Could life exist around black holes?
They warp time and space, but could they also nurture life?
From the coldest to the hottest regions, life on Earth has filled every conceivable ecological niche. It’s very likely life has evolved elsewhere, though despite our best efforts we haven’t seen any signs of it yet. What is truly amazing is that scientists have speculated that life could exist on planets in the neighbourhood of black holes.
Black holes are the most extreme and aweinspiring objects in the universe. At the centre of virtually every galaxy, including our own Milky Way, resides a menacing black hole that greedily consumes everything that falls into it – from vast dust clouds and planets to whole solar systems.
In the early evolution of galaxy formation, stellar material spinning around black holes formed flattened accretion discs billions of times the mass of our Sun, and great jets of gas fired out from them. These jets are like powerful beacons, 1,000 times brighter than our Milky Way, and are known as quasars. Much of the gas that fuelled quasars has run out in mature galaxies, but quasars can still be observed in very distant young galaxies.
The black hole itself is formed when a massive star has exhausted all its hydrogen and helium gases, causing it to violently explode and triggering its inner core to collapse in on itself. The outermost region of the black hole is the event horizon, and beyond that point the escape velocity to get out of the hole’s gravitational pull is greater than the speed of light. Therefore nothing can escape this point of infinite density, known as the singularity.
If you do fall into a black hole it is a gruesome experience, as the intense gravitational field of the singularity will pull you into a long, thin strand. This process is rather aptly called spaghettification. Any object can be spaghettified, including entire stars.
The chance of a life-supporting black hole planet seems impossible, but scientists are never one to shirk a challenge. A big boost to such speculation was the fictional Miller’s planet in Christopher Nolan’s 2014 film Interstellar. This was mainly because it used advice from theoretical physicist Dr Kip Thorne about the properties of wormholes and black holes. Although the science was praised, it was thought a planet so close to a black hole, with the time-dilation effects shown, would easily be blasted with lethal radiation and would be in serious danger of being drawn into the hole.
A group of astrophysicists led by Pavel Bakala of the Silesian University in Opava decided to take another look at this problem. Considering the thermodynamics of the situation, they postulated that the black hole would act as a heat sink for unusable waste heat – equivalent to the cold of space surrounding Earth. The usable energy would come from the cosmic microwave background (CMB) rather than a star like our Sun.
The CMB as a source of energy does not sound very promising, as it is relatively weak, but a supermassive black hole could compress and funnel it into optical wavelengths. Its narrow beam of light would look like a bright star near the shadow of the black hole to an observer on a nearby planet.
Four years after putting forward that idea, the team refined it to account for the weakness of the CMB. This could be catered for if the planet is in an orbit close to the black hole, the danger of course being that it might be drawn into its singularity. To avoid this they calculated that a planet would be able to be in a close, stable orbit with strong enough CMB light if the black hole spun at a 100 millionth less than the speed of light.
The black hole in our Milky Way, although having a mass around 4 million times larger than the Sun, would be too small, and would rip apart any nearby planet. For a planet to evade that fate it would have to orbit a supermassive black hole that only rips up planets when they go beyond its event horizon. Such black holes would have to have a mass of at least 163 million times that of our Sun.
Another consideration is that the black hole would need to be in a fairly old galaxy, with plenty of empty space around it. Otherwise any matter in the area that is sucked into the black hole would blast out deadly radiation as it spirals to certain doom inside the singularity.
Instead of the weak CMB, the energy from the accretion disc surrounding a black hole could be an alternative. Jeremy Schnittman, a research astrophysicist at NASA’s Goddard Space Flight Center, who was also inspired by Interstellar, has considered that the bright, hot gas from the