Something Mysterious Passed Between Earth and a Distant Star, and It's Barely the Mass of Three Moons

A star in the Large Magellanic Cloud, the Milky Way's satellite galaxy, briefly brightened on the night of December 18, 2019, in a smooth, symmetrical rise and fall lasting about an hour before returning to normal. Astronomers from Swinburne University in Melbourne, who spotted the event in data from a high-cadence survey of the Large Magellanic Cloud, identified it as a case of gravitational microlensing — a phenomenon predicted by Einstein's general theory of relativity in which a massive compact object passing between Earth and a distant star briefly magnifies the star's light in a distinctive way. They named the object responsible Phoebe.

There are three possibilities for what Phoebe could be: a free-floating planet ejected from its home solar system and now wandering the Milky Way; the same kind of object but belonging to the Large Magellanic Cloud itself, which would make it the first extragalactic microlensing planet ever found; or 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.

The duration of the brightening is the key clue, since microlensing timescales depend on the mass of the lensing object: the lighter the object, the faster it crosses the line of sight and the shorter the brightening lasts. Working backward from the roughly 60-minute event, the team calculated Phoebe's mass at approximately three times that of the Moon — far too small to be any kind of planet, and far too small to be a stellar remnant black hole, which has a minimum mass of roughly five times that of the Sun.

The team calculated the probability that the lensing object belonged to each possible population — Milky Way stars, Large Magellanic Cloud stars, or the dark matter halo around and between them — and found the dark matter halo was about 100,000 times more likely than either stellar population. If that interpretation holds, Phoebe would be among the oldest objects ever detected, having drifted through the dark for roughly 13 billion years since forming before the first stars.