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Active Galactic Nucleus Tori: Potential Birthplace to Millions of Planets

This paper proposes that the outer regions of Active Galactic Nucleus (AGN) disks serve as a prolific birthplace for millions of planets and even stars, where dust condensation triggers streaming instability to form massive planetesimals that rapidly grow via pebble accretion into objects ranging from Earth-sized planets to stellar-mass bodies.

Original authors: Bhupendra Mishra, Wladimir Lyra, Barry McKernan, Mordecai-Mark Mac Low, K. E. Saavik Ford, Harrison E. Cook

Published 2026-05-20

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CC BY 4.0

Original authors: Bhupendra Mishra, Wladimir Lyra, Barry McKernan, Mordecai-Mark Mac Low, K. E. Saavik Ford, Harrison E. Cook

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the center of a galaxy not as a quiet, empty void, but as a bustling cosmic construction site. Usually, we think of planets forming around stars, like seeds growing in a garden. But this paper suggests that the swirling disks of gas and dust around Supermassive Black Holes (the "Active Galactic Nuclei" or AGN) are actually massive, hidden nurseries capable of birthing millions of planets—and even stars and black holes.

Here is the story of how this happens, broken down into simple concepts:

1. The Setting: A Cosmic "Dusty Torus"

Think of a Supermassive Black Hole as a giant, hungry vacuum cleaner. Around it spins a massive disk of gas.

The Inner Disk: Close to the black hole, it's so hot that dust turns into gas (like ice melting into steam). No planets can form here.
The Outer Ring (The Torus): Farther out, the temperature drops. It's cool enough for dust to survive. This is the "Dusty Torus," a giant, donut-shaped ring of cosmic dust and gas surrounding the black hole.

2. The Spark: Dust Bunnies Getting Too Big

In our own solar system, planets start as tiny dust grains that stick together to form pebbles, then boulders, then planets.

The Problem: In these galactic disks, the dust usually blows apart or drifts away before it can grow.
The Solution: The authors propose that magnetic fields act like a safety net, holding the disk together and keeping it stable.
The "Streaming Instability": Imagine a crowd of people walking through a hallway. If they start walking in a specific rhythm, they naturally bunch up into dense clusters. In the AGN disk, dust grains do the same thing. They clump together into massive, dense "filaments" (like giant cosmic dust bunnies).

3. The Birth: Millions of "Planetesimals"

Once these dust filaments get dense enough, their own gravity takes over. They collapse instantly.

The Result: Instead of forming one or two planets, a single filament collapses into tens of millions of objects.
The Size: These aren't just tiny rocks. They range in size from Earth-like planets to Super-Jupiters (planets much bigger than Jupiter).
The Scale: The paper estimates that a single galaxy's black hole could host tens of millions of these objects. It's not just a garden; it's a whole forest of planets.

4. The Growth: Eating Pebbles and Gas

Once these "baby planets" are born, they start eating.

Pebble Accretion: They sweep up the surrounding dust pebbles, growing rapidly.
Gas Accretion: They also suck in the surrounding gas.
The Twist: In our solar system, planets stop growing when they clear a gap in the disk. But in these AGN disks, the gas is so abundant that the planets keep growing past the size of stars.
The Outcome: Some of these "planets" grow so massive that they ignite nuclear fusion and become stars. Because they are made of dust (heavy elements) rather than just hydrogen, they are essentially "rocky stars."

5. The Ultimate Fate: From Planets to Black Holes

The paper suggests a wild chain reaction:

Dust clumps into Planets.
Planets eat gas and become Stars.
These stars grow so massive (hundreds of times the mass of our Sun) that they collapse directly into Intermediate-Mass Black Holes.

These new black holes could then feed the central Supermassive Black Hole, helping it grow even larger.

Summary of the Paper's Claims

Where: In the cool, dusty outer rings of active galaxies.
How: Magnetic fields stabilize the disk, allowing dust to clump via "streaming instability."
What: This creates a massive population of objects ranging from Earth-sized planets to stars and black holes.
Why it matters: It suggests that the universe might be filled with "hidden" planets and stars born around black holes, and that black holes might grow by "eating" the stars they helped create.

What the paper does NOT say:

It does not claim we can visit these planets (they are too far away and the environment is hostile).
It does not suggest these planets could support life.
It does not claim this happens in every galaxy, only in those with active black holes and specific magnetic conditions.

In short, the paper paints a picture of the universe's most violent centers as surprisingly fertile grounds, where the dust of destruction becomes the seed for a new generation of planets, stars, and black holes.

Technical Summary: Active Galactic Nucleus Tori as a Birthplace for Millions of Planets

Problem Statement
While planet formation is well-established in circumstellar disks, the potential for planet formation in the extreme environments of Active Galactic Nucleus (AGN) disks has remained an open question. A primary obstacle is the thermal state of AGN disks; traditional models (e.g., Sirko & Goodman 2003) predict temperatures exceeding dust sublimation limits across most of the disk, rendering them dust-free and inhospitable to planet formation. Furthermore, the mechanisms for angular momentum transport and gravitational stability in these hot, ionized disks differ significantly from the cold, weakly ionized circumstellar disks where standard planet formation theories apply. This paper addresses whether the outer, cooler regions of AGN disks—specifically the dusty torus—can support the physical conditions necessary for dust coagulation, streaming instability, and subsequent planetesimal formation.

