Theoretical astrophysicists have calculated that the dense dust rings swirling around supermassive black holes could spawn not just a handful of planets, but tens of millions of them, ranging from Earth-mass rocks to objects heavier than Jupiter. The finding, built on streaming-instability models applied to active galactic nucleus (AGN) tori, challenges the assumption that planets form only in the relatively calm disks around ordinary stars. If the models hold, the most violent neighborhoods in the universe may also be its most prolific planetary factories.
Why millions of black-hole planets would reshape planetary science
Planet formation theory has long focused on protoplanetary disks, the flat rings of gas and dust orbiting young stars. Supermassive black holes, weighing millions to billions of solar masses, sit at the centers of galaxies and are surrounded by their own thick tori of dust and gas. These tori share some physical traits with protoplanetary disks, including dense concentrations of solid grains and strong rotational shear, but they operate at far larger scales and under far more extreme gravitational fields.
The new research led by McKernan and colleagues applies streaming-instability calculations to these AGN tori. Streaming instability is a well-studied mechanism in ordinary planet formation: when small dust grains clump together under the influence of gas drag and gravity, they can rapidly collapse into solid bodies called planetesimals. The team’s models indicate that the same process, scaled up to the environment around a supermassive black hole, could produce tens of millions of planetesimals spanning Earth to super-Jupiter masses, according to their preprint. Rapid pebble accretion could then push some of these objects to even larger sizes.
The practical consequence is stark. If these calculations are correct, every active galaxy with a dusty torus could harbor a planetary population orders of magnitude larger than any single stellar system. That would mean the total number of planets in the universe is dramatically higher than current estimates based on star-hosted worlds alone. It would also force a rethinking of what counts as a habitable or interesting environment for future observation.
One testable implication deserves attention. If millions of massive bodies orbit within an AGN torus, their collective gravitational influence, known as dynamical friction, should create a periodic drag on the surrounding gas and dust. That drag could modulate the black hole’s accretion rate in ways that show up as subtle, repeating patterns in X-ray and optical brightness over decade-long timescales. Existing AGN monitoring archives from missions like the Rossi X-ray Timing Explorer and ground-based optical surveys already contain decades of variability data. Searching those records for the predicted periodic signatures could offer an indirect test of whether these planet populations actually exist, though no group has yet reported such an analysis.
Three independent models point toward the same conclusion
The idea did not emerge from a single paper. Three separate theoretical efforts, spanning several years and using different approaches, converge on the same broad prediction: supermassive black holes can host planets.
The earliest of the three, a study accepted by The Astrophysical Journal, modeled the growth of sub-micron icy dust grains into Earth-sized bodies several parsecs from supermassive black holes in low-luminosity AGN. That work argued that the “radial drift barrier,” a problem that slows planet formation in ordinary protoplanetary disks because grains spiral inward before they can grow large enough, is less severe in AGN tori. The weaker radial drift allows grains to stick together and grow more efficiently.
A follow-on study by Wada and collaborators extended those results and coined the term “blanets” for black-hole planets. Their parameter-space exploration predicted mass ranges of tens to thousands of Earth masses with formation timescales as short as tens of millions of years in some configurations. A separate institutional summary from Japan’s National Astronomical Observatory reported that one specific model yielded roughly 10,000 bodies of about 10 Earth masses orbiting at approximately 10 light-years from a 10-million-solar-mass black hole, with a formation timescale of a few hundred million years.
The newest preprint by McKernan and colleagues goes further. By applying magnetized streaming-instability models directly to AGN torus conditions, it arrives at populations not in the thousands but in the tens of millions. According to reporting by Phys.org, the team also found that rapid accretion could push some objects beyond planetary masses entirely, potentially reaching stellar masses.
