Planet Formation around Supermassive Black Holes in the Active Galactic Nuclei

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 Earthsized 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 (∼ 10 8 yr) for low mass (M BH ∼ 10 6 M ) SMBHs.

The Astrobiology of Alien Worlds: Known and Unknown Forms of Life

“…Lineweaver, Fenner and Gibson [123] claimed that life would be extremely unlikely near a galactic center, based on the low metallicity and the high-energy radiation outbursts at that proximity to a black hole, though life may be protected from that radiation if it is far enough below the surface. Others [124][125][126] have argued that these planets may not necessarily be uninhabitable. However, Schnittman [127] suggested that the blueshift of light would make life near a black hole very challenging because incoming light would be amplified due to the relativistic effects to much higher ultraviolet frequencies.…”

The Habitability of the Galactic Bulge

“…The center of the Milky Way hosts an SMBH (called Sgr A*), that is currently in a quiescent phase. Simulations show [47] that some planets might exist around it, orbiting 10 to 30 light-years from the nucleus, thus at a safe distance from the strong gravitational field and tidal forces. Standard exoplanet observing techniques are not applicable for hunting these worlds.…”

“…According to the unified model of AGNs [ 45 , 46 ], gas and dust are found as a geometrically and optically thick torus, obscuring the broad emission line (width of several 1000 km s ) region around the central accretion disk. It has been suggested that this disk can be the site of planetary formation [ 47 ] around low luminosity Seyfert-type AGNs rather than in quasar-type high luminosity ones with massive SMBHs. These planets would typically be of masses ten times larger than Earth.…”

The impact of classical and General Relativistic obliquity precessions on the habitability of circumstellar neutron stars' planets

“…It should be recalled that the existence of a large number of neutron stars in the neighborhood of, e.g., the SMBH in Sgr A * at the Galactic Center, was hypothesized long ago (Morris & Serabyn 1996;Genzel, Eisenhauer & Gillessen 2010;Kim & Davies 2018), and their search is currently actively pursued (Cordes & Lazio 1997;Cushey, Majid & Prince 2017;Rajwade, Lorimer & Anderson 2017;Bower et al 2019). Moreover, it was recently shown that a large number of (starless) potentially habitable Earth-sized planets, dubbed "blanets" by their proponents, can form in SMBHs' accretion disks (Wada, Tsukamoto & Kokubo 2019, 2021b.…”