The idea of black holes as cosmic destroyers is a captivating yet oversimplified view. While they do indeed devour matter, the reality around supermassive black holes (SMBHs) is far more intricate and fascinating. Recent research reveals that under specific conditions, accretion disks surrounding these SMBHs can give birth to giant planets, challenging our conventional understanding of planetary formation.
The study, published in The Astrophysical Journal, delves into the unique environment of outer AGN disks, where temperatures are similar to those of circumstellar disks, allowing dust condensation. This phenomenon opens up the possibility of planet formation and growth through mechanisms akin to those in circumstellar disks. The lead author, Wladimir Lyra, and their team propose that these disks can foster the creation of giant planets, with masses exceeding that of Jupiter, up to and beyond the hydrogen-burning limit.
The mechanism behind this planetary formation is streaming instability, a process where solid matter, in the form of dust and pebbles, coagulates into planetesimals. This occurs when gas in the disk drags on these particles, causing them to spiral inward, but in the absence of a star, the SMBH takes center stage. The authors highlight that the dust grain sizes required for streaming instability are easily attained through coagulation, leading to the formation of solar masses of dust filaments, which then collapse into tens of millions of 'planetesimals' ranging from Earth to super-Jupiter masses.
One intriguing aspect of these exoplanets is their composition. Unlike planets in protoplanetary disks, these dust planets are not differentiated and are made solely of accumulated dust. The researchers predict a population of exotic objects directly formed above the hydrogen-burning limit, yet of pure dust. They suggest that these objects might have degenerate cores, heated by the radioactive decay of short-lived radionuclides, resulting in a magma ocean with an outgassed atmosphere, earning them the nickname 'degenerate lava drops'.
The potential for these dust planets to evolve is significant. They could eventually transition into stars or even black holes under the right conditions. The authors propose that massive seed planets in the AGN disk can accrete enough material to exceed thermal and isolation masses, potentially transitioning into stars and eventually black holes. This process could also lead to the formation of elusive intermediate-mass black holes (IMBHs), which are challenging to observe due to their mass and the mass segregation effect in AGN disks.
In conclusion, AGN disks present a compelling case for the growth and formation of various astrophysically interesting objects, from Jupiter-mass planets to stars and stellar- or intermediate-mass black holes. The outer regions of these disks, governed by dust dynamics and efficient accretion mechanisms, bear a resemblance to protostellar disks, albeit on vastly larger scales. This research not only strengthens the theoretical foundation for the existence of millions of Jupiter-mass planets but also suggests a potential channel for IMBH formation, bridging the fields of planet formation and black hole growth.
