The TRAPPIST-1 system is a science-fiction writer’s dream. Seven Earth-sized worlds orbit a red dwarf star just 40 light-years away. Three of those worlds are within the habitable zone of the star. The system spans a distance less than 25 times that of the distance from the Earth to the Moon. Oh, what epic tales a TRAPPIST civilization would have! That is, if life in such a system is even possible…
Therein lies the problem. Although the vast majority of potentially habitable worlds orbit red dwarf stars, that doesn’t mean most inhabited worlds have a red dwarf sun. Red dwarfs are known to be violently active in their youth. They emit powerful flares that might strip nearby planets of their atmospheres, and even if a planet can hold on to its sky, it would still be bathed in powerful radiation. Only when a red dwarf matures is it calm and stable. This is very different from larger stars such as our Sun, which are reasonably calm throughout their lives. Since potentially habitable red dwarf planets must orbit very close to their stars, there is a worry that even in the best conditions, life on such a world could never get a foothold. The environment is just too harsh. But a new study gives exobiologists some surprising hope.
The study focuses on red dwarf superflares and the radiation they emit. These flares emit a great amount of x-rays and ultraviolet radiation. For a young red dwarf planet with an atmosphere, most of the x-rays would never reach the surface, but the young world would still be bathed in UV radiation. The team wanted to know how hostile that UV would be to early life, so they bathed microbes in UV.
The study looked at two types of bacteria. Deinococcus radiodurans is a variety known to be UV tolerant, while Escherichia coli is known to be susceptible to radiation. They bathed each variety in ultraviolet radiation levels that would be typical at the distances of the TRAPPIST worlds e, f, and g, which are the most potentially habitable. The results weren’t good for the E. coli variant, as a simulated flare sterilized them below the limit of detection for the innermost world and some survival for the most distant one. But the D. radiodurans did fairly well. Only about 1 in 600 million survived a simulated flare for the closest world, but given the typical time span between flares, the bacteria would maintain a foothold. And, of course, with regular flares, there would be an evolutionary pressure to become more UV resistant.
So it seems that while early life in the TRAPPIST system might have a tough evolutionary road, the superflares wouldn’t sterilize the planets. Life might be common for red dwarf worlds after all.
Reference: Abrevaya, X C, et al. “An experimental study of the biological impact of a superflare on the TRAPPIST-1 planets.” Monthly Notices of the Royal Astronomical Society (2024): stae2433.