Is Proxima B Habitable? Red Dwarf Stellar Eruptions May Have Made Closest Earth-Like Planet Ill-Suited For Life, Study Suggests

Life, as we currently understand it, is fragile. A lot of things need to be just right for life on a planet to emerge — the most important among them being a planet’s distance from its parent star — and several others need to be at an optimum level for it to thrive and evolve.

This is why astronomers looking for signs of life beyond the solar system limit their hunt to planets located in the “Goldilocks zone” of a star — a region where the temperature is just right for liquid water to exist.

However, in a new study published in the latest edition of the Astrophysical Journal Letters, a team of researchers has argued that the criteria we currently use to determine a star’s habitable zone is not wide enough. They argue that in addition to a star’s heat and light output — factors scientists have traditionally considered while defining habitable zones — astronomers should also take into account how a planet’s atmosphere is affected by high-energy particles emitted by the parent star.

“If we want to find an exoplanet that can develop and sustain life, we must figure out which stars make the best parents,” lead author Vladimir Airapetian, a solar scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said in a statement released Wednesday. “We’re coming closer to understanding what kind of parent stars we need.”

For the purpose of their study, the researchers focused on red dwarfs, which are the most common and the longest-lived stars in the universe, and are, therefore, most likely to have planets locked in orbit around them. Proxima Centauri, the star closest to the sun, is also a red dwarf.

“On the downside, red dwarfs are also prone to more frequent and powerful stellar eruptions than the sun,” study co-author William Danchi, a Goddard astronomer, said in the statement. “To assess the habitability of planets around these stars, we need to understand how these various effects balance out.”

High-energy X-ray and ultraviolet emissions in stellar eruptions and storms can erode the atmospheres of planets located close to the red dwarfs, breaking molecules into atoms and then ionizing atmospheric gases. As the number of electrons generated through this process increases, the number of positively charged ions escaping the atmosphere also rises — a process that takes place on a smaller scale on Earth, whose middle-aged star is less prone to superflares.

Using a computer model to simulate the atmospheric conditions of planets around red dwarfs, the researchers found that these stars emitted enough high-energy radiation to enable the escape of oxygen and nitrogen — the building blocks of life. Hydrogen, which, along with oxygen, is needed to create water on a planet’s surface, is already known to be extremely vulnerable to ion escape.

“The more X-ray and extreme ultraviolet energy there is, the more electrons are generated and the stronger the ion escape effect becomes,” co-author Alex Glocer, a Goddard astrophysicist, said. “This effect is very sensitive to the amount of energy the star emits, which means it must play a strong role in determining what is and is not a habitable planet.”

The research has implications for those trying to ascertain the habitability of Proxima b — a planet that orbits Proxima Centauri, and, seems — at first glance — a perfect fit for life as we know it to emerge and thrive.

If the findings of the study are to be believed, Proxima b either never became habitable — as any water on its surface would have been eliminated before life had a chance to develop — or it became uninhabitable within a few tens to a hundred million years.

Either way, the study pegs the chances of life — as we know it, at least — currently existing on Proxima b at extremely slim.

“We have pessimistic results for planets around young red dwarfs in this study, but we also have a better understanding of which stars have good prospects for habitability,” Airapetian said. “As we learn more about what we need from a host star, it seems more and more that our sun is just one of those perfect parent stars, to have supported life on Earth.”