A team of scientists is astounded to have discovered that bright and turbulent regions of galaxies — called active galactic nuclei, which are powered by feeding supermassive black hole engines — could be the birthplace of millions of planets. And these regions are brilliant. They often outshine the combined light of every star in their wider home galaxy.
Active galactic nuclei (AGNs) occur when supermassive black holes are surrounded by vast amounts of gas and dust that swirl around them in flattened, platter-shaped clouds called accretion disks. These accretion disks gradually feed some matter to the black hole. Meanwhile, other matter is channeled to the poles of the black hole, from where it is blasted away as high-energy plasma jets travelling at near-light speeds. The immense gravity of the central supermassive black holes, which have masses of millions or even billions of times that of the sun, generates intense friction in the gas and dust within accretion disks, causing them to glow brightly across the electromagnetic spectrum.
The discovery is so surprising because even though AGNs are rich with gas and dust — the building blocks of planets — the turbulent conditions within the disks wouldn’t generally be considered ideal for forming planets. However, the edges of these disks may have temperatures and conditions akin to the planet-forming protoplanetary disks found around infant stars. Over time, could enough dust clump together and grow into planets?
To investigate this possibility, these scientists created a computer model of a supermassive black hole and its accretion disk and added data about the conditions at the edges of these disks. They then observed how rapidly dust clumped together and how the budding planets grew over millions of years.
“We discovered millions of Jupiter-mass planets could form at a distance of tens of parsecs [one parsec is around 3.3 light-years] from supermassive black holes, which are also AGNs,” team member and University of Colorado Boulder researcher Bhupendra Mishra told Space.com. “These are dust giants exceeding Jupiter‘s mass. They will look like lava balls.”
Mishra added that because the disk around an AGN supermassive black hole is more gas-rich compared to those that would exist around a star like the sun during its infancy, the potential of planet formation is enhanced from a few possible worlds around stars to maybe millions of planets around a supermassive black hole. He explains that the underlying mechanism of planet formation around supermassive black holes would be a phenomenon called “streaming instability” that allows multiple large filaments of dust to form. These are the birthplaces of vast amounts of planets. That eventually leads to millions of planets lurking in the outskirts of an AGN disk.
However, such planets may fly the nest quite quickly. The team’s estimate confirms that these are stable planets — but while these planets will survive, they will likely migrate radially away from the supermassive black hole and the edge of the AGN.
“We were astonished! This has not been found in AGN disk context before using a streaming instability model,” Mishra said. “My colleague Wladimir Lyra, an astronomy professor at New Mexico State University (NMSU), is world-renowned in the field of planet formation, and we both were totally amazed when we noticed this mass and size range of planet formation.”
Mishra added that the outskirts of AGN disks are not very well understood, meaning the team’s findings could help develop a much clearer picture of the hearts of active galaxies. Of course, it is early days for the team’s theory, and the detection of planets around supermassive black holes would be a helpful confirmation of the team’s conclusion. A useful tool in this investigation would be the curvature and the amplification of light from a background object that happens when a massive foreground object sits between it and Earth, a phenomenon known as gravitational lensing.
“Gravitational lensing could help to identify the cluster of these planets in the outskirts of the AGN disk. However, finding such an AGN is not easy unless we get lucky,” Mishra concluded. “I believe we could detect these planets, but we have to study this model further.”
A preprint version of the team’s research is available on the paper repository site arXiv.