Artist’s rendition of a supermassive black hole. Credit: WikiMedia Commons
At the center of most large galaxies sits a supermassive black hole — an object so massive that nothing, not even light, can escape its gravity once it gets too close. When these black holes are actively feeding, pulling in surrounding gas and dust, they become some of the most energetic objects in the known universe, outshining entire galaxies and blasting out powerful jets of radiation. Astronomers call them active galactic nuclei (AGN).
These are most likely the last place you’d expect to find planets forming.
And yet, according to a new study accepted in The Astrophysical Journal, the rings of dust orbiting these cosmic monsters may harbor more planets than anyone ever thought.
“Our approximate model suggests that AGN dust tori host the largest populations of planets in the universe,” wrote the team.
How planets normally form
To understand why this is surprising, it helps to know how planets are thought to form in ordinary circumstances. When a new star is born, it’s surrounded by a rotating disk of gas and dust — leftover material from the cloud that collapsed to form the star. Over millions of years, dust grains in that disk collide and stick together, gradually building up into pebbles, then rocks, then larger bodies, and eventually full-sized planets. Our own solar system formed this way, roughly 4.5 billion years ago.
This process requires a relatively calm, cool environment. Dust grains need to survive long enough to clump together, which means the temperature can’t be too extreme.
AGNs are anything but calm. Surrounding the central black hole is a swirling disk of superheated material spiraling inward, and encircling that at greater distances is a thicker doughnut-shaped ring of gas and dust called the torus. The whole system is flooded with radiation and subject to enormous gravitational forces.
But the researchers noticed something important: the outer parts of the torus, far enough from the black hole, are actually cool enough for dust grains to condense and hold together. The temperatures there are comparable to those found in the planet-forming disks around young stars. Same basic ingredient, very different neighborhood.
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Credit: NASA, ESA, Leah Hustak (STScI)
Clumping on a massive scale
Using a detailed computer model of how material behaves in an AGN torus, the team found that dust grains in these rings can grow through simple collisions and sticking, just as they do around ordinary stars. Once the grains reach a certain size, a phenomenon called streaming instability kicks in where the interaction between the dust and surrounding gas causes the dust to clump into enormous dense filaments.
When those filaments become heavy enough, gravity causes them to collapse. What they collapse into is staggering: the model predicts the formation of tens of millions of planetary building blocks within a single torus, spanning a huge range of sizes — from objects roughly Earth’s mass all the way up to giants several times Jupiter’s mass.
In our solar system, the distinction between a planet and a star is clear-cut. However, in an AGN torus, the model suggests that this distinction can break down entirely.
As a forming planet grows, it typically reaches a point at which it has consumed all the material available to it in its orbit, and growth stops. In an AGN torus, however, that stopping point doesn’t exist in the same way. The reservoir of material is so vast that some objects can just keep growing, eventually accumulating enough mass to trigger nuclear fusion and become stars. The authors describe this as a previously unknown pathway for star formation, one driven not by a collapsing gas cloud but by a growing planet that simply never stopped eating.
Somewhat even more unusually, the model predicts some objects that reach stellar masses but are made entirely of solid dust, with no gas — something that has no known equivalent anywhere in the universe.
There are hundreds of billions of galaxies in the observable universe, and a large fraction of them have hosted active galactic nuclei at some point in their history. If each one of those AGN tori can produce tens of millions of planets, the total number of planets formed this way across cosmic history would be enormous — potentially more than all the planets orbiting ordinary stars combined.
However, it’s also important to note that the study is theoretical, and the authors acknowledge their model involves significant simplifications. Actually observing these planets directly would be extraordinarily difficult given how distant and compact these systems are.
But the findings open up a genuinely new way of thinking about where planets can exist — and just how many of them the universe may have made.