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An exoplanet orbiting a supermassive black hole is probably one of the most inhospitable places for life that you could possibly think of.
Assuming the planet doesn't fall in, who — or what — could survive in such a place? There is possibly a (hypothetical) exception. Physicist Pavel Bakala, of the Hlohovec Observatory and Planetarium in the Czech Republic, and colleagues have figured out a way that life could flourish on an exoplanet orbiting just on the edge of a black hole’s event horizon. Never mind that the event horizon is the point of no return. If this is a supermassive black hole of the right size, a planet could somehow host life that survives even on the fringe of extreme cosmic hostility. Even Interstellar proves this in a way.
“The basic condition for the existence of imaginable biological forms is a sufficient supply of energy, and therefore a reasonable surface temperature of the exoplanet,” Bakala and his team said in a study recently published in The Astrophysical Journal.
Remember when the crew of the Endurance in Interstellar touched down on an exoplanet dangerously close to the supermassive black hole Gargantua? The movie had inspired a related study Bakala was involved in. Gargantua was immense and rotated extremely fast, with Miller’s planet orbiting just about as close to the event horizon it could get without being shredded by intense gravitational forces and lost to time and space (though some scientists would disagree about that). The planet’s precarious orbit and zooming around close to the speed of light kept it safe from Gargantua’s gaping maw.
That still doesn’t answer the question of how life could survive there if any extraterrestrial life-forms had (hypothetically, again) appeared in the movie. Enter the cosmic microwave background.
The cosmic microwave background (CMB) is made up of the remnants of electromagnetic radiation from the Big Bang. It is also the most powerful radiation source in the entire universe. Because most of it lurks in the microwave region of the electromagnetic spectrum, the CMB is invisible to us because microwaves are far past the realm of visible light since they are too long for the human eye to process.
“The [CMB] heating regime is very similar to the regime of the planet heated by a standard star, when almost all of the incoming energy can be converted to useful work and thus drive the life processes,” Bakala explained.
The usually freezing CMB could actually heat a planet orbiting a supermassive black hole, like a substitute for the Sun, while the shadow of the black hole would act as a heat sink, absorbing excess heat like the night sky does for Earth. CMB light gains energy as it falls into a black hole. Approaching the event horizon, it can gain so much energy that it becomes blindingly bright and unbearably hot. The superfast spin of the black hole (which would have to be at least 37 times the mass of our own supermassive black hole, Sagittarius A*) would focus this light into a narrow beam that would appear like a faraway orb as the Sun does.
Now you know why the impossibly fast rotation of Gargantua gives this theory a surprisingly accurate sci-fi example to go off of.
CMB heating also explains why there is such a narrow habitable zone within which a planet can orbit. Too far back, and temperatures will plummet. Too close to the black hole, and you can probably guess what happens. Bakala’s research team used Earth, Mars and Venus as a mirror. Mars is a frozen desert and Venus a toxic inferno. Earth somehow got it right. Life as we know it needs liquid water to survive, so the habitable zone needs to be that area in between where water stays in its liquid state. It would vaporize into plasma in excessive heat, and solidify into ice in excessive cold, neither of which create ideal living conditions, even for a tardigrade.
“Exoplanets orbiting around the rotating black holes can be heated by a CMB amplified in a strong gravitational field,” Bakala said, adding that the resulting light and heat “will therefore play a role analogous to the [Sun].”
Aliens could still be living it up breathing methane or crawling around on ice for all we know. At least life on our own planet, except maybe a few actual freaks of nature, gives us an idea of what to look for, even if we find it perilously close to a supermassive black hole.