Gaia Search Finds No Black Holes Lurking in the Sun's Neighborhood, For Now
Astronomers checked five candidate isolated black holes within 49 light-years against infrared and radio surveys and concluded they are unlikely to be real, with a big new radio dataset due in 2027.
Scientists estimate the Milky Way holds between about 100 million and 1 billion stellar-mass black holes. Finding them is hard. Supermassive black holes are easier to spot, either because they feed on gas and shine as active galactic nuclei or quasars, or because their gravity bends the orbits of nearby stars, which is how Sagittarius A*, the Milky Way's central black hole, was detected. Stellar-mass black holes are far more numerous and much quieter, and models predict that many of them drift alone through space, including some near the sun.
A study in The Astrophysical Journal tried to find some of those close neighbors. Led by Abdurakhmon Nosirov of Fudan University in Shanghai, the team searched data from the European Space Agency's Gaia mission for isolated black hole candidates within 15 parsecs, about 49 light-years, of the solar system. The paper is titled "Searching for Isolated Black Hole Candidates within 15 pc of the Solar System in Gaia DR3."
The idea is that an isolated black hole moving through gas could pull in some of that material and give off detectable radiation. The authors wrote that within the Local Interstellar Clouds, which fill 5% to 20% of this local volume, an isolated black hole could produce detectable electromagnetic emission, given current or near-future observatories. Outside those clouds there is too little to feed on. Their models put the resulting signal well below detection thresholds there. The solar system is moving through the Local Interstellar Cloud, a pocket of partially ionized gas, near its edge.
Predicting the glow is difficult because of what the authors called large uncertainties in the expected accretion spectra. Detecting a black hole by its gravitational pull on nearby stars is also unlikely to work here, they said, because the local stellar density is too low.
Gaia's data produced six candidates, and one was excluded as a brown dwarf. All five that remained lie close to the galactic plane, which makes them likely to be spurious astrometric solutions, caused for instance by unmodeled background sources or hidden binary companions. Gaia collected enough spectroscopic information to flag candidates, but not enough to confirm them.
So the team compared the five against other sky catalogs: CatWISE, an infrared catalog from NASA's Wide-field Infrared Survey Explorer, and two radio surveys, RACS from the ASKAP telescope and VLASS from the Very Large Array. Both of their models agreed that if the sources were black holes accreting from interstellar gas, they should show up in CatWISE. No sources matched the Gaia candidates. "Our search for infrared and radio emission from these sources further suggests that they are unlikely to be black holes accreting from the interstellar medium," the authors wrote.
The search will continue. Any isolated black hole feeding on gas would be very faint, with a fairly featureless spectrum, and would probably be invisible to any single instrument. The authors call for follow-up campaigns across radio, optical and X-ray bands. Next they plan to look in radio and X-ray catalogs for black holes feeding on warm, partially ionized interstellar gas. A new tranche of Square Kilometre Array data, due in early 2027, "appears especially promising for this endeavor," they conclude.
