This is just an illustration of a supermassive black hole. Real ones could have orbiting planets NASA Goddard

We are used to thinking of planets orbiting stars. But it turns out they might exist around supermassive black holes too.

People have floated the idea of planets orbiting smaller black holes before, but far less is known about supermassive black holes, which are millions of times more massive than our Sun. So Keiichi Wada at the Kagoshima University in Japan and his team applied models of planet formation to them to see what would happen.

“This is the very first study that claims a possibility of ‘direct’ formation of planet-like objects which are not associated with stars, but with supermassive black holes,” says Wada.


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Planet formation is usually thought to begin with a disk of dust and gas around a star. Gradually this material clumps together and its gravity then draws in more material, building a planet.

Wada and his team examined how similar disks known to surround supermassive black holes would behave and showed that roughly the same process could operate. “Basically it is the same as the formation of normal planets around stars,” says Wada.

Time dilation

Because of their huge mass and gravitational pull, supermassive black holes warp space time in strange ways. However, orbiting planets might not feel any strange effects, such as time dilation. Wada says they would probably orbit at huge distance of between 10 and 30 light years, where the extreme effects of general relativity would be “negligible”.

Planetary systems around a supermassive black hole might not be like star systems though. “The total amount of dust is enormous,” says Wada. This means the typical mass of planets would probably be large – about 10 times more massive than Earth – and there could be as many as 10,000 of them around a single black hole, says Wada.

Sean Raymond at the University of Bordeaux says the team’s logic is plausible. But he says it might be possible for planets to form closer to a supermassive black hole and to exist in even higher numbers. “It is theoretically possible for millions of planets to orbit a supermassive black hole, but it requires a lot of things to be just perfect,” he says.

Direct detection of such planets would be difficult because of the vast distances involved. But it might be possible to get indirect evidence of them, perhaps by using infrared astronomy to see the protoplanetary disk.

Reference: arxiv.org/1909.06748