Blanets: Planets Forming Around Supermassive Black Holes in AGN Disks

Overview

In this Astrum episode, Alex McColgan discusses a controversial and fascinating idea: millions of planets could form around active galactic nuclei (AGN) inside the accretion disk of a supermassive black hole. Researchers simulate how dust and gas might clump together in the outer regions of the disk, potentially creating planetary systems far from any star.

• Planet formation in AGN disks via pebble accretion may occur at tens of parsecs from the hole
• The outer dusty torus provides cooler, dust-rich conditions that can foster dust growth and clumping
• Planets could migrate outward, becoming rogue planets or, in theory, stars
• Detecting such worlds would be challenging, requiring indirect methods like gravitational lensing or radio transit studies

Introduction: Blanets and AGN Disks

The video introduces the concept of blanets, planets that might form not around a star but within the accretion disk of a supermassive black hole, specifically in the dusty outer torus of AGN disks. It contrasts the harsh inner disk with the calmer, cooler outskirts where planet formation could potentially occur.

The AGN Environment

Galactic centers often host supermassive black holes surrounded by luminous accretion disks. The inner regions reach temperatures and radiation intense enough to vaporize ices and ionize gas, creating extreme magnetic turbulence. Materials flow in, creating a thick, dynamic environment known as the accretion disk. AGNs emit across the electromagnetic spectrum and can outshine the entire galaxy, posing a dramatic contrast to the relatively placid protoplanetary disks where planets in our solar system form.

Why Planets Are Thought Impossible in AGNs

In protoplanetary disks, cold temperatures and calm conditions favor dust grains sticking together. In AGN disks, high temperatures and strong ionization produce magnetorotational instabilities that drive turbulent collisions, which tend to fragment material rather than enable growth. This turbulence, driven by magnetic fields and the Lorentz force, disrupts the gentle aggregation required for planetesimal formation.

How Blanets Could Form: The Outer Disk as a Nursery

Mishra and colleagues modeled SMBH accretion disks and found that the outer edges of AGN disks, in the dusty torus region, could be cool enough and dust-rich enough to enable dust grains to grow and coagulate. Once grains reach millimeter to centimeter sizes, streaming instability can create dense filaments that collapse into planetesimals, eventually forming a population of planetary bodies ranging from Earth-mass to several Jupiter masses. This process resembles pebble accretion and filamentary collapse seen in protoplanetary disks, but in a far more extreme context.

Implications: From Planets to Stars

The model suggests that in AGN tori, planets could accumulate material until their reservoirs allow growth beyond the typical planetary ceiling. In principle, some bodies might continue accreting until they reach stellar masses, or even ignite fusion, creating a class of objects without a star in the traditional sense. The idea extends earlier speculation about blanets coined by Kei Wada in 2020 and adds nuance through Mishra’s streaming-instability framework.

Observational Fingerprints and Challenges

Direct detection is unlikely because the central AGN glare and disk brightness obscure individual planets. Instead, researchers propose looking for indirect evidence, such as stellar populations and chemical signatures in AGN disks, or using gravitational lensing to detect a swarm of solid bodies in front of X-ray emitting regions. In our Milky Way, young massive stars near Sagittarius A* and metal-rich gas around AGNs provide contextual clues that star formation in disks could occur in extreme environments, supporting the plausibility of planet formation in similar settings.

What We Still Don’t Know

Key uncertainties include how quickly dust grains can survive and coagulate in the outer torus, how often streaming instabilities trigger filament collapse, and how long AGN disks persist. The timescales are constrained by AGN lifetimes, the duration of the torus’s dust-rich conditions, and the balance between collision rates and disk lifetime. While not yet observed, the concept expands the possible environments in which planets might arise and prompts new observational strategies for future missions and analyses.

Closing Thoughts

Whether these black hole planets exist as a common outcome of AGN disk physics or remain a theoretical curiosity, the exploration highlights the richness of planetary formation pathways and the surprises the Universe may hold even in its most hostile corners. The discussion invites further inquiry into extreme astrochemistry, disk dynamics, and the potential diversity of planetary populations across the cosmos.