The closest known black hole may have been found a cosmic stone鈥檚 throw from Earth, just 1500 light years away. Called Gaia BH1, it is estimated to be about 10 times the mass of our sun.
Although it can鈥檛 be seen directly because it emits no light, data from the European Space Agency鈥檚 Gaia space telescope revealed the gravitational tug the black hole exerts on its orbiting companion star, which is similar in age and mass to our own sun.
Several candidates for nearby black holes have been found before, but most haven鈥檛 stood up to scrutiny. However, at the Harvard-Smithsonian Center for Astrophysics in Massachusetts and his colleagues say their discovery is the best candidate yet for such an object.
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鈥淭his one is solid,鈥� says team member at Tel Aviv University in Israel. 鈥淚鈥檓 ready to bet my life on it.鈥�
Many black holes have been discovered before, such as merging black holes seen by the gravitational waves they produce, and others eating stars in binary systems, making them shine brightly in X-rays. Astronomers using the Event Horizon Telescope have also directly imaged two of these cosmic behemoths.
Dormant black holes like Gaia BH1 have been harder to find, however, owing to their near-invisibility. 鈥淭hese black holes are far from any source of food,鈥� says at the University of Warsaw in Poland 鈥� Gaia BH1鈥檚 star orbits the black hole at about the same distance Earth orbits the sun.
It isn鈥檛 clear how this system formed. One possibility is that the black hole was originally a much more massive star that expanded into a red supergiant and then collapsed, perhaps with an accompanying supernova, although it is unlikely that its companion star would have survived. Another scenario is that the black hole could actually be two black holes, making this originally a triple-star system. Alternatively, the companion star could have been captured by the black hole when passing.
El-Badry hopes to find out with follow-up observations using other telescopes, looking for evidence of a binary black hole or even planets orbiting the star, which could suggest there had been no explosive event. 鈥淚t could definitely have planets,鈥� he says. 鈥淚f you lived on a planet around the star, the black hole would be as bright as Jupiter鈥�, as it eats a small amount of the star鈥檚 solar wind, he says.
In a few billion years, the star is expected to expand into a red giant, like our sun, which will massively increase the fuel into the black hole and make it appear much brighter in any nearby planet鈥檚 sky. 鈥淥ne hundred thousand times brighter than the sun,鈥� says El-Badry.
Upcoming observations from Gaia are expected to find dozens more of these dormant black holes in our galaxy, of which there could be tens of thousands. Systems like Gaia BH1 would be prime targets for learning more about black holes. 鈥淲e can think of studying the black hole itself,鈥� says Wyrzykowski. 鈥淲e don鈥檛 normally get chances to study these extremes of physics.鈥�
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