Active Galactic Nucleus Tori: Potential Birthplace to Millions of Planets

TL;DR

This paper explores the potential for planet formation within the dust-rich tori of active galactic nuclei, suggesting they could host the largest populations of planets in the universe.

Contribution

It introduces a model showing how dust in AGN disks can lead to the formation of millions of planets through streaming instability and pebble accretion.

Findings

Dust grains in AGN disks can coagulate to form filaments containing solar masses.

Planetesimals ranging from Earth to super-Jupiter masses can form in AGN tori.

Objects can grow to stellar masses, potentially leading to star formation.

Abstract

The outer regions of AGN disks have temperatures similar to those of circumstellar disks, permitting dust condensation. Therefore, planet formation and growth could be active in these dust tori through similar mechanisms. We aim at quantifying the parameter space for the occurrence of streaming instability, and its outcomes in terms of the masses of the objects formed, their total number, and their continued growth via pebble accretion. We use a a recently proposed disk model with strong magnetization to keep the disk gravitationally stable. We find that the dust grain sizes required for streaming instability are easily attained through coagulation; the dust filaments it produces can contain solar masses, collapsing into tens of millions of planetesimals ranging from Earth to super-Jupiter masses. These planets are usually born in the 3D Bondi regime of pebble accretion, and have mass-doubling times from 10^3 to 10^7 yrs, though 3D Hill and geometric accretion are also realized. Gas accretion occurs concurrently, and crossover mass can be attained while still in the planetary mass range. As a result, vigorous accretion can occur, leading to objects with stellar masses -- defining a core accretion channel for star formation. The pebble isolation mass is beyond the hydrogen burning limit, so accretion is limited by stellar feedback instead of gap carving. Our model also predicts a population of exotic objects directly formed above the hydrogen burning limit, yet of pure dust. Our approximate model suggests that AGN dust tori host the largest populations of planets in the universe.