Planet Formation around Super Massive Black Holes in the Active Galactic Nuclei

TL;DR

This paper proposes a new mechanism for planet formation around supermassive black holes in active galactic nuclei, showing that planetesimals and planets can form despite the harsh environment, with growth timescales comparable to AGN lifetimes.

Contribution

It introduces a novel pathway for planet formation in AGN environments, highlighting the role of dust growth and gravitational instability outside the snow line.

Findings

Planets can form around SMBHs in AGNs through dust aggregation.

The growth process is unaffected by the radial drift barrier.

Formation timescales are comparable to AGN lifetimes for low-mass SMBHs.

Abstract

As a natural consequence of the elementary processes of dust growth, we discovered that a new class of planets can be formed around supermassive black holes (SMBHs). We investigated a growth path from sub-micron sized icy dust monomers to Earth-sized bodies outside the "snow line", located several parsecs from SMBHs in low luminosity active galactic nuclei (AGNs). In contrast to protoplanetary disks, the "radial drift barrier" does not prevent the formation of planetesimals. In the early phase of the evolution, low collision velocity between dust particles promotes sticking; therefore, the internal density of the dust aggregates decreases with growth. When the porous aggregate's size reaches 0.1--1 cm, the collisional compression becomes effective, and the decrease in internal density stops. Once 10--100 m sized aggregates are formed, they are decoupled from gas turbulence, and the aggregate layer becomes gravitationally unstable, leading to the formation of planets by the fragmentation of the layer, with ten times the mass of the earth. The growth time scale depends on the turbulent strength of the circumnuclear disk and the black hole mass $M_{BH}$, and it is comparable to the AGN's lifetime ($\sim 10^8$ yr) for low mass ($M_{BH} \sim 10^6 M_\odot$) SMBHs.