Scientists Think They Found a 'Unicorn' Black Hole Close to Earth

Located 1,500 light years away, the 'Unicorn' is the closest known black hole and is also unusually small.
April 23, 2021, 1:00pm
​Image: PASIEKA via Getty Images
Image: PASIEKA via Getty Images
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Scientists have discovered a black hole just 1,500 light years from Earth, which makes it “the closest known black hole candidate,” according to a new study. 

In addition to being the nearest black hole, the newly detected object is only three times as massive as the Sun, which makes it one the tiniest black holes ever detected. 

This smol singularity in our cosmic neighborhood has been nicknamed “the Unicorn,” both because it offers a rare look at a low-mass black hole and because of its location in the unicorn constellation Monoceros. 

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Astronomers led by Tharindu Jayasinghe, a doctoral student and presidential fellow at The Ohio State University, were searching for bright stars accompanied by “non-interacting” black holes, which are black holes that aren’t tugging material off their stellar companions, when they suddenly saw signs of the Unicorn around a red giant star in Monoceros.

“The discovery of the Unicorn was serendipitous!” said Jayasinghe in an email. “However, we have been working on finding these non-interacting black holes for a while now, and this discovery was spurred by this ongoing project,” he added, referencing a dedicated search for these elusive black holes at The Ohio State University.

Given that light cannot escape black holes, they are largely invisible to light-based telescopes. That said, black holes that share binary systems with stars sometimes produce high-energy light shows if they are actively devouring gas from their stellar companions. Non-interacting black holes do not produce these bursts of radiation, but it’s possible to track these elusive objects down by their more subtle gravitational pull on companion stars.

Jayasinghe and his colleagues spotted just this type of gravitational influence, known as a tidal distortion, affecting a bright dying star called V723 Mon. This hint prompted the team to search for the signs of the star’s potential companion, which had “to be both much less luminous and significantly more massive than the giant, V723 Mon,” according to a study published on Wednesday in the Monthly Notices of the Royal Astronomical Society.

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The tidal tug of the companion was visible in observations captured by the Kilodegree Extremely Little Telescope (KELT), the All Sky Automated Survey (ASAS), and NASA's Transiting Exoplanet Survey Satellite (TESS). The team also confirmed the detection with follow-up observations from the Remote Observatory Atacama Desert (ROAD), the Neil Gehrels Swift Observatory, and the Keck Observatory, among others.

As a result, they concluded that “the bright red giant V723 Mon has a dark, massive companion that is a good candidate for the closest known black hole,” according to the study. 

The Unicorn’s low mass also places it in the mysterious “mass gap” between the most massive neutron stars (roughly 2.2 times the mass of the Sun) and the smallest black holes (roughly five times the mass of the Sun). 

“There appears to be a theoretical maximum to the mass of a neutron star at around three solar masses (in reality, this limit should be somewhat less than that), so the Unicorn is truly one of the smallest black holes possible,” Jayasinghe said. 

“Mass gap compact objects are rare!” he continued. “Finding these systems will help scientists understand the formation mechanisms of black holes/neutron stars. It will also help understand the characteristics of these compact objects in the Milky Way Galaxy.”

This is not the first time that scientists have announced the discovery of a black hole with unrivaled proximity to Earth. In 2020, another team published observations of a potential black hole just 1,000 light years away, in a star system called HR 6819, which is significantly closer than the Unicorn. However, those findings were challenged by a study published a few months later, which proposed an alternate model for the observations that did not invoke a black hole. While the identity of the object at HR 6819 is in limbo, the Unicorn has stepped in to claim the mantle of the closest known black hole, according to Jayasinghe and his colleagues.

“We have been following the literature on this object (HR 6819), and multiple research groups have now shown that it is in fact not the host to a black hole,” Jayasinghe said. “I do agree with these studies—there is strong evidence that HR 6819 is not a [black hole] host.” 

The discovery of the Unicorn raises many new questions about the abundance and behavior of non-interacting black holes. Jayasinghe and his colleagues estimate that the Milky Way might contain 100 to 1,000 “Unicorn-like” systems. Observing more of these systems could shed light on how black holes in the mass gap form, and how often they end up in binary systems with various types of stars.

“While this is a relatively new field of study (looking for non-interacting black holes), we should expect to see more discoveries in the near future,” Jayasinghe said. “There's a lot to do, but I am excited for all of it!”