Newswise —
Researchers from the Max Planck Institutes for Solar System Research and for Chemistry, together with the University of Göttingen, have discovered that stars rich in heavy elements are less conducive to the development of complex life than stars with low metal content. The team demonstrated the correlation between a star’s metallicity and the ability of its planets to create an ozone layer for protection against harmful ultraviolet light emitted by the star. This finding provides valuable information for scientists searching for habitable star systems using space telescopes. Additionally, the study suggests a surprising conclusion: the universe becomes progressively less hospitable to the emergence of complex life on newly formed planets as it ages.
Over the past few years, researchers have increasingly concentrated on the gas envelopes of distant planets in their search for habitable or inhabited worlds. They examine observational data to determine whether these planets possess an atmosphere, and whether it includes gases like oxygen or methane, which are primarily produced by lifeforms on Earth. In the coming years, the James Webb Telescope, developed by NASA, will expand these observations to unprecedented levels. It will allow researchers to not only characterize the atmospheres of large gas giants, such as Super-Neptunes, but also to scrutinize the much fainter spectrographic signals emanating from rocky planet atmospheres for the first time.
The study, which was recently published in Nature Communications, employed numerical simulations to examine the ozone content of exoplanet atmospheres. Like on Earth, this molecule, composed of three oxygen atoms, can safeguard the planet’s surface and its resident life forms against harmful ultraviolet (UV) radiation. Therefore, an ozone layer is a critical prerequisite for the emergence of complex life. “Our aim was to determine the characteristics of a star that must exist for its planets to generate a protective ozone layer,” Anna Shapiro, the first author of the study and a researcher at the Max Planck Institute for Solar System Research, stated in outlining the study’s fundamental concept.
As is often the case in scientific research, the concept of the current study was prompted by a previous discovery. Three years ago, a team of scientists from the Max Planck Institute for Solar System Research examined the variations in the Sun’s brightness in comparison to those of hundreds of similar stars. The outcome revealed that the visible light intensity of many of these stars fluctuated significantly more than that of the Sun. Alexander Shapiro, who participated in both studies, remarked, “We observed enormous intensity spikes,” and he suggested that the Sun might be capable of producing similar fluctuations. “In such cases, the ultraviolet light intensity would also increase significantly,” he added. Sami Solanki, co-author of both studies and the director of the Max Planck Institute for Solar System Research
Dual role of UV radiation
The researchers focused their calculations on the subgroup of stars, approximately half of all stars, around which exoplanets have been observed to orbit, and whose surface temperatures range from approximately 5,000 to 6,000 degrees Celsius. The Sun, with a surface temperature of around 5500 degrees Celsius, is also a member of this subgroup. “Ultraviolet radiation from the Sun plays a dual role in the atmospheric chemistry of Earth,” explains Anna Shapiro, whose previous research has concentrated on the effects of solar radiation on the Earth’s atmosphere. Ozone can be created and destroyed through reactions with individual oxygen atoms and oxygen molecules. Long-wave UV-B radiation destroys ozone, while short-wave UV-C radiation generates protective ozone in the middle atmosphere. “It was therefore plausible to assume that ultraviolet light might have a similarly intricate impact on exoplanet atmospheres,” the astronomer notes. The precise wavelengths of radiation are critical.
To determine the impact of ultraviolet light on exoplanet atmospheres, the researchers conducted calculations that precisely identified the wavelengths of the ultraviolet light emitted by stars. They also took into account the effect of metallicity, a property that characterizes the ratio of hydrogen to heavier elements, which are often referred to as “metals” by astrophysicists. The Sun, for example, has a ratio of more than 31,000 hydrogen atoms to one iron atom. The study also considered stars with lower and higher iron content. This is the first time that metallicity has been factored in to such calculations.
Simulated interactions of UV radiation with gases
After identifying the ultraviolet light wavelengths emitted by stars and considering the effect of metallicity, the researchers went on to investigate how this calculated UV radiation would impact the atmospheres of planets orbiting at a life-friendly distance around these stars. Life-friendly distances refer to those orbits where the temperature is moderate enough to support liquid water on the planet’s surface. Using computer simulations, the team investigated the processes triggered in the planet’s atmosphere by the parent star’s characteristic UV light.
To compute the composition of planetary atmospheres the researchers used a chemistry-climate model that simulates the processes that control oxygen, ozone, and many other gases, and their interactions with ultraviolet light from stars, at very high spectral resolution. This model allowed the investigation of a wide variety of conditions on exoplanets and comparison with the history of the Earth’s atmosphere in the last half billion years. During this period the high atmospheric oxygen content and the ozone layer were established that allowed the evolution of life on land on our planet. “It is feasible that the history of the Earth and its atmosphere holds clues about the evolution of life that may also apply to exoplanets” says Jos Lelieveld, Managing Director of the Max Planck Institute for Chemistry, who was involved in the study.
Promising candidates
The simulations yielded unexpected results for the researchers. It was found that in general, stars with lower metallicity emit more ultraviolet (UV) radiation than their higher metallicity counterparts. However, the proportion of UV radiation that produces ozone (UV-C) compared to that which destroys it (UV-B) is critically dependent on the metallicity. In stars with lower metallicity, UV-C radiation dominates, resulting in the formation of a dense ozone layer. In contrast, in stars with higher metallicity, UV-B radiation predominates, resulting in a much sparser protective envelope. “These findings suggest that, contrary to expectations, stars with lower metallicity may offer more conducive conditions for the emergence of life,” concludes Anna Shapiro.
The researchers were surprised by the outcomes of their simulations, which revealed that, in general, stars with lower metallicity emit more ultraviolet (UV) radiation than their higher metallicity counterparts. However, the ratio of UV radiation that produces ozone (UV-C) to that which destroys it (UV-B) is critical and varies based on metallicity. Stars with lower metallicity exhibit higher levels of UV-C radiation, which leads to the formation of a denser ozone layer. In contrast, stars with higher metallicity have more UV-B radiation, which results in a sparser protective envelope. “These findings suggest that stars with lower metallicity may offer more favorable conditions for the emergence of life, contrary to expectations,” says Anna Shapiro.
Paradoxical conclusion
In addition, the study draws an almost paradoxical conclusion: as the universe evolves, it may become less hospitable to life. Heavy elements and metals are synthesized in stars towards the end of their multi-billion-year lifetimes and are then released into space either via stellar wind or a supernova explosion. This material becomes the building blocks for the formation of the next generation of stars. “Thus, each new star has more metal-rich material available than its predecessors, and stars in the universe become more metal-rich with each generation,” explains Anna Shapiro. The new study indicates that the likelihood of star systems producing life decreases as the universe ages. However, there is still hope in the search for life, as many host stars of exoplanets have similar ages to our Sun, which is known to support complex and diverse lifeforms on at least one of its planets.
Max Planck Society (Max-Planck-Gesellschaft)
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