Ames Research Centre / NASA
A star in the Large Magellanic Cloud experienced a brief increase in brightness, possibly due to a gravitational microlensing event
Something that has drifted silently through the darkness for 13 billion years briefly announced its presence by bending the light of a distant star for an hour one night in December 2019.
On the night of December 18, 2019, a star in our satellite galaxy, the Large Magellanic Cloud, momentarily became brighter.
Not in a dramatic or explosive way — just a smooth ascent and descent and symmetrical luminosity, lasting about an hour, as if something had passed in front of it and bent its light towards us. Then it went back to normal and was never observed to vary.
This something was given the name “Phoebe”. And figuring out what it actually is proves to be one of the most fascinating enigmas in modern astronomy. The phenomenon at the center of this story is called gravitational microlensing and it is one of the most elegant predictions of Einstein’s general theory of relativity.
When a compact, massive object passes between us and a distant star, its gravity acts like a lensbriefly amplifying the star’s light in a very characteristic way.
O profile of this increase in brightness is unmistakable and does not resemble, in any way, that produced by a variable star, a solar flare or an asteroid.
When a team of astronomers from Swinburne University in Melbourne detected Phoebe in data from their high-cadence survey of the Large Magellanic Cloud, they had no doubt that they were experiencing a genuine microlensing event.
The question is: What is Phoebe after all??
There are three possibilities. The first is that it is a wandering planet — a world expelled from its solar system long ago, wandering the Galaxy alone.
The second possibility is that the planet belongs to the Large Magellanic Cloud itself and not to our Milky Way, which would make it the first extragalactic planet ever discovered by microlensing.
The third hypothesis is considerably more exotic: the mysterious Phoebe could be a primordial black hole — a microscopic black hole formed not by the collapse of a star, but by density fluctuations in the first fractions of a second after the Big Bang, before any stars existed.
A event duration is the determining clue, and since microlensing timescales depend on the mass of the object responsible for the effect, the lighter the object, the faster it crosses our line of sight and the shorter the increase in brightness.
Lasting around 60 minutes, Phoebe is right on the threshold than current surveys can detect. Working the physics in reverse, the team calculated its mass to be about three times that of our Moon — making it much smaller than any planet and too small to be any type of black hole resulting from a stellar remnant.
NASA Ames/JPL-Caltech/T. Pyle
Gravitational microlensing of light from a distant background star caused by a rogue exoplanet passing toward it
Stellar black holes have a minimum mass of about five times that of the Sun. Phoebe is several orders of magnitude below that threshold. The only type of black hole this size is those that were formed in the Big Bang itself.
The team to probability that the lensing object belongs to each of the possible populations — stars in the Milky Way, stars in the Large Magellanic Cloud, or the halo of dark matter between and around them.
O 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 ordinary stellar matter.
It will be confirmed, Phoebe is one of the oldest objects ever detected —formed before the first stars, before the first atoms, in the violent chaos of the early universe.
Something that Drift silently through the darkness 13 billion years ago briefly announced its presence by bending the light of a distant star, for an hour, on a December night in 2019.
