Scientists believe they have narrowed down the star systems most likely to harbor life

With the number of confirmed exoplanets in the Milky Way risingwe need to be more selective about the targets we choose to search for life signs.

A team of astronomers led by Anna Shapiro of the Max Planck Institute for Solar System Research in Germany has narrowed down the options.

According to a new analysis, Earth-like exoplanets orbiting Sun-like stars with a relatively low metal content are more likely to be shielded from harmful UV radiation that could hinder life by putting it at risk of genome damage.

This may seem counterintuitive since stars with lower metal content emit more ultraviolet light. But the team’s work shows that a planet with an oxygen-rich atmosphere has a thicker one ozone Layer that offers more protection to a world orbiting a metal-poor star than one with a metal-rich host.

“Our results,” they write in their newspaper“imply that planets hosted by low-metallicity stars are the best targets to search for complex land life.”

Not all stars are created equal. They can be small, cool, and dark, or large, hot, and blazing. And while they share some basic elements, their chemical composition can vary widely.

That’s because very early in the history of the universe, there were no heavy elements. Hydrogen and helium were pretty much all there was; From these elements the first stars were born, their hearts huge engines that smashed atoms together to create larger, heavier atoms.

When these stars died, the violent process created even heavier elements and spat and seeded those elements into space to be absorbed new stars born from interstellar clouds of dust and gas.

These elements change the radiative power of the star. Stars with a higher proportion of elements heavier than hydrogen and helium, or with a higher metallicity, emit less ultraviolet radiation than stars made of lighter material. And we know from our lives here on earth that UV radiation can damage and cause damage to sensitive terrestrial organisms different types of DNA damage.

The role of UV radiation on the potential habitability of alien worlds had not yet been explored, so Shapiro and her colleagues studied Earth as a model.

An extraterrestrial civilization viewing the solar system from a great distance might find Earth unfriendly to life. At our current distance from the Sun, radiation levels from the UV-C and UV-B wavelength bands are ‘well above the maximum tolerable level for terrestrial life’, according to the researchers.

But our atmosphere blocks most of it: oxygen or O₂in the upper atmosphere absorbs most UV-C and the ozone layer or O₃in the middle atmosphere the UV-B absorbs.

UV radiation is involved in the formation and destruction of ozone. Wavelengths below 240 nanometers break O apart₂ molecules; Free-floating O atoms can then collide with and bind to O₂ Molecules to form O₃. However, longer wavelengths break up the O₃ through photodissociation. The resulting O atoms can then recombine to form O₂.

Several factors affect a star’s UV output, including its metallicity and temperature. Shapiro and her team modeled Earth-like worlds orbiting Sun-like stars and tweaked the parameters that would affect UV radiation to see what effect it would have on the orbiting exoplanet.

They found that metallicity was more important than temperature in affecting the exoplanet’s habitability, but in a way completely opposite to what one might assume. The stars with lower metallicity and more UV radiation were more likely to have habitable worlds.

That’s because the way UV radiation interacts with the oxygen in the atmosphere has created better protection, resulting in less of that radiation reaching the exoplanet’s surface.

“Paradoxically, while higher metallicity stars that appeared later in the history of the Universe emit less UV radiation, the associated stellar radiation spectrum leaves less O₃ Formation that improves UV penetration and makes conditions on planets orbiting these stars less friendly to the land biosphere,” write the researchers.

“We therefore find that the surface of planets orbiting metal-rich stars are exposed to more intense UV radiation than the surface of planets orbiting metal-poor stars. Therefore, planets in the habitable zones of low-metallicity stars are the best targets for finding complex land life.”

It is not enough to be able to rule out stars with higher metallicity now. But analyzing and characterizing exoplanet atmospheres with instruments like the James Webb Space Telescope will help scientists determine if their findings are on track and bring us a tiny step closer to finding signs of life on an alien world.

The research was published in nature communication.