Two prominent astronomers, Harvard’s Avi Loeb and NASA’s Jeremy Schnittman, have proposed that inhabited planets might exist around the black holes harbored at the center of most galaxies similar to the fictional waterworld planet Miller, the first planet in the system orbiting the supermassive black hole Gargantua in the movie Interstellar. It’s even possible that life may form on some of these planets, given that extremeophiles on Earth have adapted to boiling heat, freezing cold, and acidic, highly salty and even radioactive environments.Our own galaxy, the Milky Way, harbors a black hole Sgr A* (Sgr stands for Sagittarius), with an innermost stable circular orbit that’s about the size of the orbit of Mercury. The downside to life near a black hole, writes Loeb, is the heat released by accreting supermassive black holes, posing an existential threat to their civilizations.
In the paper with John Forbes, of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, Loeb showed that a significant fraction of all planets in the universe are vulnerable to their atmospheres being stripped or their oceans being boiled off as a result of having been close to an active galactic nucleus sometime during their lives.
Elsewhere, MIT’s Technology Review reports that Schnittmann asked what kind of energy sources could generate these kinds of temperatures on a planet orbiting a black hole., suggesting that the energy source would have to be entirely different from Earth’s: without a sun, he said, the incoming light would vanish, removing almost all the energy for life on Earth. “Without its constant heat flux, the oceans would likely freeze over in a matter of days.”
“Replacing the sun with an accreting black hole might not be the end of life on Earth”
But it turns out, reports the MIT Technology Review, that there other sources of energy for a planet orbiting a supermassive black hole. ”Most of what we know about black holes comes from observing the electromagnetic radiation coming from gas as it accretes onto the black hole,” says Schnittman. “One could naturally imagine that replacing the sun with an accreting black hole might not be the end of life on Earth after all.”
Supermassive black holes writes Schnittman are the brightest persistent sources of radiation in the universe, particularly in the ultraviolet region where the radiation peaks. They are surrounded by a hot accretion disc of gas that is falling into the black hole that is too extreme to support liquid water, but Schnittman says “they can be made more comfortable by imagining that the accretion rate of the black hole is a tiny fraction of the observed value,” which means that that any planet orbiting close to a supermassive black hole would do so in a cloud of hot gas creating a black-body radiation field that would “hardly hospitable to life.”
But further out from the disk, the gas would be cooler. For it to be room temperature, reports MIT, the planet would have to orbit at distance that is 100 times the gravitational radius of the black hole, where liquid water would be possible. “All known life forms require an energy gradient in order to survive, so an all-pervasive black-body radiation background would probably not be very conducive to complex life,” says Schnittman.
A bigger problem says Schnittman “is that if the accretion rate were lower, the density of the disc would also be lower, making it more difficult to radiate. And without this radiation, the accretion disc would just heat up beyond the temperature of liquid water. So there is a paradox at the heart of this argument that ultimately invalidates it.”
Schnittman calculates that a planet orbiting just beyond the gravitational radius would experience enough heating from the cosmic microwave background to do the trick. “This would be like orbiting a white dwarf at a distance of 0.2 AU,” providing enough energy for liquid water, but would also bathe the planet in dangerous levels of ultraviolet.
Then there is the light from o the density of stars at galaxy centers where the night sky of the alien black hole planet would be 100,000 times brighter than on Earth, providing a significant background of UV light and x-rays. Schnittman imagines, reports Technology Review that a civilization that is sufficiently advanced to construct a sort of “reverse Dyson sphere” that reflects this energy. “This would allow habitability much closer to the host supermassive black hole, even in the face of overwhelming background UV or x-ray radiation.”
Specter of nature’s silent killer
“Yet even with such a protective shield, there is still the specter of nature’s silent killer: neutrinos.” Schnittman says. But, while neutrinos do not interact strongly with matter they could lead to geothermal heating. “And unlike the harmful UV or x-ray flux from this blue-shifted electromagnetic radiation, neutrino heating of the planet’s core could lead to a thriving population of lifeforms similar to those found near deep ocean vents on Earth,” says Schnittman.
Schnittman concludes that gravitational waves of the black hole would provide a steady hum of destructive vibrations that along with dark matter could provide “a rich tapestry of doom.”
Read more at Emerging Technology from the arXiv
The Daily Galaxy, Max Goldberg, via MIT Technology Review and Scientific American
Image credit top of page:The NASA/ESA Hubble Space Telescope image at top of the page shows the bright star-forming ring that surrounds the heart of the barred spiral galaxy NGC 1097, with its supermassive black hole 100 million times the mass of our Sun gradually sucking in the matter around it. The bright ring of stars around it are 5000 light-years across, although the spiral arms of the galaxy extend tens of thousands of light-years beyond it.