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Red dwarf superflares batter formerly 'habitable' exoplanet

Kepler-438b: Decidedly less livable than previously thought

Kepler-438b takes a pasting. Image by Mark A Garlick / University of Warwick

Humanity may have to rethink colonisation plans for exoplanet Kepler-438b, since what was hailed earlier this year as a candidate to support life as we know it, has now been declared decidedly inhospitable.

The planet - a probably rocky world with a diameter 12 per cent bigger than Earth - was identified back in January, circling red dwarf Kepler-438. It's in the habitable zone around its star, with a 70 per cent chance that liquid water could theoretically exist on its surface.

However, just as we were all packing our bags for the 470 light-year jaunt to Kepler-438, killjoys from the University of Warwick's Astrophysics Group have warned that regular "superflares" - and associated coronal mass ejections (CMEs) - from the red dwarf have most likely stripped away any atmosphere and continue to bombard the planet with extremely unhealthy levels of radiation.

Lead researcher Dr David Armstrong elaborated: "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."

It probably doesn't help that Kepler-438b is much closer to its star than we are to our own sizzling ball of gas. That this is the case and it's still in the habitable zone, is because the red dwarf is much smaller and cooler than the Sun.

Confirming that this is not a healthy orbit, Chloe Pugh, of Warwick Uni's Centre for Fusion, Space and Astrophysics, said: "Coronal mass ejections are where a huge amount of plasma is hurled outwards from the Sun, and there is no reason why they should not occur on other active stars as well.

"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.

"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."

The superflares erupting from Kepler-438 are "approximately ten times more powerful than those ever recorded on the Sun", and each unleashes energy "equivalent to 100 billion megatons of TNT", the researchers note.

The Warwick University research, entitled The Host Stars of Kepler’s Habitable Exoplanets: Superflares, Rotation and Activity, will appear in the Monthly Notices of the Royal Astronomical Society. ®

Bootnote

In July, NASA declared it had discovered "the closest thing yet to another Earth" - Kepler-452b. Once again, scientists put the kibosh on the prospect of colonisation by noting that the Kepler-452 solar system's extreme age - six billion years - meant the planet is probably suffering as its venerable G-type star increases its energy output en route to expanding into a red giant.


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