In 2019, astronomers recorded an unexpected phenomenon on a distant star.
Over the course of about an hour, its brightness gradually decreased and then returned to its base level.
Its behavior did not match any obvious stellar phenomenon - too long for a stellar flare, too short for a supernova, and too smooth for most known types of stellar variability.
Now, after carefully studying the properties of this event, astronomers say it may be a signal from one of the most elusive objects in the Universe: a tiny primordial black hole, weighing about as much as three Earth's satellites.
A black hole of that mass would have an event horizon roughly the same size as the period at the end of this sentence.
A team of astronomers led by Renee Kay of Swinburne University of Technology in Australia argues that no other explanation fits the statistics of this event so well, and so they have named the black hole candidate Phoebe.
«"Phibe suggests a population of compact, moon-mass objects associated with the distribution of dark matter in the Milky Way and potentially opens a new window into the physics of inflation," the researchers write in a preprint published on arXiv.
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We tend to think of black holes as very heavy, large objects—with masses ranging from at least several Suns to tens of billions of Suns.
This is explained by the peculiarities of their formation, which begin with the death of a massive star, whose giant core then collapses under the influence of gravity, giving rise to one of the densest known objects in the Universe.
However, immediately after the Big Bang, ideal conditions for the formation of much, much smaller black holes may have existed. Quantum fluctuations in spacetime could have created excess density in the expanding universe, which then collapsed, much like a stellar core might collapse today.
These black holes are known as primordial black holes, and their existence is currently only known theoretically.
This may be because they are difficult to detect. A primordial black hole with the mass of Earth would have a diameter of only 1.8 centimeters (0.7 inches).
The actual size of the black hole is 5 Earth masses, according to a 2019 paper speculating on the nature of Planet Nine. (Scholz and Unwin, arXiv, 2019)
Even if such a black hole were to perform an accretion event, the light emitted by the material caught in its gravitational grasp would be barely noticeable—undetectable from Earth with our current instruments.
But this is not the only way to detect a primordial black hole.
Even at very small diameters, the gravity around these objects would be strong enough to warp spacetime beyond the event horizon.
This region of highly curved spacetime can act as a cosmic lens, and any background light passing through it will be amplified, causing a brief, soft increase in brightness before it returns to normal levels—a phenomenon known as microlensing.
This is exactly the signal that the Dark Energy Camera (DECam) detected in 2019 when it looked towards the Large Magellanic Cloud, located approximately 163,000 light-years from Earth.
The event occurred on December 18, when the DECam telescope was operating for five consecutive nights as part of the Asteroid Mass Primordial Black Hole Microlensing (AMPM) project.
Over the course of about 60 minutes, the brightness of the star in the Large Magellanic Cloud increased, while the brightness of nearby light sources remained unchanged.
An image taken at the moment of Phoebe's peak brightness, with light curves showing how its brightness increased dramatically while the brightness of nearby stars remained constant. (Key et al., arXiv, 2026)
Microlensing events are rare, but not unknown. Microlensing has previously been linked to stellar black holes, tiny dim stars and the worlds surrounding them, and rogue exoplanets drifting through space without a star.
To determine whether Fibe could be a black hole, the researchers first had to rule out instrument malfunctions, stellar flares, contamination from other stars, and stellar fluctuations.
They then had to simulate different microlensing scenarios: a free-floating exoplanet in the Milky Way; a free-floating exoplanet in the Large Magellanic Cloud; and a primordial black hole in the Milky Way's extended dark matter halo, far from the concentration of matter in the galactic plane.
The Milky Way's halo is an extended region around the disk of the galaxy. (Melissa Weiss/Center for Astrophysics | Harvard and Smithsonian)
According to their calculations, the lensing body, Phoebe – whatever it is – is five orders of magnitude more likely to belong to the Milky Way's dark matter halo than to known stellar populations in any of these galaxies.
The most likely explanation is that Phoebe is a primordial black hole, about three times the mass of the Moon, located about 59,630 light-years away.
This doesn't rule out the existence of a rogue exoplanet in the Milky Way's halo. In fact, the existence of a rogue exoplanet remains quite probable, given that, at least observationally, rogue exoplanets are much more likely to exist and be detected.
However, in the Milky Way's halo, which is sparsely populated at best, a black hole is much more likely than a rogue exoplanet, which are generally thought to be more numerous in regions of space with more stars.
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This discovery coincided with another discussion.
In February 2026, astronomers from the United States and Japan, analyzing data from the Subaru Telescope, identified 12 candidate microlensing events toward Andromeda, which they say could be caused by primordial black holes.
Then another group of researchers from the University of Warsaw (Poland) reanalyzed the same data and published a refutation in March, establishing that all of these events could be attributed to ordinary, known stars.
On the topic: Scientists say LIGO may have detected the first primordial black hole.
This new discovery is the reason for this discussion.
Kay and her colleagues say their discovery confirms the original interpretation of the Subaru data, which held that the events are consistent with phenomena occurring in primordial black holes.
This means only one thing: we will need a more sensitive telescope.
«"Our discovery prompted the microlensing programs at the Roman and Vera K. Rubin Observatories to conduct high-frequency and long-term observations to improve sensitivity to low-mass microlenses," the researchers write in their paper.
We are looking forward to it.
Preprint available on arXiv.
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