An isolated black hole has been identified for the first time.
Researchers used data from the Hubble telescope and the Gaia probe to study this mysterious object. Located in the constellation Sagittarius, it was detected through its effect on the light of a distant star, a phenomenon known as gravitational microlensing.
Initially, another team had suggested the object might be a neutron star. However, additional analyses revealed a mass about seven times that of the Sun, ruling out this possibility. Only a black hole could explain these observations.
This discovery is significant because all previously identified black holes were in binary systems with a star. Their presence was inferred from interaction with their stellar companion.
Recent data helped refine the object's mass, confirming its status as a black hole. A second research team also revised its initial conclusions, agreeing on this identification after more precise measurements.
The future Nancy Grace Roman Space Telescope, scheduled for launch in 2027, could help discover other isolated black holes. These observations pave the way for a better understanding of these objects.
Image taken in 2022 showing the field around the OGLE-2011-BLG-0462 event. The source star and its brighter neighbor are indicated. A nearby faint star does not affect the measurement. Green circles show star positions in 2011, red ones in 2022. Stars in this region move about one pixel in 11 years.
Credit: The Astrophysical Journal (2025). DOI: 10.3847/1538-4357/adbe6e
This confirmation marks an important milestone in astronomy. It shows that black holes can exist in isolation, without being bound to a companion star, and offers new opportunities for their study.
How is an isolated black hole detected?
Isolated black holes are detected through the gravitational microlensing effect. When a black hole passes in front of a distant star, its gravity distorts and amplifies the star's light. This effect allows astronomers to spot its presence despite the absence of light emitted by the black hole itself.
Gravitational microlensing is a powerful tool for studying dark objects in the Universe. It relies on Einstein's theory of general relativity, which predicts that mass curves spacetime. This technique was crucial in confirming the existence of this first discovered isolated black hole.
Observations require space telescopes like Hubble and Gaia, capable of precisely measuring the tiny displacements of stars. These instruments help distinguish black holes from other compact objects, such as neutron stars, by measuring their mass and gravitational effect.