Black hole radiation may actually be a catalyst for life

At the center of most large galaxies, including the Milky Way, resides a supermassive black hole. Periodically, this cosmic entity accumulates interstellar gas, switching into an active galactic nucleus (AGN), a state in which it releases intense radiation throughout the galaxy. 

Given that environment, one would not expect any life-form to thrive under such intense energy bombardment. 

Yet a new study published in The Astrophysical Journal proposes that AGN radiation may paradoxically offer life a boost, supporting certain conditions that encourage organisms to flourish instead of perish.

Effects of black hole radiation 

In this joint effort by Dartmouth College and the University of Exeter, researchers used computer models to evaluate how ultraviolet (UV) light from AGNs changes the atmospheres of Earth-like planets. 

Their findings demonstrate that although black hole radiation can be catastrophic in some cases, it may assist life once sufficient oxygen has accumulated in the planet’s atmosphere.

“Once life exists, and has oxygenated the atmosphere, the radiation becomes less devastating and possibly even a good thing,” said Kendall Sippy, the lead author of the paper. “Once that bridge is crossed, the planet becomes more resilient to UV radiation and protected from potential extinction events.”

These simulations encompassed scenarios in which an Earth-like planet experiences the ultraviolet output from an AGN. If the atmosphere is high in oxygen, the radiation triggers chemical processes leading to a denser ozone layer. This thicker ozone layer, in turn, shields the surface from harmful UV rays. 

Such phenomena mirror what occurred on Earth roughly two billion years ago when the first microbes began generating oxygen, eventually fueling a feedback cycle that improved environmental habitability.

Supporting the conditions for life

In line with earlier research on how solar radiation shaped the early Earth, this study’s results confirm that the effect of radiation relies heavily on the planet’s distance from its source and whether life is already in progress. 

On a planet devoid of oxygen, black hole radiation would be devastating. But with oxygen in the atmosphere, new ozone creation would mitigate the radiation’s most harmful effects.

“If life can quickly oxygenate a planet’s atmosphere, ozone can help regulate the atmosphere to favor the conditions life needs to grow,” said co-author Jake Eager-Nash, a postdoctoral researcher at the University of Victoria. “Without a climate-regulating feedback mechanism, life may die out fast.”

The rapid building of an ozone layer under AGN-type conditions was unexpected. “With modern oxygen levels, this would take a few days, which would hopefully mean that life could survive,” Eager-Nash says. “We were surprised by how quickly ozone levels would respond.”

Evaluating various galactic settings

In reality, the Milky Way’s central black hole, known as Sagittarius A*, has a negligible effect on Earth. Yet, the researchers wished to explore a hypothetical situation in which Earth is located much nearer an active black hole, experiencing billions of times more intense radiation. 

With no initial oxygen present (similar to Earth’s distant past), this would all but preclude life from beginning. But as the atmosphere gradually oxygenated, an ozone shield would form, stabilizing conditions sufficiently for living organisms.

They also examined different galactic types, including “red nugget relic” galaxies where stars are clustered more tightly together, making the AGN’s influence lethal to potentially habitable planets. Larger elliptical galaxies like Messier-87, or a spiral galaxy like ours, more thinly scatter their stars, lowering the potential for dangerous AGN interference.

Black hole radiation studies 

The project’s origin was, in part, serendipitous. Ryan Hickox, a professor at Dartmouth, went on a sabbatical trip aboard the Queen Mary 2 with his dog and met Nathan Mayne from the University of Exeter. 

Their conversation revealed overlapping research interests, and they soon realized Mayne’s PALEO software for modeling exoplanet atmospheres could be repurposed for black hole radiation studies.

This collaboration enabled Sippy to work together with Eager-Nash, previously a graduate student in Mayne’s lab. 

“It models every chemical reaction that could take place,” Sippy said. “It returns plots of how much radiation is hitting the surface at different wavelengths, and the concentration of each gas in your model atmosphere, at different points in time.”

Accreting neutron star research

McKinley Brumback, a Dartmouth-trained astrophysicist and assistant professor at Middlebury College, also contributed her expertise on X-ray binaries – systems in which matter falls from a regular star onto a neutron star, producing X-rays. 

While AGNs switch between active and dormant over millions of years, neutron star binaries do so in mere days or months, allowing researchers to scrutinize similar physical mechanisms at much shorter timescales.

Rethinking galactic habitability

Overall, the researchers’ findings prompt a fundamental reassessment of how extragalactic radiation could shape planetary environments. An AGN’s energy might not be purely destructive: under suitable conditions, it could encourage the buildup of an ozone layer, safeguarding evolving lifeforms. 

Moreover, the scenario in which Earth receives billions of times more radiation than it does today lends new perspectives on the range of possible planetary evolutions across the universe.

Thus, cosmic forces – once presumed purely detrimental – can become catalysts for ecological and biological complexity, as long as certain life-critical thresholds, like oxygenation, have already been reached.

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