Scientists Work Out Which Stars May Host Life on Their Planets

With billions of stars in our home galaxy and billions more galaxies out in the gargantuan universe, the task of finding exoplanets that may host life might seem an insurmountable task.

Now, however, scientists may have found a way to narrow down the list; According to a study published in the journal Nature Communications on April 18, stars with higher levels of heavy elements including metals may be less friendly to the evolution of life.

The study, led by Anna Shapiro of the Max Planck Institute for Solar System Research in Germany, investigated the amount of ultraviolet radiation emitted from stars of varying "metallicity" (levels of heavier elements like metals) to determine which type of star might be more ideal for life. They found that the metallicity of a star is related to the ability of a planet to form a protective layer against ultraviolet (UV) light.

"We wanted to understand what properties a star must have in order for its planets to form a protective ozone layer," Shapiro said in a statement.

Ozone, or O3, is a chemical that can absorb UV light. In our own atmosphere, the ozone layer is crucial to mopping up much of the harmful radiation from the sun, which is why the ozone hole is such a concern. O3 absorbs the energy from longer wavelength UV-B light, splitting into O2 and lone O ions. O2 oxygen also absorbs some UV, splitting into two O ions after interacting with mainly shorter wavelength UV-C.

For an ozone layer to form in an oxygenated atmosphere, however, enough powerful UV light is needed to create the O ions that will eventually combine to form O3. Essentially, an ozone layer is formed by a star releasing a certain level of UV light, which then in turn protects the planet's surface from further UV light.

The scientists found that while stars richer in heavy elements like metals produced less UV light, those that were metal-poor allowed for the formation of an ozone layer, and were therefore more ideal for the formation of life, due to less UV actually reaching the surface of the planet.

"We thus find that the surface of planets orbiting metal-rich stars is exposed to more intense UV radiation than the surface of planets orbiting metal-poor stars," the authors wrote in the paper.

UV light is bad news for life, as it ionizes molecules. If those molecules are DNA or other genetic code, atoms being ionized may cause mutated genes, which may lead to cancers in multicell organisms like humans, or merely kill a single-celled organism.

"Therefore planets in the habitable zones of stars with low metallicity are the best targets to search for complex life on land," the authors wrote. "For the stellar types considered, metallicity has a larger impact on the surface UV than the stellar temperature. The atmospheric oxidation (cleaning) capacity is found to be stable and life-supporting, almost independent of stellar metallicity at an oxygen volume fraction above 1 percent."

Therefore, an exoplanet is much more likely to be habitable to life if it is in the Goldilocks zone of its star—orbiting at a distance where the temperature allows liquid water to exist—and if its host star has a lower level of heavier metal elements, allowing a protective ozone layer to form.

The paper also suggests a strange conclusion that stars born more recently are less likely to be capable of harboring life on their planets.

"Each newly forming star therefore has more metal-rich building material available than its predecessors. Stars in the universe are becoming more metal-rich with each generation," Shapiro said.

Therefore, newer stars have heavier elements, produce less UV light, and therefore its orbiting planets cannot develop an appropriate ozone layer.

This revelation is hoped to aid researchers in their hunt for life elsewhere in the universe. Higher-metallicity stars may not be a write-off for life, but the results of this study will be strengthened by the analysis of exoplanet atmospheres using telescopes such as the James Webb Space Telescope.

"With James Webb, you open up the amount of colors and regions, and colors equates to what kind of molecules you can detect," Néstor Espinoza, an astrophysicist and astronomer at the Space Telescope Science Institute (STScI) in Baltimore, previously told Newsweek.

"So we'd have all those like a tiny feature of water that you could see. But now with James Webb, you open up the whole chemical inventory, [it] can detect water, you can come to take carbon dioxide, and so on. So that really, it's a game changer in the precision, because the telescope is so large and so stable," he said. "The fact that we can not only explore this but explore it very precisely, is something that we have not accessed before."

Do you have a tip on a science story that Newsweek should be covering? Do you have a question about exoplanet life? Let us know via science@newsweek.com.