A recent study detected an invisible object passing close to the Milky Way in an event that lasted about an hour, called Phoebe. It could be a primordial black hole in the galaxy’s halo or a rogue planet in the nearby Large Magellanic Cloud.
Astronomers spotted something invisible and unknown crossing the outskirts of the Milky Way at very high speed. The phenomenon, named Phoebe, lasted only about an hour and is described as one of the fastest and lowest-mass signals ever detected in the history of astronomy.
The detection was made with a high-resolution camera installed in Chile, which captured the brightness of a star increasing smoothly and symmetrically for around 60 minutes. The object is so faint that it could not be seen directly: scientists only noticed its passage through the effect of its gravity on light, a phenomenon called gravitational microlensing. Hypotheses range from a primordial black hole to a wandering planet.
How Phoebe was spotted near the Milky Way
If it is located in the Large Magellanic Cloud, Phoebe could be a rogue planet with a mass much greater than Jupiter.
The incident took place in December 2019, when an international team used the DECam camera, installed on the four-meter Blanco telescope, at the Cerro Tololo Observatory, in Chile.
For five nights, researchers photographed around 10 million stars in the Large Magellanic Cloud every minute, looking for small increases in brightness that would indicate the passage of an invisible object. That’s how, in a single night, they found Phoebe.
The event was quick and was not repeated. According to the study, published in the journal Monthly Notices of the Royal Astronomical Society (MNRAS) and led by researcher Renee Key, it is one of the fastest and lowest mass microlensing signals ever recorded, with a characteristic duration of around 60 minutes.
The name Phoebe is not for nothing: it combines the acronyms in English for “free floating planet” and “primordial black hole”, precisely the main hypotheses about what crossed the outskirts of the Milky Way.
A primordial black hole in the Milky Way’s halo?
Statistical analysis gives a strong clue. According to the authors, the object is five orders of magnitude, or about 100,000 times, more likely to belong to the dark matter halo of the Milky Way than to the star content of our or neighboring galaxy.
Therefore, the main hypothesis is that Phoebe is a primordial black hole, considered by scientists to be the best candidate ever found for this type of object.
A primordial black hole is a hypothetical object that would have formed in the first moments of the Universe, right after the Big Bang, and not from the collapse of a star. He is one of the leading candidates to explain mysterious dark matter.
In this scenario, Phoebe would have a mass equivalent to about three times that of our Moon, or just 0.032 times the mass of Earth, and would be among the oldest objects ever detected, wandering in the dark for billions of years.
Or a rogue planet in the Large Magellanic Cloud?
According to information on the NSC portal, there is, however, an alternative explanation. If Phoebe is located in the Large Magellanic Cloud, a neighboring galaxy located about 163 thousand light years away, its mass would be much greater, around 0.1 times that of our Sun.
In this case, it could be a wandering planet, that is, a world that does not orbit any star and wanders alone through space, or a low-mass object.
This possibility would also be historic. If confirmed as a wandering planet in the Large Magellanic Cloud, Phoebe would be the first extragalactic exoplanet discovered using this method.
The decisive clue is the duration of the event: the lighter the object, the faster it crosses the line of sight and the shorter the flash, which helps scientists estimate which scenario it is approaching.
Why it’s hard to confirm and why it matters
The big problem is that microlensing events like this don’t happen again, which makes it almost impossible to confirm Phoebe’s nature with certainty.
Before reaching hypotheses, the team needed to rule out equipment failures, stellar explosions and contamination from other stars. Even so, it is still a single episode, observed for about an hour, close to the Milky Way.
It is worth being cautious: primordial black holes were for a long time treated as a marginal idea and have returned to fashion as the search for dark matter remains unanswered, but concrete evidence of their existence is still scarce.
On the other hand, a Japanese team recently reported 12 similar events in the direction of the Andromeda galaxy, some possibly caused by similar objects in the Milky Way’s halo.
If confirmed, Phoebe could help understand what dark matter is made of, whether as a rogue planet or an ancient primordial black hole.
An invisible object crossing the Milky Way, which could be either a black hole from the beginning of time or a rogue planet, is the kind of mystery that shows how much there is still to discover in the Universe.