Astronomers from New Mexico State University, Nicolaus Copernicus Astronomical Center and CUNY Borough of Manhattan Community College propose that the dust-rich tori surrounding active galactic nuclei (AGN) — the energetic cores of galaxies powered by feeding supermassive black holes — behave much like the protoplanetary disks that spawn planets around young stars.
Mishra et al. suggest that AGN dust tori host the largest populations of planets in the Universe. Image credit: ESO.
“Planets form within disks of gas and dust surrounding young stars,” said New Mexico State University astrophysicist Wladimir Lyra and his colleagues.
“However, an additional and compelling environment for planet formation has emerged: the disks surrounding AGN.”
“In recent years, significant analogies have been drawn between the physical processes within circumstellar disks and within AGN disks.”
“These analogies suggest that mechanisms traditionally associated with planet formation may also operate in the more extreme environments of disks around supermassive black holes.”
Using computer modeling of a strongly magnetized AGN disk, the researchers found that dust grains a few nanometers to a fraction of a millimeter across, drifting in from the interstellar medium, can coagulate and trigger a process called the streaming instability.
This mechanism concentrates dust into dense filaments that collapse under their own gravity, seeding objects ranging from Earth-mass bodies to super-Jupiters, and, in some cases, objects at or beyond the hydrogen-burning limit, the threshold at which an object is massive enough to become a star.
The scientists estimate that such disks could host on the order of tens of millions of planetary-mass objects, with pebble accretion and gas accretion driving continued growth over the roughly 1-to-10-million-year lifetime of an AGN episode.
“We’re finding objects that are a thousand times the mass of the Earth, but built of pure dust,” Dr. Lyra said.
“And not only that, but also some of these objects are approaching the mass of the Sun.”
“We came up with the hypothesis that low mass black holes orbiting the disk around supermassive black holes would behave just like planetary embryos, protoplanets behave around the Sun.”
“They would migrate, they would change their orbit, they would collide with other protoplanets and make bigger things.”
“That’s a completely different way to form heavy black holes than anything else in the Universe.”
“We’ve worked out this idea into a full-fledged theory. It’s known as the ‘AGN channel’ and it has accumulated compelling observational evidence.”
