Planet Formation around Supermassive Black Holes in the Active Galactic Nuclei

• Published in: The Astrophysical journal 2019-12, Vol.886 (2), p.107
• Main Authors: Wada, Keiichi, Tsukamoto, Yusuke, Kokubo, Eiichiro
• Format: Article
• Language: English
• Subjects: Active galactic nuclei; Aggregates; Astrophysics; Black holes; Cosmic dust; Density; Dust; Dust particles; Exoplanet formation; Interstellar dust; Luminosity; Planet formation; Planetary system formation; Planets; Protoplanetary disks; Protoplanets; Radial drift; Snow line; Space telescopes; Supermassive black holes; Turbulence
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ab4cf0

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 submicron 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 10 times the mass of the Earth. The growth timescale depends on the turbulent strength of the circumnuclear disk and the black hole mass MBH, and it is comparable to the AGN's lifetime (∼108 yr) for low mass (MBH ∼ 106M ) SMBHs.