Supermassive Black Holes May Act as Universal Planet Factories

Tens of millions of planetary-mass objects could be forming right now in the centers of active galaxies. While we typically think of star formation as the top-down collapse of massive gas clouds, the environments surrounding supermassive black holes appear to favor a completely different, bottom-up mechanism.

The efficiency of dust agglomeration

Stellar-mass black holes are likely too hostile for stable planetary systems. The supernova events that create them often fling away the very matter needed to sustain planets, and the resulting accretion disks produce high-energy x-rays and thermal radiation capable of stripping atmospheres or even frying worlds from a distance.

Supermassive black holes, however, present a much more fertile landscape. A galaxy's central black hole is often surrounded by a colossal accretion disk and a doughnut-shaped cloud of cold, dark dust known as a torus. Astronomers investigating these structures have found that the outer regions of these disks—hundreds of billions of kilometers away from the event horizon—provide the perfect raw material for planetary growth. In these cooler zones, dust particles can glom together, aggregating from minuscule grains into pebbles, then boulders, and eventually planetary-mass objects.

A spectrum of rocky worlds and massive stars

This cumulative growth process is remarkably efficient and produces a staggering range of results. According to research recently accepted for publication in the Astrophysical Journal, this mechanism can generate objects spanning the entire cosmic hierarchy: from Earth-mass planets to full-fledged stars with 300 times the mass of our sun.

These worlds would be fundamentally different from anything in our solar system. Because the gas and dust in the outer disk are decoupled and noninteracting, these objects would form almost entirely from dust with little to no gas content. Even a gas giant like Jupiter would, by definition, be a "terrestrial" world—composed almost entirely of rock.

If these findings hold, the bustling cores of galaxies may actually be the most fecund places in the universe for planet formation. This discovery shifts our understanding of galactic evolution, suggesting that the centers of galaxies are not just gravitational anchors, but massive, bottom-up manufacturing hubs for rocky worlds and new stars alike.

Source: Planets aplenty may lurk around supermassive black holes
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