Astronomers have probably found the closest known black hole to Earth, a strange little object called “The Unicorn” that lurks just 1,500 light years away from us.
The nickname has a double meaning. Not only does the black hole candidate live in the constellation Monoceros (“the unicorn”), its incredibly low mass – about three times larger than the sun – makes it almost one of a kind.
“Because the system was so unique and so strange, you know, it definitely justified the nickname of ‘The Unicorn,'” Tharindu Jayasinghe’s discovery team leader, astronomer Ph.D. student at Ohio State University, he said in a new video the school did to explain the discovery.
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“The Unicorn” has an inflated companion a huge red star that is nearing the end of its life. (Our sun will swell like a red giant in about five billion years.) That companion has been observed by a variety of instruments over the years, including the All Sky Automated Survey and NASA Exoplanet Transmission Survey Satellite.
Jayasinghe and his colleagues analyzed that large dataset and noticed something interesting: The red giant’s light shifts in intensity from time to time, suggesting that another object grabs the star and changes its shape.
The team determined that the object making the pull is likely to be a black hole – one harboring only three solar masses, based on the details of star speed and light distortion. (For perspective: The supermassive black hole at the heart of our Milky Way galaxy packs around 4.3 million solar masses.)
“Just as the gravity of the moon distorts the Earth’s oceans, causing the seas to swell to and from the moon, producing a high tide, so too does the black hole star to a football-like shape with one axis longer than the other, “studied co-author Todd Thompson, chair of Ohio State’s astronomy department, he said in a statement. “The simplest explanation is that it’s a black hole – and in this case, the simplest explanation is the most likely.”
That explanation, while likely, is not set in stone; “The Unicorn” remains a black hole candidate right now.
Few such very light black holes are known, because they are very difficult to find. Black holes are famous for lifting everything, including light, so astronomers have traditionally found them by noticing the effect they are having on their environment (although we have recently first direct image of a black hole, thanks to Horizon Event Telescope). And the smaller the black hole, the smaller the impact.
But efforts to find very low-mass black holes have increased significantly in recent years, Thompson said, so we could learn much more about these mysterious objects soon.
“I think the field is pushing towards this, to map out how much low mass, how much intermediate mass and how many high mass black holes there are, because every time you find one it’s to give you a clue as to which stars are falling, which are exploding and which are in between, “he said in the statement.
Jayasinghe and his team report finding “The Unicorn” in a paper that has been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society. You can read it for free at the online printout site arXiv.org.
Mike Wall is the author of “Out there“(Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook.