Planets orbiting stars with less “metals” —a term for elements heavier than hydrogen and helium— might unexpectedly have more favorable conditions for the emergence of complex life than worlds hosted by metal-rich stars, according to a study published in ‘Nature Communications’. Research by German scientists indicates that the chemical composition of a star greatly influences the ultraviolet radiation it emits into space, and therefore the conditions for the emergence of life in its system.
A team from the Max Planck Institute for Solar System Research and Chemistry as well as the University of Göttingen also suggests that as the universe ages “it becomes less and less conducive to the emergence of complex life on new planets”. The metallicity of a star is related to the ability of its planets to surround themselves with a protective layer of ozone, which is an important prerequisite for the emergence of complex life.
In search of star systems most likely to support life
The study thus provides scientists looking for important habitable star systems in the sky about where this endeavor could be especially promising. With the help of numerical simulations, the study focuses on the ozone content of exoplanet atmospheres. That compound can protect the planet’s surface (and the life forms that reside on it) from cell-damaging ultraviolet (UV) radiation.
“We wanted to understand what properties a star must have in order for its planets to form a protective ozone layer”, says Anna Shapiro, from Max Planck and one of the study’s signatories. The team focused on stars that have exoplanets and whose surface temperatures range from 5,000 to 6,000 degrees, a group to which the Sun belongs.
Influence of metallicity on possible life conditions
These researchers calculated exactly which wavelengths make up the ultraviolet light emitted by stars and, for the first time, also took into account the influence of metallicity. That property describes the ratio between hydrogen and the heavier elements (metals) in the star’s building material.
In the case of the Sun, there are more than 31,000 hydrogen atoms for every iron atom. The study also considered stars with lower and higher iron content. In addition, they investigated how the calculated UV radiation would affect the atmospheres of planets orbiting at a distance suitable for life, and simulated what processes the characteristic ultraviolet light from its star sets in motion in the planet’s atmosphere.
To calculate the composition of planetary atmospheres, the researchers used a chemistry-climate model that simulates the processes that control oxygen, ozone, and other gases, as well as their interactions with ultraviolet light from stars. This model allowed to investigate a great variety of conditions in the exoplanets and to compare them with the history of the Earth’s atmosphere in the last 500 million years.
Metal-poor stars would allow the formation of a dense ozone layer
In general, metal-poor stars emit more UV radiation than their metal-rich counterparts. But the ratio between ultraviolet C radiation, which generates ozone, and ultraviolet B, which destroys ozone, also depends to a large extent on metallicity.
In metal-poor stars, UV-C radiation predominates, which allows the formation of a dense ozone layer, and in the others, UV-B is abundant, which makes this protective envelope much scarcer. “Contrary to expectations, metal-poor stars should thus offer more favorable conditions for the appearance of life”, concludes Shapiro, quoted by Max Planck.
Also, as the universe ages, it is likely to become increasingly hostile to life, as metals and other heavy elements form inside stars at the end of their lives. Depending on the mass of the star, these materials are released into space in the form of stellar winds or in a supernova explosion, constituting the material from which the next generation of stars will form.
