Black Holes Discovered by the Gaia Mission

Although black holes cannot be observed directly—they emit no light—their presence can be inferred from the influence they exert on their surroundings. By measuring the positions and motions of stars in the Milky Way with unprecedented precision, the Gaia spacecraft has enabled the discovery of systems where a star moves along a peculiar orbit… as if it were circling an invisible companion.

This is how Gaia BH1, Gaia BH2, and Gaia BH3 were identified—the first “dormant” black holes, meaning they do not accrete matter and reveal themselves solely through their gravitational influence on a companion star. The abbreviation in their names comes from the first letters of the English term black hole. The closest of them, Gaia BH1, lies only about 2,000 light-years from Earth and has a mass of roughly 10 Suns. Meanwhile, Gaia BH3, announced in 2024, is a record-holder with a mass of 33 solar masses, making it the most massive black hole of its kind in our Galaxy. Scientists from the Astronomical Observatory of the University of Warsaw participated in these discoveries.

But that’s not all. Researchers from the same institution have long studied gravitational microlensing, where the light from a background star is bent by an unseen foreground object. When the lens is a black hole, its mass and distance can be determined even without directly seeing it. Yet it was only through the precise measurements provided by Gaia that subtle apparent shifts in the positions of background stars caused by lensing could be detected. This has made previously impossible discoveries feasible—namely, the detection of isolated black holes. Using this technique, astronomers have identified several candidates likely to be such objects.

These are floating black holes, unbound to any star, and their detection represents a potential first step toward mapping the “dark population” of black holes in the Milky Way. Thanks to Gaia, galactic archaeology—the study of the Milky Way’s history—has advanced significantly. Isolated black holes are today invisible but numerous remnants of former stars.

The cosmos is alive and constantly changing: stars are born, evolve, and die. Observations by Gaia have revealed not only the current structure of our Galaxy but also provided clues about its past evolution, deepening our understanding of the Milky Way and the life cycles of stars.