Earth-sized planet Proxima b unlikely to host life: NASA

The newly discovered Earth-sized planet Proxima b is unlikely to support life despite being in the 'habitable zone' of its host star, due to the exposure to frequent stellar eruptions, NASA scientists have found.
The search for life beyond Earth starts in habitable zones, the regions around stars where conditions could potentially allow liquid water to pool on a planet's surface.
The study suggests that some of these zones might not actually be able to support life due to frequent stellar eruptions - which spew huge amounts of stellar material and radiation out into space - from young red dwarf stars.

"If we want to find an exoplanet that can develop and sustain life, we must figure out which stars make the best parents," said Vladimir Airapetian, a solar scientist at NASA's Goddard Space Flight Centre in the US.
To determine a star's habitable zone, scientists have traditionally considered how much heat and light it emits.
Stars more massive than our sun produce more heat and light, so the habitable zone must be farther out. Smaller, cooler stars yield close-in habitable zones.

However, along with heat and visible light, stars emit X-ray and ultraviolet radiation, and produce stellar eruptions such as flares and coronal mass ejections - collectively called space weather.

One possible effect of this radiation is atmospheric erosion, in which high-energy particles drag atmospheric molecules - such as hydrogen and oxygen, the two ingredients for water - out into space.
The new model for habitable zones now takes this effect into account.
The research has implications for the recently discovered planet orbiting the red dwarf Proxima Centauri, our nearest stellar neighbour.
Researchers applied their model to the roughly Earth-sized planet, dubbed Proxima b, which orbits Proxima Centauri 20 times closer than Earth is to the Sun.
Considering the host star's age and the planet's proximity to its host star, scientists expect that Proxima b is subjected to torrents of X-ray and extreme ultraviolet radiation from superflares occurring roughly every two hours.

They estimate oxygen would escape Proxima b's atmosphere in 10 million years. Additionally, intense magnetic activity and stellar wind - the continuous flow of charged particles from a star - exacerbate the harsh space weather conditions.
The scientists concluded that it is quite unlikely Proxima b is habitable.
"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," he added

This research was published in The Astrophysical Journal Letters.