The possibility of life on planets outside our solar system has long intrigued scientists. Recent research has explored Earth-like exoplanets, which orbit stars emitting stronger solar radiation than the sun. A study published in Astrobiology on June 12th provides new insights into how life might endure under such conditions.
The study focused on Clavascidium lacinulatum, a type of lichen found in the Mojave Desert. Researchers discovered that this lichen could survive three months of exposure to high levels of solar radiation, previously thought lethal. Although injured, the lichen was able to recover and replicate, suggesting that photosynthetic life might exist on planets exposed to intense solar radiation.
Henry Sun, Associate Research Professor of Microbiology at DRI and one of the lead authors, explained: “The study was motivated by a curious observation... I noticed that the lichens growing there aren’t green; they’re black... And that pigment turned out to be the world’s best sunscreen.”
Earth's atmosphere filters UVC rays, known for their damaging effects. These rays are used for sterilization due to their ability to damage DNA and prevent reproduction. Many Earth-like exoplanets orbit M and F stars that emit intense UVC radiation during solar flares.
Sun commented on the shift in focus from Mars to these exoplanets after advancements like the James Webb Space Telescope: “We’re talking about planets that have liquid water and an atmosphere.”
Together with his then-graduate student Tejinder Singh, Sun collected Mojave Desert lichen for experimentation. They subjected it to UVC light in a lab setting for three months. Half of the algal cells remained viable upon rehydration.
Singh stated: “In order for a microorganism to persist on a planet, it has to last longer than a day... We also wanted to go beyond just activity and demonstrate viability.”
Further investigation with chemists from the University of Nevada revealed how lichen acids function similarly to UV-resistant additives in plastics. The top layer of lichen acts as a protective shield against radiation.
Despite Earth's atmosphere filtering out UVC rays before lichens evolved, they possess this protection as an additional benefit due to UVA and UVB protection capabilities.
To assess different atmospheric impacts, researchers placed lichens in an oxygen-free environment with UVC light. This setup reduced radiation damage further.
The study concludes: “They may be teeming with colonial microorganisms that... are ‘tanned’ and virtually immune to UVC stress,” indicating potential habitability beyond Earth.
Tejinder Singh emphasized life's resilience: “This work reveals the extraordinary tenacity of life even under harshest conditions... we inch closer to understanding where life might be possible beyond this planet we call home.”
For more information, access "UVC-Intense Exoplanets May Not Be Uninhabitable: Evidence from a Desert Lichen" via Astrobiology at https://doi.org/10.1089/ast.2024.0137
Authors include Tejinder Singh (DRI), Christos Georgiou (U. of Patras), Christopher Jeffrey (UNR), Matthew J. Tucker (UNR), Casey S. Philbin (UNR), Tanzil Mahmud (UNR), Christopher P. McKay (NASA Ames), Henry J. Sun (DRI).
DRI is Nevada’s non-profit research institute established in 1959 focusing on significant scientific questions worldwide while boosting local economies through grants supporting research endeavors across its campuses.