Formation of "Blanets" from Dust Grains around the Supermassive Black Holes in Galaxies

TL;DR

This paper explores the formation of 'blanets', a new class of planets around supermassive black holes, focusing on dust coagulation, physical conditions, and the impact of radial advection, suggesting they can form in low-luminosity AGNs within 100 million years.

Contribution

It provides a detailed analysis of dust coagulation and physical conditions for blanet formation, incorporating radial advection effects, which was not previously considered.

Findings

Blanets can form within 70-80 million years at several parsecs from SMBHs.

Blanet masses range from 20 to 3000 Earth masses depending on conditions.

Radial advection significantly increases the potential mass of blanets.

Abstract

In Wada, Tsukamoto, and Kokubo (2019), we proposed for the first time that a new class of planets, "blanets" (i.e., black hole planets), can be formed around supermassive black holes (SMBHs) in the galactic center. Here, we investigate the dust coagulation processes and physical conditions of the blanet formation outside the snow line ($r_{snow} \sim $ several parsecs) in more detail, especially considering the effect of the radial advection of the dust aggregates. We found that a dimensionless parameter $\alpha = v_t^2/c_s^2$, where $v_t$ is the turbulent velocity and $c_s$ is the sound velocity, describing the turbulent viscosity should be smaller than 0.04 in the circumnuclear disk, to prevent the destruction of the aggregates due to collisions. The formation timescale of blanets $\tau_{GI}$ at $r_{snow}$ is, $\tau_{GI} \simeq$ 70-80 Myr for $\alpha = 0.01-0.04$ and $M_{BH} = 10^6 M_\odot$. The mass of the blanets ranges from $\sim 20 M_E$ to $3000 M_E$ in $r < 4$ pc for $\alpha = 0.02$ ($M_E$ is the Earth mass), which is in contrast with $ 4 M_E-6 M_E$ for the case without the radial advection. Our results suggest that \textit{blanets} could be formed around relatively low-luminosity AGNs ($L_{bol} \sim 10^{42}$ erg s$^{-1}$) during their lifetime ($\lesssim 10^8$ yr).