It is believed that the so-called 'habitable zones' – regions around stars where conditions are potentially suitable for the existence of liquid water on planets, as the main criterion for the possible existence of life – are optimal for the search for life outside the Earth.
A new NASA study significantly limits the list of such planets. Agency scientists point out that some of these zones cannot actually support life due to the frequent stellar eruptions inherent in young red dwarfs – the release of huge amounts of stellar matter and radiation into space.
Having laid this concept at the basis of a new model, an interdisciplinary team of NASA scientists set itself the task of finding out exactly how habitable zones can be determined taking into account the influence of stellar activity, which threatens the exoplanet's atmosphere and leads to oxygen loss. The study was published in Astrophysical Journal Letters on February 6, 2017.
“If we want to find an exoplanet that can develop and support life, we must find out which stars are the best candidates for this,” said Vladimir Hayrapetyan, lead author of the article, a scientist at NASA's Mission Control Center in Greenbelt. “Only by figuring out what kind of parent stars we need can we get closer to understanding this issue.”
To determine the habitable zone around a star, astronomers have traditionally looked at the degree of heat and light radiation from a star. It is believed that stars that are more massive than our Sun produce more heat and light, so their habitable zone is farther away, while the habitable zone of less massive and colder stars, on the contrary, is closer.
In addition to heat and visible light, the new study suggests that stars emit X-rays and ultraviolet radiation, as well as stellar eruptions such as flares and coronal mass ejections that characterize the space weather of orbiting planets. One possible consequence of this radiation is so-called atmospheric erosion, during which high-energy particles drag atmospheric molecules such as hydrogen and oxygen, the ingredients of water, into space. Hayrapetyan's new team model for habitable zones now takes this effect into account.
“Although our results for planets in orbits of young red dwarfs are quite pessimistic, we now have a better understanding of which stars have more habitable prospects,” Hayrapetyan said. “The more demands we make on a parent star, the more we come to understand that our Sun is one of the best examples of parent stars that support life on Earth.”
Sources: NASA
