On the night of 18 December 2019, a star in our satellite galaxy, the Large Magellanic Cloud, briefly got brighter. Not dramatically nor explosively, just a smooth symmetrical rise and fall in brightness lasting about an hour, as though something had passed in front of it and bent its light toward us. Then it returned to normal and was never seen to vary again.

That something has been named Phoebe. And working out what it actually is turns out to be one of the most intriguing puzzles in modern astronomy. The phenomenon at the heart of the story is called gravitational microlensing and it’s one of the most elegant predictions of Einstein's general theory of relativity. When a massive compact object passes between us and a distant star, its gravity acts like a lens, briefly magnifying the star's light in a very characteristic way. The shape of the brightening is distinctive and entirely unlike anything produced by a variable star, a flare, or an asteroid.

The Large and Small Magellanic Clouds (Credit : ESO/S. Brunier)

When a team of astronomers from Swinburne University in Melbourne spotted Phoebe in data from their high cadence survey of the Large Magellanic Cloud, they were in no doubt they were looking at a genuine microlensing event. The question is what exactly is Phoebe?

There are three possibilities. The first is a free floating planet, a world ejected from its solar system long ago, now wandering alone through the Galaxy. The second is the same thing, but belonging to the Large Magellanic Cloud itself rather than our own 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 event is the critical clue and, since microlensing timescales depend on the mass of whatever is doing the lensing, the lighter the object, the faster it crosses our line of sight and the shorter the brightening lasts. At roughly 60 minutes, Phoebe sits right at the edge of what current surveys can detect. Working backwards through the physics, the team calculated its mass at approximately three times that of our Moon making it far smaller than any planet, and far too small to be any kind of stellar remnant black hole.

Gravitational microlensing of the light of a distant background star by a passing rogue exoplanet (Credit : NASA Ames/JPL-Caltech/T. Pyle)

Stellar black holes have a minimum mass of roughly five times that of the Sun. Phoebe is orders of magnitude below that floor. The only kind of black hole that small is one that formed in the Big Bang itself.

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 is 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.

Source : AMPM II. A Lunar-Mass Primordial Black Hole Microlensing Candidate in the Milky Way Halo