The three studies disagree on specifics. The earlier Japanese model predicts roughly 10,000 bodies of about 10 Earth masses, while the McKernan preprint predicts tens of millions spanning a much wider mass range. Formation timescales also diverge: tens of millions of years in some blanet models versus a few hundred million years in others. These differences reflect different assumptions about torus density, temperature structure, magnetic fields, and the efficiency of pebble accretion. But despite the spread in numbers, all three efforts agree that the basic physics of dust growth and gravitational collapse should operate in AGN tori, not just in stellar disks.
What life would be like near a black-hole planet
Even if AGN tori are prolific nurseries, that does not mean they are friendly places. The central black hole can flood its surroundings with high-energy X-rays, ultraviolet light, and particle radiation. Any planet embedded in the torus would likely be shrouded in thick dust and gas, bathed in a faint glow from reprocessed radiation rather than direct starlight. Temperatures in some regions may be cold enough for ices to dominate, while others closer in could be heated to hundreds of degrees.
Habitability, in the terrestrial sense, seems unlikely in the inner torus, where radiation and tidal forces are strongest. Farther out, at distances of several light-years, conditions might be gentler, but the planets would still orbit in a crowded, dynamically active environment. Frequent gravitational encounters with neighboring bodies could pump up their orbital eccentricities, leading to tidal heating and potentially intense internal activity. Any moons or ring systems would have to survive in the face of constant perturbations.
Yet the sheer number of potential worlds changes the conversation. Even if only a tiny fraction of black-hole planets occupy relatively benign niches-shielded by thick atmospheres, buried oceans, or magnetic fields-the absolute number of such refuges across the universe could be large. For astrobiology, the existence of blanets would broaden the search space far beyond the familiar realm of Sun-like stars and their habitable zones.
How astronomers might detect blanets
Directly imaging a planet embedded in an AGN torus is beyond current capabilities. The regions where blanets are predicted to form lie at distances of light-years from the black hole but at cosmological distances from Earth, making them effectively unresolvable points of light. Instead, astronomers will have to rely on indirect signatures.
One route, already hinted at by the McKernan team, is to look for the dynamical fingerprints of millions of orbiting bodies. Their combined gravity could stir the torus gas, altering its turbulence and viscosity. This might leave characteristic imprints on the broad emission lines produced by fast-moving gas near the black hole, or on the long-term flickering of the AGN’s brightness.
Another possibility is microlensing. As a blanet passes in front of a bright background source within the AGN, such as a compact region of the accretion disk, its gravity could briefly magnify the light. In principle, high-cadence monitoring of strongly lensed quasars might reveal short-lived brightening events caused by individual planets in the lensing galaxy’s central torus. Extracting those signals from the noisy variability of AGN, however, will be challenging.
Future X-ray and infrared observatories with improved sensitivity and time resolution could help. By combining long-term monitoring with sophisticated variability models that include the effects of embedded planets, researchers may be able to distinguish blanet-induced patterns from intrinsic disk turbulence. If even one robust case emerges, it would validate the broader idea that planet formation is a generic outcome of rotating, dusty disks, whether around young stars or supermassive black holes.
From curiosity to new cosmic census
For now, black-hole planets remain a theoretical curiosity, supported by detailed calculations but untested by observation. The convergence of multiple independent models, though, suggests that the concept is more than a speculative flourish. If AGN tori truly manufacture tens of millions of planets per galaxy, they could rival or exceed the output of conventional stellar nurseries.
Confirming or refuting that picture will require creative use of existing data and careful planning for future surveys. Either outcome would be illuminating. A positive detection would expand the known habitats for planets into some of the universe’s most extreme environments. A null result, if it stands up against the models, would force theorists to revisit their assumptions about dust physics, turbulence, and magnetic fields in AGN tori.
In the meantime, the notion of worlds orbiting in the shadow of supermassive black holes underscores a broader shift in astronomy: planets may be far more ubiquitous, and far more diverse, than the first exoplanet discoveries ever hinted. From the quiet suburbs of ordinary stars to the crowded hearts of active galaxies, the cosmos appears determined to build planets wherever physics allows.
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*This article was researched with the help of AI, with human editors creating the final content.
