The two closest black holes to Earth have been discovered. Both the black holes are located in the Milky Way galaxy, and are of a new kind.
The European Space Agency's (ESA's) Gaia mission helped discover this new family of black holes by tracking the orbits of some stars.
A team of astronomers studied these orbits and noticed that some of the stars 'wobbled' in the sky, according to the ESA. This implies that the stars were being gravitationally influenced by massive objects.
Other telescopes looked for the objects that influenced the stars, but could not find any light. This led astronomers to conclude that the objects are black holes.
What are the black holes called? How far are they from Earth?
The new family of black holes resides in Earth's cosmic backyard. ESA states that many more black holes may be lurking in the shadow.
The black holes are called Gaia BH1 and Gaia BH2.
Gaia BH1 is the closest black hole to Earth, and is located 1,560 light years from the planet in the direction of the constellation Ophiuchus. Gaia BH2 is the second-closest black hole to Earth and is located 3,800 light years from the planet in the constellation Centaurus.
On Thursday, March 30, the astronomers published a paper in the journal Monthly Notices of the Royal Astronomical Society describing Gaia BH2. They had separately described Gaia BH1 in the same journal in November 2022. Both objects were discovered last year, before being described as black holes in the respective papers.
How were the black holes discovered?
Astronomers discovered the two black holes by studying the movement of their companion stars. Since there was a strange 'wobble' in the movement of the stars in the sky, it was implied that they were orbiting a very massive object.
The objects did not seem to emit any light, and hence, astronomers ruled out the possibility of them being double-star systems.
All black holes found so far have been discovered by the emission of light, usually at X-ray and radio wavelengths. The light is emitted by the material falling into a black hole.
What makes the two black holes unique?
The new black holes are unique because they are truly black, and can only be detected by their gravitational effects.
Another unique characteristic of this new family is that the distances of the stars to the respective black holes, and the orbits of the stars around the black holes, are much longer than the distances and orbits of any other known binary systems of black holes and stars.
Star-black hole pairs in which the star and black hole are close to each other are called X-ray binaries. These pairs are very bright and emit X-ray and radio light, making it easier to find them.
The two newly discovered black holes suggest that colossal behemoths in wider binaries are more common.
In an ESA statement, Kareem El-Badry, the researcher who discovered the new black holes, said the new group of black holes is different from the ones already known because of the separation of each black from its respective companion star.
El-Badry, who is a researcher at the Harvard-Smithsonian Center for Astrophysics, and the Max-Planck Institute for Astronomy, explained that the black holes likely have a completely different formation history than X-ray binaries.
More about Gaia's work
According to the ESA, Gaia accurately measures the positions and motions of billions of stars, which can provide important information about objects that gravitationally influence these stars.
Exoplanets, black holes, and stars are objects that can gravitationally influence stars.
While Gaia provided accurate measurements of the movement of the stars in three directions, additional radial velocity (the velocity of an object along the line of sight from an observer to the object) measurements were required to understand more precisely how the stars moved away and towards Earth.
The radial velocity measurements for the newly found black holes were obtained with the help of ground-based observatories. All these findings helped astronomers conclude that they had detected black holes.
Why are the two black holes invisible?
Mostly, black holes are not completely invisible because when material falls into them, they emit light in radio waves and X-rays. NASA's Chandra X-ray and the South African MeerKAT radio telescope on the ground searched for light from Gaia BH2, but they were unable to spot any signal.
Yvette Cendes from the Harvard-Smithsonian Center for Astrophysics, who helped discover Gaia BH2, said even though the team of astronomers detected nothing, this information is "incredibly invaluable" because it tells them a lot about the environment around a black hole. A lot of particles were seen coming off the companion star in the form of stellar wind.
Cendes explained that since the team did not see any radio light, it implies that the black hole is not a great eater, and not many particles are crossing at its event horizon, or the boundary marking the outer edge of a black hole.
According to the ESA, the black holes do not emit any light, and this is what makes them practically invisible. The probable reason the black holes do not emit light is that they are much farther away from their companion stars.
The two black holes closest to Earth have the most widely separated orbits of all known black holes.
More such black holes may be lurking near Earth
There is a possibility that many more similar black holes in wide binaries are residing in Earth's cosmic backyard.
Cendes said the discovery is very exciting because it implies that these black holes in wide orbits are common in space, and probably more common than binaries where the black hole and star are closer.
Cendes added that Gaia's next data release, which will come out in 2025, will contain many more of these stars with mystery black hole companions.
Based on 66 months of observations, the next data release will contain improved information on the orbits of stars.