A beneficial glow is emerging: stars that are comparatively poor in heavy elements have a light spectrum that promotes the formation of an ozone layer on their planet that protects against UV radiation, according to a study. In contrast to metal-rich stars, they offer more favorable conditions for the emergence of life as we know it. The results could thus help to specifically target star systems that are particularly promising in the search for signs of atmospheric life, say the researchers.
The universe is teeming with stars with planetary systems - astronomical research has shown this more and more clearly in recent years. The focus is now increasingly on the question of whether an earth-like environment can perhaps also be detected on such exoplanets. Indications of life could be provided by evidence of certain substances in the atmospheres of rocky planets, which one hopes to find through the sharp view of NASA's new James Webb telescope. To do this, of course, the most promising candidates for potential life should be examined under the magnifying glass. Astronomers are guided by the factors that contributed to the emergence of life on Earth.
On the trail of the formation of ozone layers
The researchers around Anna Shapiro from the Max Planck Institute for Solar System Research in Göttingen (MPS) have dealt with the ozone layer in this regard. As is well known, this atmospheric structure protects the planet's surface from cell-damaging UV radiation. This is due to the special characteristics of the three-atom oxygen compound. At least for the emergence of complex life on land, the ozone layer was an important prerequisite for us. "We now wanted to gain information about the properties a star must have so that its planets can form a protective ozone layer," says Shapiro.
The researchers looked at a category of stars that also includes our sun: They have surface temperatures of between around 5000 and 6000 degrees Celsius. As the team explains, the formation of an ozone layer is strongly linked to the radiation a planet receives from its parent star. "Ultraviolet radiation from the sun plays a dual role in Earth's atmospheric chemistry," says Shapiro. While the long-wave UV-B radiation destroys ozone, the short-wave UV-C radiation contributes to the formation of the three-part oxygen molecule in the middle atmosphere. "It was therefore obvious to assume that ultraviolet light could also have a similarly complex influence on the atmospheres of exoplanets," says the astronomer.
Using model calculations, the researchers have now investigated the extent to which the so-called metallicity of stars influences how strongly they emit the different wavelengths of UV light. This feature describes the ratio of hydrogen and heavier elements in the building material of a star. The sun has more than 31,000 hydrogen atoms for every iron atom. It is thus a comparatively metal-poor star. The researchers' calculations initially showed that stars with a low metal content emit more UV radiation than metal-rich ones. But with them, the ozone-generating UV-C radiation predominates. In the more metal-rich versions, however, the ozone-destroying UV-B radiation is ahead.
Better chances of a protective layer
The researchers then investigated the extent to which the calculated UV radiation would affect the atmosphere of planets orbiting model stars at a habitable distance. To do this, they used a chemistry-climate model that simulated the interactions between gases and ultraviolet light in the atmosphere. It was found that in metal-poor stars, the ratio of ozone-forming UV-C to ozone-depleting UV-B radiation actually favors the development of a thick ozone layer. "Contrary to our initial expectations, metal-poor stars should therefore offer better conditions for the emergence of life," sums up Shapiro.
The study can therefore serve astrobiological research: "It provides valuable information as to which stars we should observe with particular attention," says co-author Laurent Gizon from the MPS. In addition, another interesting aspect is emerging, the researchers conclude: the universe seems to become more and more hostile to life as it ages. Because the building material of the stars is increasingly rich in metal. This is because heavy elements are formed inside stars at the end of their lives and then released into space by stellar winds or supernova explosions. "Each emerging star therefore has more metal-rich tree material available than its predecessors," says Shapiro.
Source: Max Planck Institute for Solar System Research, Article: Nature Communications, doi: 10.1038/s41467-023-37195-4