A brief brightening of a distant star has led astronomers to identify amysterious object that passed between Earth and the Large Magellanic Cloud. The event lasted only about an hour, yet researchers say it could represent either an unusual free-floating planet or a candidate primordial black hole, one of the most elusive objects proposed by modern cosmology.
The discovery was made using the Dark Energy Camera (DECam) as part of a search for compact objects that are difficult or impossible to observe directly. Rather than detecting light emitted by the object itself, the team relied on a phenomenon known as gravitational microlensing.
The findings have been presented in a study posted on arXivand have not yet undergone peer review. While the researchers examined multiple explanations for the event, their analysis suggests that the object is far more likely to belong to the Milky Way’s dark matter halo than to the stellar populations of either the Milky Way or the Large Magellanic Cloud.
A Brief Flash From A Distant Star
The event occurred while astronomers were monitoring stars in the Large Magellanic Cloud, a satellite galaxy of the Milky Way. During their observations, they detected a star that briefly brightened as an unseen object passed across the line of sight.
The effect is known as gravitational microlensing. According to NASA, the presence of mass warps space-time, allowing an intervening object to act like a lens and temporarily magnify the light from a more distant source. Although the phenomenon is often associated with stars and galaxies, planets and other compact objects can also produce measurable microlensing signals.
The newly detected event lasted approximately one hour. In their paper, the researchers reported the discovery of what they described as “a one hour-long microlensing event.” The object responsible for the signal was given the name Phoebe. Because microlensing does not require the lensing object to emit light, it has become an important tool for identifying otherwise invisible bodies.
The Location Of Phoebe Changes Everything
One of the main challenges facing the researchers is determining where Phoebe is located. The answer has major implications for its identity. As reported by the study, if Phoebe resides within the Large Magellanic Cloud, its estimated mass would be around 0.1 solar masses. Under that interpretation, the object could be a free-floating planet or a planet orbiting a host star at a very large distance.
The authors noted that such a finding would be remarkable. If confirmed, it would represent the first known extragalactic microlensing exoplanet detected beyond the Milky Way. Yet they argue that another scenario appears significantly more likely.
If Phoebe is instead located within the Milky Way’s galactic halo, much closer than the Large Magellanic Cloud, its estimated mass falls dramatically to about three times the mass of the Moon. That lower mass creates a different set of questions, since it would place the object outside the range expected for ordinary stellar black holes.
New Evidence for Primordial Black Holes?
The team compared several galactic models to evaluate the probability of Phoebe’s origin. As stated by the paper, their optical-depth analysis indicates that the object is five orders of magnitude more likely to belong to the Milky Way’s dark matter halo than to the stellar content of either the Milky Way or the Large Magellanic Cloud.
Based on that result, the researchers argue that Phoebe is currently “the best candidate for a PBH,” referring to a primordial black hole. Unlike black holes formed by collapsing stars, primordial black holes are hypothetical objects thought to have formed during the first moments of the universe.
The detection adds weight to the possibility that a population of low-mass compact objects may exist within the Milky Way’s dark matter distribution. The paper stated that:
“Phoebe suggests a population of compact, lunar-mass objects associated with the dark matter distribution of the Milky Way,” said the study team, adding that: “and potentially opens a new window to the physics of inflation.”
The researchers also write that the finding could “potentially open a new window to the physics of inflation.” At the same time, they emphasize that no primordial black hole has been confirmed. The event remains a single observation, and further detections of similar short-duration microlensing events will be needed to clarify whether Phoebe is an unusual planet, a compact dark object, or something even more unexpected.
