Atmospheric ozone and oxygen protect the biosphere of our planet against harmful ultraviolet radiation. In new research, astronomers modeled atmospheres of Earth-like exoplanets hosted by stars with near-solar temperatures — between 5300 and 6300 K — and a broad range of metallicities covering known exoplanet host stars; they found that paradoxically, although metal-rich stars emit substantially less ultraviolet radiation than metal-poor stars, the surface of their planets is exposed to more intense ultraviolet radiation.
Shapiro et al. show that the development of complex life on planets in the habitable zone can be sustained from a few percent of oxygen upward, being robust for a large range of stellar characteristics and against major extraterrestrial cataclysms. Image credit: MPS / Hormesdesign.de.
Complex, multicellular life on land requires oxygen from which ozone forms, leading to a tolerable ultraviolet radiation level at the surface for its development and evolution.
Stellar emission and planetary ultraviolet protection depend on the effective temperature of the host star.
While for a young planet ultraviolet exposure can be essential for abiogenesis, high levels of ultraviolet light trigger genomic damage and are a threat to all life forms.
“We wanted to understand what properties a star must have in order for its planets to form a protective ozone layer,” said Dr. Anna Shapiro, an astronomer at the Max Planck Institute for Solar System Research.
“We saw huge peaks in intensity. It is therefore quite possible, that the Sun, too, is capable of such spikes in intensity. In that case, also the intensity of the ultraviolet light would increase dramatically.”
“So naturally we wondered, what this would mean for life on Earth and what the situation is like in other star systems,” added Dr. Sami Solanki, director at the Max Planck Institute for Solar System Research.
In their research, the authors investigated the dependence of planetary surface ultraviolet on the atmospheric concentration of oxygen and stellar metallicity for stars of three spectral types: G2V (5800 K, representing solar case), G5V (5300 K), and F7V (6300 K).
“In the Earth’s atmospheric chemistry, ultraviolet radiation from the Sun plays a dual role,” Dr. Shapiro said.
“In reactions with individual oxygen atoms and oxygen molecules, ozone can both be created and destroyed.”
“While long-wave UV-B radiation destroys ozone, short-wave UV-C radiation helps create protective ozone in the middle atmosphere.”
“It was therefore reasonable to assume that ultraviolet light may have a similarly complex influence on exoplanet atmospheres as well. The precise wavelengths are crucial.”
“The results were surprising,” the astronomers said.
“Overall, metal-poor stars emit more UV radiation than their metal-rich counterparts. But the ratio of ozone-generating UV-C radiation to ozone-destroying UV-B radiation also depends critically on metallicity: in metal-poor stars, UV-C radiation predominates, allowing a dense ozone layer to form.”
“For metal-rich stars, with their predominant UV-B radiation, this protective envelope is much more sparse.”
“Contrary to expectations, metal-poor stars should thus provide more favorable conditions for the emergence of life.”
“Moreover, our study yields an almost paradoxical conclusion: as the Universe ages, it is likely to become increasingly hostile to life,” they added.
“Metals and other heavy elements are formed inside stars at the end of their several-billion-year lifetimes and — depending on the mass of the star — are released into space as stellar wind or in a supernova explosion: the building material for the next generation of stars.”
“Each newly forming star therefore has more metal-rich building material available than its predecessors,” Dr. Shapiro said.
“Stars in the Universe are becoming more metal-rich with each generation.”
“According to our study, the probability that star systems will produce life thus also decreases as the Universe ages.”
“However, the search for life is not hopeless.”
“After all, many host stars of exoplanets have a similar age as the Sun.”
“And this star is indeed known to harbor complex and interesting lifeforms on at least one of its planets.”
The study was published this week in the journal Nature Communications.
_____
A.V. Shapiro et al. 2023. Metal-rich stars are less suitable for the evolution of life on their planets. Nat Commun 14, 1893; doi: 10.1038/s41467-023-37195-4
