Most scientists believe that supermassive black holes hinder the formation of stars and planets by pulling from space the matter from which they could form. However, a recent study suggests that the opposite may be true.
Dusty Worlds in the Accretion Disks of Black Holes
A common myth about black holes is that they act like giant cosmic vacuum cleaners, sucking in everything around them. However, research by Wladimir Lyra has revealed a new mechanism around supermassive black holes that resembles a cosmic nursery where planets more massive than Jupiter are born. This was reported by Phys.org.
Lyra, an associate professor of astronomy at New Mexico State University, published his paper, “Active Galactic Nuclei Tori: A Potential Birthplace for Millions of Planets,” together with Bhupendra Mishra, who currently works at the Santa Fe Preparatory School.
Astronomers have hypothesized that stellar-mass black holes, moving in a gas disk around a supermassive black hole, can migrate under the influence of the disk material, change their orbits and approach each other more often. Such approaches can end in mergers, which result in the formation of increasingly massive black holes. This scenario became the basis of the so-called “active galactic nucleus channel” — one of the possible mechanisms for the hierarchical growth of black holes. However, this is still a theoretical scenario..
Lyra’s team found that conditions in the outer regions of accretion disks around supermassive black holes may resemble those observed in accretion disks around young stars. Like protoplanetary disks, disks around supermassive black holes contain objects with masses comparable to those of planets and composed entirely of dust.
Using a computer model, Lyra’s team simulated conditions in the outer regions of these disks and determined how dust condensed into clumps and how the emerging planets grew over millions of years.
Turning Giant Planets into Stars
“The biggest surprise was the number of planets and their sizes — how enormous and massive they can become over the lifetime of active galactic nuclei,” Mishra said.
Computer simulations show that the outer regions of the accretion disk of an active galactic nucleus (AGN) may have conditions favorable for the formation of large dust bodies. The energy released by matter as it falls into a supermassive black hole heats the gas and dust in the disk, and its structure and temperature determine where such objects can form and grow.
According to calculations, their mass in some cases can significantly exceed the mass of ordinary planets and approach the masses of brown dwarfs or even stars. If such a body accumulates enough matter, it can theoretically turn into a brown dwarf or star, launching the corresponding thermonuclear reactions.
“This is a mechanism of star formation that we have discovered for the first time,” Lyra said. “Usually, stars form through what is called gravitational collapse. It is a top-down process: first there is gas, a large cloud of gas forms, and it becomes too dense, so it collapses under its own weight. Usually, everything begins with something large that collapses to form a star. Our mechanism works in the opposite direction. Formation occurs from the bottom up. First the building blocks form, then gas joins them, and suddenly — boom! — a star forms.”
Supermassive Black Holes and Their Possible Detection by LISA
Lyra’s team believes that such an environment would be ideal for the formation of large stars that could collapse into black holes. Over time, these black holes could eventually merge, and form supermassive black holes with masses several hundred times greater than that of the Sun.
“These black holes are simply gigantic,” Mishra said. “They are hundreds or thousands of times larger than the Sun, and if they begin moving toward the center, they will also generate a signal that will probably be detected by LISA (Laser Interferometer Space Antenna) — it will be a gravitational-wave signal.”
LISA is a future space observatory that the European Space Agency plans to launch in the mid-2030s. It will consist of three identical spacecraft moving in the form of an equilateral triangle and exchanging laser beams with one another. LISA is designed to detect and measure gravitational waves — invisible ripples in the fabric of spacetime caused by large-scale cosmic events.
Microlensing as a Method for Testing the Theory
Astronomers want to test their theory using the phenomenon of microlensing. It occurs when a massive, often invisible object acts like a cosmic magnifying glass, increasing the brightness of a background star and thereby making it possible to test for the existence of newly predicted theoretical objects.
In this case, by predicting a unique, observable “brightness pattern” — a light curve — corresponding to a particular theoretical object, scientists can search astronomical data for the matching “fingerprint” and thereby prove their theory.
“Einstein believed that this would never be observed because the ‘fingerprint’ was too weak, but his theory gave rise to an entirely new field,” Lyra said. “This is one of the main techniques we use to search for exoplanets. There is now a billion-dollar NASA mission — the Nancy Grace Roman Space Telescope — that will use this technique to map an entire population of exoplanets.”
The researchers suggest that this effect could also be used to test the theory. If planet-like objects are indeed forming in the disk of an active galactic nucleus (AGN), then some of them might occasionally find themselves close to the line of sight between Earth and the bright central region of the AGN. Their gravity would then act as a microlens, temporarily changing the apparent brightness of the nucleus. The nature of such changes could be compared with the model predictions.
Next Simulations of Supermassive Black Holes
NASA plans to launch the Roman Space Telescope in late August. Its infrared camera has a field of view about 100 times larger than Hubble’s infrared instruments, and the array itself has about 300 megapixels. This will allow Roman to conduct large-scale surveys of the sky and simultaneously detect a large number of faint objects and microlensing events.
But the team is not waiting for that. It is already preparing further computer models. In them, the scientists want to reproduce in more detail the physical conditions near a supermassive black hole: the movement of gas in the accretion disk, its turbulence and the complex structure of magnetic fields. Such calculations will help predict what electromagnetic signals might accompany the merger of black holes in the disk of an active galactic nucleus and whether future observations will be able to detect them.
