Intense radiation from host stars could strip away the ozone layer of Earth-like exoplanets and render them uninhabitable, according to a new study presented at the EWASS 2018: European Week of Astronomy and Space Science in Liverpool, UK.
This illustration shows a red dwarf star orbited by a hypothetical exoplanet. Red dwarfs tend to be magnetically active, displaying gigantic arcing prominences and a wealth of dark sunspots. Red dwarfs also erupt with intense flares that could strip a nearby planet’s atmosphere over time, or make the surface inhospitable to life as we know it. Image credit: NASA / ESA / STScI / G. Bacon.
Astronomers now know of around 4,000 planets in orbit around other stars. A handful of these are both Earth-sized and in the habitable zones of the stars they orbit, where the temperature is right for liquid water.
But many candidate Earth-sized worlds are in orbit around red dwarf stars, much smaller and cooler than our own.
To be in the habitable zone, the planets need to be much closer to their stars than we are to the Sun.
The problem, however, is that red dwarfs can produce significant X-ray emission, and often have large flares of radiation and eruptions of particles in coronal mass ejections (CMEs).
To try to assess the risk, Dr. Eike Guenther of the Thueringer Observatory in Germany and colleagues are intensively monitoring low-mass stars where flares might take place.
In February 2018, they observed a giant flare from a red dwarf called AD Leonis, located 16 light-years away in the constellation Leo.
Also known as Gliese 388, AD Leonis has a giant planet orbiting 3 million km away, and it may have Earth-sized worlds further out in its habitable zone.
The astronomers are working to establish what the flare did to the known giant planet and any hypothetical planets further out.
Their initial results suggest the giant planet was unaffected, and that unlike similar events on the Sun, the radiation flare was not accompanied by a CME.
This is potentially good news for life further out, as CMEs are thought to have a role in stripping away the atmosphere of smaller planets.
From their monitoring, the team believes CMEs are generally less common in smaller stars.
On the other hand, X-ray radiation is dangerous.
“X-rays would cut through the atmosphere and reach the surface of an Earth-like planet,” the astronomers said.
“Life on land would be badly affected by a stellar flare and might only survive in the oceans.”
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Eike Guenther. Flares and CMEs in M-stars. EWASS 2018, abstract # 125
