Black hole discovered near Earth could be ‘tip of an exciting iceberg’

7 May 2020

An artist’s impression shows the orbits of the objects in the HR 6819 triple system. Image: ESO/L Calçada

Astronomers have detected the closest black hole to Earth – just 1,000 light years from the planet we call home.

One of the most destructive forces in the universe has been discovered in the galactic equivalent of our own backyard. Astronomers from the European Southern Observatory (ESO) and other institutes published a study revealing a black hole closer to our solar system than any other seen to date, located just 1,000 light years from Earth.

As part of a triple system, the black hole could be just the tip of the iceberg, according to astronomers, with many more similar black holes potentially being discovered in the future. Located in the constellation of Telescopium, the system in which it was discovered is so close to us that it can be viewed from the southern hemisphere on a dark, clear night with the naked eye.

The researchers had originally observed the system, called HR 6819, as part of a study of double-star systems. However, as they looked closer, they were stunned to find a third, undiscovered body in the form of a black hole.

‘There must be hundreds of millions of black holes out there, but we know about only very few’

It soon became apparent that one of the two visible stars in the system orbits an unseen object every 40 days, while the second star is at a large distance from the inner pair. The hidden black hole in HR 6819 is believed to be one of the very first stellar-mass black holes discovered that is truly black because it does not react violently with its environment.

ESO scientist Thomas Rivinius said that an invisible object with a mass of at least four times that of our own sun could only be a black hole. While dozens of black holes have been discovered in our own Milky Way galaxy, all of them have made their presence known by releasing powerful x-rays as they obliterate planets and stars surrounding them.

Hundreds of millions of black holes

However, scientists estimate that over the Milky Way’s lifetime, many more stars have collapsed into black holes as they ended their lives. Now, the discovery of a silent, invisible black hole in HR 6819 can help astronomers find where other silent black holes might be found.

“There must be hundreds of millions of black holes out there, but we know about only very few. Knowing what to look for should put us in a better position to find them,” Rivinius said.

Adding to this, astronomer Dietrich Baade said that finding a black hole in a triple system so close indicates we are just seeing “the tip of an exciting iceberg”.

Astronomers will now focus their telescopes on another system, called LB-1, that has the potential to be a triple system. Co-author of the research, Marianne Heida, added: “LB-1 is a bit further away from Earth but still pretty close in astronomical terms, so that means that probably many more of these systems exist.

“By finding and studying them, we can learn a lot about the formation and evolution of those rare stars that begin their lives with more than about eight times the mass of the sun and end them in a supernova explosion that leaves behind a black hole.”

Colm Gorey was a senior journalist with Silicon Republic