Last updated April 4, 2018 at 12:42 pm
Even if alien life existed on some exoplanets, X-rays could have stripped the planet’s atmosphere and killed all the aliens.
Artist’s impression of a red dwarf star orbited by an exoplanet. Credit: NASA/ESA/G. Bacon (STScI)
Over the past few years there have been many announcements of the discovery of planets which could support life. In the habitable zone around stars, there have been around 4000 found which are a similar size to Earth and have a temperature suitable for liquid water.
They’re thought to be our best chance of finding alien life.
However, at a conference in the UK, an astronomer has suggested that even if life did exist on some of these planets, there’s a chance it’s already been killed off by a huge bombardment of X-rays.
The problem is that 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.
This, according to Dr Eike Guenther from the Thueringer Observatory in Germany, means they’re right in the firing line of radiation from their stars.
A coronal mass ejection from our Sun, taken on Feb. 27, 2000 by SOHO LASCO C2 and C3. A CME blasts into space a billion tons of particles traveling millions of miles an hour. Credit: SOHO ESA & NASA
In the line of fire
Red dwarfs can produce significant X-ray emission, and often have large flares of radiation.
In February this year, Guenther and his colleagues watched as a giant flare leapt from the star AD Leo, located 16 light years away in the constellation of Leo. AD Leo has a giant planet orbiting three million kilometres away (50 times closer than the Earth to the Sun), and it may have Earth-sized worlds further out in its habitable zone.
Working to establish what the flare did to the known giant planet and any hypothetical planets further out, they found that the giant planet was probably unaffected. Unlike similar events on the Sun, the radiation flare was not accompanied by an eruption of particles in so-called coronal mass ejections (CMEs).
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. As they are generally less likely to occur with smaller stars, there is less risk from them to the close-in planets.
However, the CMEs are only part of the tale. Red dwarfs also produce significant amounts of X-ray emissions, and it’s these emissions which could be dangerous to anything living on the planets.
According to Guenther’s team, the X-rays would cut through the atmosphere and reach the surface of an Earth-like planet. Life on land would be badly affected by a stellar flare and might only survive in the oceans.
“Astronomers are mounting a global effort to find Earth-like worlds, and to answer the age-old question of whether we are alone in the Universe. With sporadic outbursts of hard X-rays, our work suggests planets around the commonest low-mass stars are not great places for life, at least on dry land,” said Guenther.
The researchers are still working to refine the details of their model.
Some scientists suggest that giant radiation flares could deplete the ozone layer of a planet by 94% for two years, and that could be fatal for all life.
If they are right, then talk of ‘Earth 2.0’ may be premature.
The research was presented at EWASS 2018.
