Astronomers detect mysterious invisible object crossing Milky Way halo at extreme speed

In December 2019, astronomers watching the Large Magellanic Cloud through a high-resolution camera in Chile caught something extraordinary: the light of a distant star brightening smoothly and symmetrically for about sixty minutes, then fading. Nothing unusual appeared in the images. No object crossed the field of view. Yet something had passed between Earth and that star—something invisible, something massive enough to bend light itself.

The team, led by researcher Renee Key, had been photographing roughly ten million stars each minute for five nights, hunting for the telltale signature of gravitational microlensing: a subtle brightening caused when an unseen object's gravity acts as a lens, magnifying starlight. They found one event. Just one. They named it Phoebe, a name that blends the acronyms for "free-floating planet" and "primordial black hole"—the two leading theories about what had just crossed the halo of the Milky Way.

The detection itself was remarkable enough. Phoebe ranks among the fastest, lowest-mass microlensing signals ever recorded in the history of astronomy. The entire event lasted roughly an hour. The camera that caught it, called DECam, was mounted on the four-meter Blanco telescope at Cerro Tololo Observatory in Chile. For five nights the researchers had scanned the sky methodically, looking for the faint signature of gravity bending light. On one night, they found it.

What Phoebe actually is remains uncertain, but the statistics point strongly in one direction. The object is roughly one hundred thousand times more likely to belong to the Milky Way's dark matter halo than to the visible stars of our galaxy or its neighbor. This makes the leading hypothesis a primordial black hole—a theoretical object that would have formed in the first moments after the Big Bang, not from the collapse of a dying star. If Phoebe is indeed a primordial black hole, it would have a mass about three times that of the Moon, or roughly 0.032 times the mass of Earth. It would be among the oldest objects ever detected, having wandered through the darkness for billions of years.

But there is an alternative. If Phoebe resides not in the Milky Way's halo but in the Large Magellanic Cloud itself—a neighboring galaxy about 163,000 light-years away—then it would be far more massive, perhaps one-tenth the mass of our Sun. In that case, it could be a rogue planet, a world that orbits no star and drifts alone through space. If confirmed as such, Phoebe would be the first extragalactic exoplanet ever discovered using this method.

The key to distinguishing between these scenarios lies in the duration of the brightening. The lighter the object, the faster it crosses the line of sight, and the shorter the flash of magnified light. The sixty-minute duration gives astronomers a window into which scenario fits better, though neither can be confirmed with certainty from a single observation.

Here lies the fundamental problem: microlensing events do not repeat. Once Phoebe passed, it was gone. The team had to rule out equipment failures, stellar explosions, and contamination from other stars before settling on their conclusion. Yet they were left with a single hour of observation, a single episode, and no way to watch it happen again. Primordial black holes were once considered fringe science, but as searches for dark matter continue to yield few answers, they have returned to favor—though concrete proof of their existence remains scarce.

Recently, a Japanese team reported twelve similar events in the direction of the Andromeda galaxy, some possibly caused by objects like Phoebe in the Milky Way's halo. If confirmed, Phoebe could help answer one of astronomy's deepest questions: what is dark matter made of? Whether it turns out to be an ancient black hole born at the dawn of time or a solitary planet wandering between stars, the detection points to how much remains unknown in the universe.