Planets aplenty may lurk around supermassive black holes

Planets might exist in the least likely place you’d imagine—around the outskirts of supermassive black holes

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With about 6,300 exoplanets discovered so far and more than 10,000 candidates awaiting confirmation , it’s easy to forget the ones that started it all: the very first exoplanets discovered and announced in 1992 . These worlds were not orbiting stars like the sun. Instead they circled a pulsar, the dead remains of an exploded star. This is one of the last places in the universe astronomers expected to find planets—a pulsar is the remnant from a supernova, after all—and it’s still unclear how these worlds formed.

Clearly nature excels at making planets, even under extremely hostile conditions. Just how “hostile” those conditions can be, however, no one knows yet. Planets like those in our solar system form from whirling disks of gas and dust around baby stars, but disks are common around another kind of astrophysical object, too: black holes. Could planets be born there as well or at least emigrate from elsewhere and survive in their new neighborhood?

The answer, astonishingly, is “maybe,” though it depends on what kind of black hole we’re talking about.

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For stellar-mass black holes—which, like pulsars, are forged in the collapsing core of an exploding massive star—there are some caveats. For example, when a massive star goes supernova, a majority of its matter is flung out into space. If enough matter is lost, the star’s gravity will be sufficiently weakened so that it can’t hold on to any extant planets, and they, too, will be lost to space. If any planets do survive the catastrophic explosion—or somehow form afterward from leftover debris—they can still be torn apart by tidal forces and gulped down by the black hole if they wander too close.

If material is falling into the black hole—perhaps siphoned off a companion star that orbits perilously close—the material forms an accretion disk: a flattened pancake of material whirling around the black hole. Friction heats the disk to ridiculously high temperatures that can fry planets from a substantial distance. Even worlds that are too far-off to broil could still suffer from having their atmospheres stripped away by the accretion disk’s copious emission of x-rays and other high-energy radiation.