New Calculations Suggest Large Planets Can Form Around Supermassive Black Holes

Artist's impression of planets around an active supermassive black hole. Kagoshima University

Over the last 25 years, we have discovered planets in the most peculiar and extreme environments, posing the question: What conditions are too extreme for a planet to form? Now, new calculations propose that even supermassive black holes occasionally have the right conditions. 

As reported in The Astrophysical Journal, Japanese researchers have simulated the range of potential conditions around an active (but not too active) supermassive black hole. These black holes are surrounded by huge donut-shaped clouds of dust and gas.

Away from the supermassive black hole, where the temperature is low enough, the dust in this ring can start accumulating ice crystals. And these icy dust-grains can merge with each other. The “fluffy” crumbs grow and grow and when they become the size of a building, gravity takes over and more and more material coalesces on them to form fully-fledged planets.

"Our calculations show that tens of thousands of planets with 10 times the mass of the Earth could be formed around 10 light-years from a black hole," co-author Eiichiro Kokubo, a professor at the National Astronomical Observatory of Japan, said in a statement. “Around black holes there might exist planetary systems of astonishing scale.”

The time scale for the formation of these planets depends on how chaotic the movements in the disk are. The team expects them to grow over hundreds of millions of years, which is comparable to the amount of time a supermassive black hole similar in size to the one at the center of the Milky Way might remain active.

"With the right conditions, planets could be formed even in harsh environments, such as around a black hole," stated first author Keiichi Wada, a professor at Kagoshima University. 

Just because something is possible doesn’t mean that it is probable. But given that almost every galaxy in the universe has a supermassive black hole at its core (and some more than one), planets orbiting supermassive black holes might certainly be out there.

The researchers rightly point out the usual techniques we use to detect exoplanets are hopeless for finding such objects. Indirect techniques based on changes in the black hole's emission would have to be employed but our tech might have to improve significantly before this can be achieved.  

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