Black holes are often portrayed in popular culture as star-swallowing chasms into which light and matter disappear forever. They're definitely not somewhere you'd expect to find life. But a new analysis by Tomáš Opatrný of Palacký University in Olomouc, Czech Republic, has found that black holes might be able to provide life-sustaining energy to orbiting planets, in a similar way to how our warm sun provides energy to Earth, reports New Scientist.
It might seem counterintuitive, but a cold sun (such as a black hole) can provide energy in the same way a hot sun can. That's because according to the second law of thermodynamics, energy is created from a temperature difference. On Earth that temperature difference exists between the scorching hot sun and the surrounding cold vacuum of space. But that script can be flipped. The temperature difference between a cold sun and a hot sky can also work.
This dynamic could exist around a black hole that has swallowed up all of the matter that once surrounded it. Since such a black hole would be satiated, its temperature would essentially drop to zero. This would be in contrast to the temperature of the surrounding universe, which sits at about 2.7 kelvin (or about minus 270 degrees Celsius) thanks to the heat leftover from the Big Bang.
Opatrný and his team calculated that a planet orbiting a cold sun like this could extract around 900 watts of useful power from the temperature difference. That's not a lot of energy — it's certainly not enough to power the rise of a civilization — but it could potentially support simpler life forms.
Interestingly, if we rewind the clock on the universe, it's possibel that planets orbiting cold suns with zero temperature might have been even more viable for life. That's because the heat leftover from the Big Bang, or the cosmic microwave background (CMB), would have been much hotter in the early universe. For instance, just 15 million years after the Big Bang, the universe’s background temperature would have been 300 kelvin (or 27 degrees Celsius). That would have been warm enough for a planet orbiting a black hole to host liquid water — but 15 million years is probably not enough time for life to evolve, however.
Though living around a black hole might seem grim, these frigid orbs could one day become the last bastion for life in the universe. In about 100 trillion years, all of the universe's stars are projected to have burned out. Only cold suns will remain.
“When the stars are gone, black holes will be a last-resort source of energy,” said Lawrence Krauss of Arizona State University.