At the heart of most large galaxies, including our own Milky Way, lurks a supermassive black hole. These cosmic giants periodically consume interstellar gas, switching into active galactic nucleus (AGN) mode and releasing massive amounts of high-energy radiation.
The common assumption? That this radiation would obliterate any chance for life. But a surprising new study suggests otherwise.
Black Hole Radiation: A Paradoxical Source Of Life
A study published in The Astrophysical Journal by researchers from Dartmouth and the University of Exeter challenges the idea that black hole radiation is purely destructive. Their computer simulations reveal that, under the right conditions, AGN radiation could actually nurture life rather than extinguish it.
According to Kendall Sippy, lead author of the study, the key factor is whether life has already gained a foothold on a planet.” Once life exists, and has oxygenated the atmosphere, the radiation becomes less devastating and possibly even a good thing,”explains Sippy. “Once that bridge is crossed, the planet becomes more resilient to UV radiation and protected from potential extinction events.”
The Protective Role Of Ozone Against Black Hole Radiation
The researchers simulated AGN radiation effects on Earth and Earth-like planets with different atmospheric compositions. They found that when oxygen is already present, radiation triggers chemical reactions that build up the ozone layer, which in turn shields the surface from harmful ultraviolet rays.
This phenomenon mirrors what happened on Earth nearly two billion years ago, when primitive microbes began oxygenating the atmosphere.
“If life can quickly oxygenate a planet’s atmosphere, ozone can help regulate the atmosphere to favor the conditions life needs to grow,” says Jake Eager-Nash, co-author of the study. “Without a climate-regulating feedback mechanism, life may die out fast.”
Could Earth Survive Near An Active Black Hole?
Earth is too far from Sagittarius A*—the supermassive black hole at the center of the Milky Way—to experience AGN radiation effects. However, the researchers wanted to see what would happen if Earth were much closer to an active AGN, exposing it to billions of times more radiation.
Their simulations revealed that in Earth’s early oxygen-free atmosphere, AGN radiation would have completely prevented life from forming. However, as oxygen levels approached modern concentrations, the ozone layer would develop rapidly, shielding the planet within days.
“With modern oxygen levels, this would take a few days, which would hopefully mean that life could survive,” says Eager-Nash.
Not All Galaxies Are Habitable Under Agn Influence
Not all galaxies would offer the same protective benefits. The study looked at “red nugget relic” galaxies like NGC 1277, where stars cluster tightly around their central black holes. In such environments, the researchers found that AGN radiation would be lethal for any potential life.
By contrast, in larger spiral galaxies like the Milky Way or massive elliptical galaxies such as Messier 87, stars are spread out more, reducing the impact of AGN radiation on potentially habitable planets.
A chance encounter that led to a groundbreaking study
The research began when Ryan Hickox, professor at Dartmouth, was on sabbatical and booked a trip aboard the Queen Mary 2. On the ship, he met Nathan Mayne, an astrophysicist from Exeter, who was giving a guest lecture.
Their discussion sparked the idea to use PALEO, a software developed to model solar radiation on exoplanet atmospheres, to analyze the much stronger radiation from AGNs.
