A team of astronomers at the Space Telescope Science Institute, working with one colleague from the University of St Andrews' Center for Exoplanet Science and another from the European Southern Observatory, has confirmed the existence of a lone black hole. In their paper published in The Astrophysical Journal, the group describes how they studied newer data regarding an object they had spotted several years ago to confirm its identity.

In 2022, members of essentially the same team reported the discovery of what they described as a "dark object" moving through the constellation Sagittarius. They suggested it might be a lone black hole. Shortly thereafter, a second research team challenged that result, suggesting it was more likely a neutron star. After continuing to study the object, the original research team has found more evidence backing up their original claim that it is likely a lone black hole.

Prior to this new finding, all the black holes that have been identified have also had a companion star—they are discovered due to their impact on light emitted by their companion star. Without such a companion star, it would be very difficult to see a black hole. The one identified by the team was only noticed because it passed in front of a distant non-companion star, magnifying its light and shifting its position in the sky for a short while.

The research team made their initial observations using data from Hubble over the years 2011 to 2017. This time around, they looked at data from Hubble for the years 2021 and 2022, as well as from the Gaia space probe. They found that the object under review was approximately seven times as massive as the sun, showing that it could not be a neutron star, leaving only a black hole as the sole option.

Additionally, the second research team revised their neutron star assessment in 2023, agreeing that the object was indeed a black hole. They found that the object was around six solar masses, but the higher uncertainty in their measurement still keeps it consistent with the more recent results.

Now, the latest study by the original research team marks the first time the existence of a lone black hole has ever been confirmed. They hope to find more examples with the Nancy Grace Roman Space Telescope, which is scheduled to launch in 2027.

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