Gaia’s Faintest Stars: Chasing Primordial Black Holes in the Galactic Backyard

In his 2025 Research Note, Jeremy Mould embarks on a cosmic treasure hunt using data from the European Space Agency’s Gaia spacecraft. The goal? To track down primordial black holes (PBHs)—hypothetical remnants from the early universe that could explain mysteries like dark matter and the expansion of the universe. These black holes are believed to be incredibly small, cold, and dim, which makes detecting them a massive challenge. Despite advanced theories and massive datasets, this search did not uncover any PBHs. Still, the effort sheds light on what might be hiding among the faintest stars cataloged by Gaia and what kinds of future tools will be needed to uncover these elusive objects.

Selecting the Faintest of the Faint

To begin the search, Mould selected a subset of very faint stars from the Gaia Data Release 3 (DR3), based on their reduced proper motion (RPM)—a method that helps identify dim, fast-moving stars. The criteria included stars with high parallax (meaning they’re relatively nearby) and very low brightness. These stars were then placed on a Hertzsprung-Russell (HR) diagram, a standard tool for visualizing stellar properties. Familiar stellar types like main-sequence stars and white dwarfs showed up clearly, but the stars of interest lay below the typical brightness cutoff, in a region where more exotic objects might reside.

Decoding the Dimmest Objects

Once the sample was isolated, the paper evaluated several explanations for these extremely faint stars. Many of them are likely brown dwarfs—objects too small to sustain the hydrogen fusion that powers normal stars. Others may be old white dwarfs or neutron stars nearing the end of their cooling process. But the really intriguing possibilities include free-floating planets (FFPs), which are planetary-mass objects wandering the galaxy without a host star, and PBHs. FFPs could explain some of the reddest and faintest entries, but only if they’re young and warm—conditions that are rare in the solar neighborhood.

Alternative Origins: Other Star Populations

The study also considered whether these objects could be Population II stars, which are older and have fewer heavy elements than typical stars. However, comparison with deep observations of globular clusters showed no matches for the ultra-cool dwarf (UCD) sample identified here. Another idea is that some of these faint stars are UCDs with unusually small radii, possibly explained by different equations of state in their internal physics. These smaller-radius models match the data better than standard models, but don't point clearly to PBHs.

PBHs Hiding in Plain Sight?

A particularly creative idea explored is the existence of “captive PBHs”—PBHs that have been caught by stars and now reside in their cores. Simulations suggest that a star with a PBH inside could shrink in size. This might help explain some unusually small UCDs seen in Gaia data. However, detecting such an effect is extremely difficult. PBHs might also cause helium from a star’s core to rise to the surface, but this is hard to confirm because cool stars do not show helium lines clearly in their spectra.

The Limits of Detection

So, why didn’t Gaia find any PBHs? Simply put, they’re too dim. Even if PBHs made up all of the local dark matter, one would only come close enough to be seen by Gaia about once every 40 billion years. That’s far too rare to show up in current data. Future telescopes like the Rubin Observatory might offer a better path through microlensing—when a PBH passes in front of a star and briefly magnifies its light due to gravity. This method doesn’t require the PBH to be bright, only that its gravity is strong enough to distort nearby light.

Final Thoughts: No PBHs, But Valuable Clues

While the search didn’t find any primordial black holes, it did produce a refined understanding of Gaia’s faintest stars. The most likely explanations include ultracool dwarfs with unusual sizes, a few free-floating planets, and perhaps some stars influenced by hidden PBHs. Detecting PBHs directly remains a job for future observatories and techniques, but this study is a step forward—mapping the faint edges of the known universe and narrowing down where these cosmic ghosts might still be hiding.

Source: Mould