A new study debunks a recent claim that astronomers may have detected a lunar-mass primordial black hole. In a reanalysis of observations from the Dark Energy Camera (DECam), researchers found that the star nicknamed “Phoebe” was simply doing something ordinary that many stars do: changing its brightness naturally over time. The new findings have been reported in a paper posted to the arXiv preprint server on June 17.
An intruder
In May 2026, a team of researchers reported evidence for primordial black holes (PBHs) by spotting a short-lived flicker of light from a star in the Large Magellanic Cloud. They monitored millions of stars in this galaxy using DECam. They spotted one star that briefly brightened for less than an hour. They interpreted this as a gravitational microlensing event. Microlensing occurs when a tiny object passes in front of a star and its gravity briefly bends and magnifies the star’s light like a lens.
Because the brightening was so short, they calculated the “lens” causing it must have had about the mass of the moon. This one-time, nonrepeating signal was interpreted as a microlensing event caused by a primordial black hole nicknamed “Phoebe.” A primordial black hole is a hypothetical relic from the very early universe, and its existence would mean dark matter is made of these tiny black holes.
But this claim contradicts data from another telescope survey, the Optical Gravitational Lensing Experiment (OGLE), which has been watching the same region of sky for years with high precision. If lunar-mass primordial black holes really made up dark matter, OGLE should have seen hundreds to thousands of similar flickering events.
“No convincing short-timescale microlensing events were found in the OGLE data, placing strong limits on the abundance of lunar- and planetary-mass PBHs in the Milky Way dark matter halo,” the researchers, Andrzej Udalski and Przemek Mr.óz of the University of Warsaw, who reanalyzed the same data, wrote in the new paper.
Enigmatic or ordinary?
In this new study, Udalski and Mr.óz present an independent analysis of the same public DECam data, plus extra observations from 2020 and 2021 that the previous paper didn’t include.
They found that the star brightened at least three separate times over the years—one of which was previously interpreted as a microlensing event. In addition, its average brightness also changed over time. A real microlensing event from a single passing primordial black hole cannot repeat.
To verify that the brightness changes were unique to the star, the team also tracked two nearby stars of similar brightness. Those stars remained steady throughout, confirming that the variations were real.
Taken together, the repeated changes in brightness show that the star is naturally varying in brightness—a well-known and common behavior of variable stars.
“These observational characteristics indicate that Phoebe is an ordinary variable star, rendering the claims of the discovery of a lunar-mass PBH in the Milky Way dark matter halo invalid,” they write. (The star itself is called “Phoebe” in the new paper, whereas the previous paper used the same nickname for the suspected primordial black hole passing in front of it.)
The finding rules out what had appeared to be rare evidence for a primordial black hole and restores consistency with other microlensing surveys.
“This is not the first time that a variable star has been mistaken for a short-timescale microlensing event in high-cadence time-series observations of limited duration,” the researchers note. Short-term monitoring lasting some days, they explain, simply isn’t enough to tell the difference. Distinguishing a genuine one-time gravitational event from a star’s natural flickering requires months or years of monitoring.
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Publication details
Andrzej Udalski et al, Eppur non si trovano Vol. 3: Phoebe—a Mirage of a Primordial Black Hole, arXiv (2026). DOI: 10.48550/arxiv.2606.19442
Journal information:
arXiv
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Reanalysis suggests ‘Phoebe’ is a variable star, not a primordial black hole (2026, June 28)
retrieved 28 June 2026
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