Could dark matter — the mysterious substance that makes up 85% of the mass of the universe — partially be black holes the size of planets? New research looks to the Milky Way’s neighbors for an answer.
Rogue Planets or Tiny Black Holes?
Schematic of a black hole lensing the light from a background star as the black hole passes in front of the star from the vantage point of an observer on Earth. Click to enlarge. [NASA/ESA; CC BY 4.0]
Thanks to surveys like the Optical Gravitational Lensing Experiment (OGLE), researchers have identified thousands of microlensing events. Most of these events are due to stars, brown dwarfs, and stellar remnants, but a small number seem to arise from objects the size of planets. While the majority of astronomers attribute these signals to free-floating exoplanets unmoored from their host stars, others have proposed a more speculative cause: primordial planet-sized black holes. Primordial black holes are theorized to have formed early in the universe when dense regions collapsed directly into black holes without forming stars first.
Previous research has suggested that if the observed microlensing signals are due to primordial planet-mass black holes rather than planets, as much as 10% of the mass attributed to dark matter is made up of tiny, ancient black holes. It’s an enticing theory, but a highly speculative one — how can we tell if the microlensing signals come from planets or planet-sized black holes?
OGLEing Microlensing Events
In their recent research article, Przemek Mróz (University of Warsaw) and collaborators showed that answering this question is all about looking in the right place. In the Milky Way’s disk and bulge, where stars and planets are common, it’s impossible to distinguish between microlensing signals from planets and planet-sized black holes. Away from these heavily populated areas, interference from stellar and planetary signals should be minimal, and microlensing signals could reveal primordial planet-mass black holes — should they exist — floating through the Milky Way’s dark-matter halo.
The survey’s search area overlaid on images of the Large (left) and Small (right) Magellanic Clouds. [Mróz et al. 2024]
The resulting dataset included 17.6 million light curves from stars in the Magellanic Clouds, which the team whittled down using several criteria. First, they required that the light curves show an increase in brightness but be otherwise steady, eliminating variable stars and reducing the dataset to 2,538 light curves. Then, they removed false positives and light curves affected by imaging artifacts, leaving 308 light curves. Finally, they eliminated all light curves that weren’t fit well by a microlensing model — leaving just two possible microlensing events.
And Then There Was One
The two microlensing candidates found in the authors’ survey. The top plot shows what is likely a bona fide microlensing signal, while the bottom plot shows what the authors believe is a stellar flare. [Mróz et al. 2024]
Extrapolating from their observations, Mróz’s team calculated that primordial planet-mass black holes — between roughly half the mass of the Moon up to the dividing line between planets and brown dwarfs — can make up at most just 1% of all dark matter. This in turn suggests that the microlensing signals previously attributed to primordial planet-mass black holes are instead due to the more mundane (though still fascinating) population of exoplanets without host stars.
Citation
“Limits on Planetary-Mass Primordial Black Holes from the OGLE High-Cadence Survey of the Magellanic Clouds,” Przemek Mróz et al 2024 ApJL 976 L19. doi:10.3847/2041-8213/ad8e68