It was hailed as the most habitable planet ever found beyond the solar system. But now the distant world of Kepler 438b has started to look decidedly less inviting.

Astronomers at the Harvard-Smithsonian Center for Astrophysics unveiled the small, rocky world circling the star Kepler 438 in January this year. A touch larger than Earth, in an orbit that keeps it warm enough for liquid water, but not too warm, the planet looked ripe for life.



But astronomers at the University of Warwick have found a fly in the ointment that might have been disastrous for any alien organisms looking to gain a foothold. Their measurements show that the surface of Kepler 438b is regularly blasted with radiation, enough to strip away its atmosphere and leave it bare and lifeless.



“Unlike the Earth’s relatively quiet sun, Kepler 438 emits strong flares every few hundred days, each one stronger than the most powerful recorded flare on the sun. It is likely that these flares are associated with coronal mass ejections, which could have serious damaging effects on the habitability of the planet,” said David Armstrong, an astrophysicist who studied the world.



“If the planet, Kepler 438b, has a magnetic field like the Earth, it may be shielded from some of the effects. However, if it does not, or the flares are strong enough, it could have lost its atmosphere, be irradiated by extra-dangerous radiation and be a much harsher place for life to exist”.



The superflares that erupt from Kepler 438, a red dwarf star, release as much energy as 100bn megatons of TNT every few hundred days. But the coronal mass ejections associated with the flares are more damaging for the prospects of life on the world.



“The likelihood of a coronal mass ejection occurring increases with the occurrence of powerful flares, and large coronal mass ejections have the potential to strip away any atmosphere that a close-in planet like Kepler 438b might have, rendering it uninhabitable,” said Chloe Pugh, a researcher on the study which appears in Monthly Notices of the Royal Astronomical Society.



“With little atmosphere, the planet would also be subject to harsh UV and X-ray radiation from the superflares, along with charged particle radiation, all of which are damaging to life,” she added. If the atmosphere survived, it could keep temperatures on Kepler 438b averaging about 60 Celsius.



Kepler 438b completes an orbit around its star every 35 days, making a year on the the surface pass 10 times faster than on Earth. It was discovered alongside a handful of other planets by Nasa’s Kepler space telescope which detects alien worlds as they move across the faces of their stars.