Could Planets Orbit Supermassive Black Holes?

James Maynard
Nov 25, 2019 · 3 min read

Normally, when we think of exoplanets, we picture worlds around other stars, not much different then our own Sun. Now, a pair of researchers from Japan propose the idea that some planets may orbit supermassive black holes — how would they form?

Nearly every galaxy houses a supermassive black hole — millions or billions of times more massive than our sun — at their core. Researchers at a pair of research facilities in Japan now suggest that planets may occasionally orbit these behemoth regions of space.

Typically, planets coalesce from discs of gas and dust which form around young stars. However, similar conditions also exist in other places, including around supermassive black holes at the centers of galaxies.

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An artist’s concept of planets orbiting a supermassive black hole, lit up by a highly-energetic black hole. Image credit: Kagoshima University.

“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. Around black holes there might exist planetary systems of astonishing scale,” Eiichiro Kokubo, planetary researcher at the National Astronomical Observatory of Japan.

Light from the Darkness

Astronomers currently know of more than 4,000 confirmed exoplanets orbiting other stars, and thousands more targets are awaiting confirmation. However, technologies used in searches for distant worlds are currently unable to detect any planet orbiting a supermassive black hole.

“With the right conditions, planets could be formed even in harsh environments, such as around a black hole,” states Keiichi Wada, a professor at Kagoshima University researching active galactic nuclei. These galactic cores shine with energy provided by black holes.

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An artist’s concept of a supermassive black hole, based on an image of the Pinwheel Galaxy (M 101) obtained by the Hubble Space Telescope. Image credit: NASA/JPL-Caltech

Discs surrounding supermassive black holes can contain as much as 100,000 times as much mass as the Sun. This is more than a billion times as much gas and dust as is typically found in a planetary disc around a typical star.

Energy emanating from the black hole is blocked by the dust, potentially shielding protoplanets from the intense radiation produced just outside these enigmatic objects. Researchers calculate worlds may form in a few hundred million years under these conditions. The amount of time it takes for small particles to coalesce into planets is dependent on the properties of the disc and the mass of the supermassive black hole.

“I have seen the dark universe yawning, Where the black planets roll without aim;
Where they roll in their horror unheeding, without knowledge or lustre or name.”
— H.P. Lovecraft, Nemesis

A rogue supermassive black hole, thrown from its galactic core, has been spotted by the Hubble Space Telescope. Video credit: NASA Goddard

“As a natural consequence of the elementary processes of dust growth, we discovered that a new class of planets can be formed around supermassive black holes (SMBHs). We investigated a growth path from sub-micron sized icy dust monomers to Earth-sized bodies,” researchers wrote in an article published in the Astrophysical Journal describing their study.

Although astronomers cannot currently detect these planets, future instruments could open up new worlds — literally — as astronomers begin the search for planets orbiting supermassive black holes.

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