Methodology
The authors employ a recently proposed magnetized AGN disk model (Hopkins et al. 2024) which posits that strong toroidal magnetic fields ( $\beta_m \ll 1$ ) provide the necessary pressure support to maintain gravitational stability in the outer disk, allowing temperatures to drop sufficiently for dust survival. This model replaces the assumption of a constant Toomre $Q=1$ with a magnetically supported, gravitationally stable configuration.

The study proceeds through the following analytical steps:

Disk Structure: The authors calculate radial profiles for gas density, temperature, surface density, and scale height for supermassive black hole (SMBH) masses of $10^7$ , $10^8$ , and $10^9 M_\odot$ .
Dust Growth and Streaming Instability (SI): They quantify the conditions for dust coagulation from interstellar medium (ISM) grains to pebbles. The analysis determines the maximum Stokes number ($St$) limited by either fragmentation or radial drift. The authors assess whether grains can reach the critical Stokes number threshold ( $St_{SI} \approx 10^{-2}$ or $10^{-3}$ ) required to trigger the streaming instability within the AGN lifetime.
Filament Mass and Planetesimal Formation: Upon satisfying SI conditions, the authors calculate the mass of the resulting overdense dust filaments ( $m_f$ ) and the characteristic mass of the planetesimals ( $m_p$ ) formed via gravitational collapse, utilizing scaling relations from numerical simulations (Liu et al. 2020; Klahr & Schreiber 2020).
Accretion Regimes: The study investigates the subsequent growth of these objects via pebble accretion and concurrent gas accretion. The authors determine the relevant accretion regimes (3D Bondi, 3D Hill, 2D, or geometric) by comparing the object's Bondi radius, Hill radius, and geometric radius against the dust scale height.
Population Statistics: By integrating the number of filaments per radial bin and the number of planetesimals per filament, the authors estimate the total population of planetary-mass objects in the AGN disk.

Key Results

Dust Survival and Growth: In the magnetically supported model, the outer disk (beyond $\sim$ 1 pc) cools sufficiently for dust to survive. Dust grains can grow via coagulation to sizes required for streaming instability ( $St \gtrsim 10^{-2}$ ) within timescales ( $10^3$ – $10^7$ years) that are feasible within the estimated AGN lifetimes (1–10 Myr), particularly if the fragmentation velocity threshold is $\ge 1$ m s $^{-1}$ .
Massive Planetesimal Formation: The streaming instability produces filaments containing millions of Jupiter masses of pure dust. These filaments collapse into tens of millions of "planetesimals." The characteristic mass of these objects scales strongly with radius ( $m_p \propto r^{1.5}$ ) due to the flaring of the AGN torus.
Accretion Dynamics:
- Pebble Accretion: Most objects form in the 3D Bondi accretion regime. Mass-doubling times range from $10^3$ to $10^7$ years.
- Gas Accretion: Gas accretion occurs concurrently but is initially 3–4 orders of magnitude slower than pebble accretion. It only becomes dominant once the object grows significantly in mass.
- Feedback Limit: Crucially, the pebble isolation mass (the mass at which a planet carves a gap and halts solid accretion) is calculated to be above the hydrogen burning limit ( $\sim 0.08 M_\odot$ ). Consequently, accretion is not halted by gap carving but by stellar feedback (winds and radiation) once the object ignites nuclear fusion.
Population Estimates: The model predicts the formation of tens of millions of planetary-mass objects in the outer regions of AGN disks. The number of objects increases with higher SMBH mass and larger radial distance.

Significance and Claims
The paper claims that AGN dusty tori represent a viable and potentially prolific site for planet formation, hosting the largest populations of planets in the universe. The study identifies a novel "core accretion channel" for star formation, where objects formed via planetesimal mechanisms grow massive enough to become stars and potentially intermediate-mass black holes (IMBHs) via direct collapse or core collapse, rather than through standard molecular cloud fragmentation.

The authors highlight the formation of "exotic objects": stellar-mass bodies composed entirely of heavy elements (pure dust) formed in the geometric accretion regime, which do not accrete significant gas and remain as degenerate, rocky "stars." Furthermore, the paper suggests that the hierarchical growth of these objects provides a plausible pathway for the formation of IMBHs ( $10^2$ – $10^5 M_\odot$ ), which could subsequently merge or accrete to feed the central SMBH.

The study concludes that the outer AGN disk is a compelling physical analog to protostellar disks, albeit on vastly larger scales, bridging the fields of planet formation and black hole growth. Observational signatures of this population could include occultations and microlensing events against the central X-ray emitting region of the AGN.