On the night of December 18, 2019, a star in the Large Magellanic Cloud—our galaxy's nearest satellite galaxy—briefly brightened in a smooth, symmetrical rise and fall lasting about an hour, then returned to normal and has never varied again. That fleeting brightening, astronomers now believe, was caused by something called Phoebe passing directly in front of the distant star and bending its light toward us through the invisible force of gravity.
The phenomenon is called gravitational microlensing, and it represents one of the most elegant predictions of Einstein's general theory of relativity. When a massive compact object passes between Earth and a distant star, its gravity warps spacetime like a lens, magnifying the star's light in a distinctive, characteristic pattern entirely unlike the variations produced by flares, variable stars, or asteroids. When astronomers from Swinburne University in Melbourne spotted Phoebe in data from their high-cadence survey of the Large Magellanic Cloud, they recognized immediately what they were looking at: a genuine microlensing event.
But what is Phoebe? That question has turned into one of modern astronomy's most intriguing puzzles. Three possibilities emerged from the analysis. The first is a free-floating planet, a world ejected from its solar system long ago and now wandering alone through the galaxy. The second is the same thing, but originally belonging to the Large Magellanic Cloud itself rather than the Milky Way—which would make it the first extragalactic microlensing planet ever found. The third option is considerably more exotic: a primordial black hole, a microscopic black hole formed not from a collapsing star but from density fluctuations in the first fractions of a second after the Big Bang, before any stars existed at all.
The duration of the brightening event is the critical clue. Since microlensing timescales depend directly on the mass of the lensing object, a lighter object will cross our line of sight more quickly, producing a shorter flash. At approximately 60 minutes, Phoebe sits right at the edge of what current surveys can detect. Working backward through the physics, the Swinburne team calculated Phoebe's mass at approximately three times that of Earth's moon—far smaller than any planet, and far too small to be any kind of stellar remnant black hole. Stellar black holes have a minimum mass of roughly five times that of the sun. Phoebe is orders of magnitude below that threshold. The only kind of black hole that small is one that formed in the Big Bang itself.
To determine which possibility was most likely, the team calculated the probability of the lensing object belonging to each possible population: Milky Way stars, Large Magellanic Cloud stars, or the dark matter halo between and around them. The dark matter halo wins by a factor of 100,000—Phoebe is five orders of magnitude more likely to be a dark matter object than anything associated with normal stellar matter.
If that interpretation holds, Phoebe ranks among the oldest objects ever detected, formed before the first stars, before the first atoms, in the violent chaos of the infant universe. Something that has been drifting silently through the dark for 13 billion years briefly announced itself by bending the light of a distant star for one hour on a December night in 2019.