Almost every galaxy has a black hole at its center. The black hole’s immense gravitational pull attracts and consumes surrounding matter and gas. Over time, as they accumulate enough material, they can become an active galactic nucleus (AGN). An AGN emits powerful, high-energy radiation that can spread throughout the galaxy.
A new study by researchers at Dartmouth and the University of Exeter suggests that this radiation could nurture life. Instead of dooming a species to extinction, it could help ensure its survival.
For the study, researchers used computer simulations, for the first time, to measure how ultraviolet radiation from an AGN can change a planet’s atmosphere in ways that could either support or hinder life.
Similar to findings on solar radiation, the study revealed that the impact depends on the planet’s distance from the radiation source and whether life has begun to develop there.
Kendall Sippy, ’24, the study’s lead author, said, “Once life exists and has oxygenated the atmosphere, the radiation becomes less devastating and possibly even a good thing. Once that bridge is crossed, the planet becomes more resilient to UV radiation and protected from potential extinction events.”
The research team studied how radiation affects Earth-like planets with different atmospheres. On planets with oxygen, radiation triggers chemical reactions that increase the protective ozone layer. The more oxygen, the stronger this effect.
Radiation breaks oxygen molecules into single atoms, which then recombine to form ozone (O₃). As the ozone layer grows in the upper atmosphere, it blocks more harmful radiation. A similar process began about two billion years ago when early microbes produced oxygen on Earth.
Changes to ozone levels are heating the planet more than we realize
Solar radiation helped form ozone, creating a shield that allowed life to thrive. This, in turn, led to more oxygen and ozone production, forming a feedback loop that supported complex life. This concept aligns with the Gaia hypothesis, which suggests that Earth’s systems interact in ways that sustain life.
Study co-author Jake Eager-Nash, currently a postdoc at the University of Victoria, said, “If life can quickly oxygenate a planet’s atmosphere, ozone can help regulate the atmosphere to favor the conditions life needs to grow. Without a climate-regulating feedback mechanism, life may die out fast.”
Our Earth is too far from our galaxy’s AGN to feel its effects. Still, researchers were curious to determine what could happen if Earth were much closer to a hypothetical AGN.
To do so, they recreated Earth’s oxygen-free atmosphere in the Archean. The study revealed that radiation would almost completely prevent life from forming initially. However, the planet’s ozone layer would grow as oxygen levels increased to near-modern levels.
Eager-Nash said, “With modern oxygen levels, this would take a few days, which would hopefully mean that life could survive. We were surprised by how quickly ozone levels would respond.”
To their surprise, the picture differed for an Earth-like planet in an older galaxy. In compact galaxies like NGC 1277, known as “red nugget relics,” the stars are packed closely together. This proximity makes the effects of an active galactic nucleus (AGN) particularly deadly due to intense radiation exposure. In larger galaxies, such as elliptical-shaped Messier-87 or our spiral Milky Way, stars are spread out more, creating safer distances from the AGN’s radiation.
Journal Reference
- Kendall I. Sippy, Jake K. Eager-Nash, Ryan C. Hickox, Nathan J. Mayne, and McKinley C. Brumback. UV Radiation from an AGN impacts Planetary Atmospheres and Consequences for Galactic Habitability. The Astrophysical Journal. DOI 10.3847/1538-4357/adac5d
Source: Tech Explorist
