole. Normally, we see many particles from the companion star as stellar wind. The fact that we did not detect any radio light indicates that the black hole does not isn’t a big eater and doesn’t consume a lot of stellar particles. We don’t know why yet, but we can’t wait to find out!”

More discoveries to come
According to the researchers, the discovery of Gaia BH1 and Gaia BH2 is just the beginning. The fact that these black holes are so close to Earth suggests that there are likely many other similar black holes waiting to be discovered. “It’s exciting because it implies that black holes belonging to this new family are very common,” says Cendes. “They may even be more abundant than black holes in closer binary systems.”

The problem, however, is to detect these “invisible” black holes. “The good news is that Gaia is still collecting data,” reassures Cendes. The next dataset will be based on 66 months of observations and will contain improved information about the orbits of stars. “This dataset will also contain many more stars,” Cendes explains, “and perhaps their mysterious companions.”