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

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 $\sim 20M_E$ to $3000M_E$ in $r<4$ pc for $\alpha =0.02$ ($M_E$ is the Earth mass), which is in contrast with $4M_E-6M_E$ 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).

17 pages, 7 figures, accepted by ApJ

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• On planet formation around supermassive black holes and the grain disruption barriers by radiative torques
• The impact of classical and General Relativistic obliquity precessions on the habitability of circumstellar neutron stars' planets