New research reveals that radiation from active galactic cores can strengthen the protective ozone layer in oxygen-rich atmospheres and protect life from extinction.
Black holes may not be as destructive to life as we previously thought. A surprising new study reveals that the powerful radiation from active galactic nuclei (AGN) – supermassive black holes in a particularly energetic phase – may actually help protect life on nearby planets.
This radiation creates a chain reaction that leads to the formation of ozone in oxygen-rich atmospheres, thus providing protection from harmful radiation and creating a feedback loop that makes life more resilient. The project was born from a chance encounter on a cruise ship, advanced simulations and a collaboration between astrophysicists from Dartmouth College and the University of Exeter.
Black Holes and Galactic Radiation At the heart of most large galaxies, including our own Milky Way, is a supermassive black hole. Interstellar gas drawn into the black hole's orbit powers it as an active galactic nucleus (AGN), sending high-energy radiation throughout the galaxy.
This is not an environment we expect life to inhabit, but a surprising new study published in the Astrophysical Journal reveals that AGN radiation can actually have surprisingly positive effects. Rather than leading to species extinction, it may contribute to their success.
Simulations of Effects on Life The study is perhaps the first to concretely measure, using computer simulations, how UV radiation from an active galactic core can change a planet's atmosphere in a way that supports or harms life. Similar to studies that have examined the effects of solar radiation, the researchers found that the benefit or harm depends on the planet's distance from the radiation source and whether life has already established itself on it.
"Once life is established and has created oxygen in the atmosphere, radiation becomes less destructive and may even be beneficial," says Kendall Sippy, the study's lead author. "After you cross that threshold, the planet becomes more resistant to UV radiation and more protected from events that could cause extinction."
How does UV light increase ozone? The researchers modeled the effects of AGN radiation not only on Earth, but also on Earth-like planets with different atmospheric compositions. When oxygen was already present, the radiation triggered chemical reactions that led to the thickening of the planet's protective ozone layer. The richer the atmosphere is in oxygen, the more positive the effect.
High-energy radiation breaks oxygen molecules into individual atoms, which recombine to form ozone (O3). As ozone builds up in the upper atmosphere, it reflects dangerous radiation back into space. A similar process occurred on Earth about two billion years ago, thanks to the first bacteria that emitted oxygen.
Clues from Earth's history Solar radiation helped early life on Earth create oxygen and ozone in the atmosphere. According to the Gaia hypothesis, these ozone recyclers allowed complex life to evolve.
"If life is able to rapidly oxygenate the atmosphere, ozone could regulate the conditions needed for life to grow," says Jake Iger-Nash, one of the study's authors.
What would happen if Earth were closer to a black hole? Earth is too far from the black hole at the center of the Milky Way, Sagittarius A*, but researchers have looked at what would happen if Earth were much closer. In an atmosphere without oxygen, as in the Archean period, life would have been nearly impossible. But at current oxygen levels, the ozone layer would have strengthened within days, protecting life.
In older, denser galaxies, like NGC 1277, the radiation was lethal. But in massive, sprawling galaxies like M87 or the Milky Way, the effects of AGN are milder.
A chance collaboration on the Queen Mary 2 ship Sippy began studying black holes in Professor Ryan Hickox's lab at Dartmouth, and later met Professor Nathan Main of Exeter during a cruise in England. Main was using a software called PALEO, and together they realized that it could also be applied to strong AGN radiation.
Discovering the feedback loop "The feedback discovered in the oxygenated atmosphere was completely unexpected," Hickox notes. Without the chance encounter, the study would not have taken place.
"This is the kind of insight that can only be gained from a combination of experts from different fields," he adds.
More of the topic in Hayadan:
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Life began in the cosmic soup at the center of the galaxy.
The density of stars in the center of the galaxy is much greater than in our surroundings, and the frequency of supernovae is therefore correspondingly greater. The question is whether an ozone layer as described would provide sufficient protection from extinction events at the statistical frequency of such eruptions near the planet.