UV light is often seen as the enemy. We slather on SPF, wear hats and sunglasses, and hang out in the shade all because of the threat of sunburns and skin cancer as ultraviolet rays from the sun penetrate the ever-thinning ozone layer. Someone pale enough to glow in the dark like myself usually doesn’t think of UV light being a life-giving force while at the drugstore stocking up on SPF 100+ before a trip to the beach that will still involve an umbrella.
Scientists are now seeing the rays most of us dread in a different light. In the search for life beyond Earth, scientists at the Harvard Center for Astroyphysics are convinced that part of the reason exoplanets orbiting red dwarf stars are void of aliens is that their light sources do not produce enough ultraviolet light to spark the biological processes that thrive on our planet. Ribonucleic acid is necessary for life as we know it, but certain UV levels are needed to power the chemistry that produces this elixir of life. Life cannot come into being without a catalyst.
There is not enough of that catalyst even in the habitable zones of the red dwarf stars (even the ones with liquid water) observed in the study, including Proxima Centauri, LHS 1140 and TRAPPIST-1. This only furthers the belief that the absence of enough UV light could mean the absence of anything alive. Computer models and the scientists’ knowledge of red dwarfs led to the estimate that anywhere from 100 to 1,000 times less UV light than that beaming down on Earth hits the surface of terrestrial planets in these potentially habitable zones. Chemical processes dependent on the same kind of light we try to keep our faces out of are thought to either progress much more slowly than they did on an embryonic Earth or be altogether impossible.
"It may be a matter of finding the sweet spot," said Robin Wordsworth of the Harvard School of Engineering and Applied Science, a co-author of a study recently published in the Astrophysical Journal. "There needs to be enough ultraviolet light to trigger the formation of life, but not so much that it erodes and removes the planet's atmosphere."
TRAPPIST-1 and other red dwarf star systems also face the opposite problem. Sometimes they get bombarded by UV flares intense enough to scorch the atmosphere and threaten any hypothetical life. The irony here is that in the case of a UV deficit, these flares could actually inject enough energy to break even. Red Dwarf stars have been a major focus in the search for life because they are orbited by many exoplanets that may or may not be crawling with something, but the only planet where life formed that we can compare to is our own, and we don’t even know exactly how that happened.
What if aliens did emerge even without enough of a catalyst? It could tell us something very different about how life on Earth spawned, but we need more time and technology to really shed UV light on that.