X-rays could sterilise alien planets in otherwise habitable zones

April 3, 2018, Royal Astronomical Society
Artist’s impression of a red dwarf star orbited by an exoplanet. Credit: NASA/ESA/G. Bacon (STScI)

Intense radiation could strip away the ozone layer of Earth-like planets around other stars and render them uninhabitable, according to a new study led by Dr. Eike Guenther of the Thueringer Observatory in Germany.

Dr. Guenther sets out the work in a presentation on 3rd April at the European Week of Astronomy and Space Science in Liverpool.

Astronomers now know of around 4000 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 , 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 so-called coronal mass ejections (CMEs).

To try to assess the risk, Guenther and his collaborators are intensively monitoring where flares might take place.

In February 2018, they observed a giant flare from the star AD Leo, located 16 light years away in the constellation of Leo. AD Leo has a giant planet orbiting 3 million kilometres away (50 times closer than the Earth to the sun), and it may have Earth-sized worlds further out in its .

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 believe CMEs are generally less common in smaller stars.

On the other hand, the X-ray radiation is dangerous. According to Guenther's team, these 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.

Guenther states: "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 around the commonest low-mass are not great places for life, at least on dry land."

The next stage for the research group is to refine the details of their model. Some scientists suggest that giant flares could deplete the of a planet by 94% for two years and could even be fatal for all life. If they are right, then talk of 'Earth 2.0' may be premature.

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not rated yet Apr 03, 2018
What would an ozone layer do close to a red dwarf? If the dwarf is stable (probably not existing as such) then it would emit very little UV. If it is not stable it would kill the ozone layer).

Now this may not be so important if instead of an ozone layer there would be a tholin haze keeping direct radiation from reaching the ground. I don't know how well it would work with X-rays though. And then life would have to be different as oxygen would be really toxic to such a planet.

But then... red dwarfs are there to stay for thousands of billions of years, plenty of time for life much different than the kind we know of to develop.
5 / 5 (1) Apr 03, 2018
What exactly would a lack of ozone layer mean to, say, subterranean or deep sea life? Probably not much. All it might do is sterilize the top layer of oceans and prevent land based life from forming. Life is not limited to 'surface dwelling'. This is always forgotten when talking about 'habitable zones'

I think the entire term 'habitable zone' is just so much BS...anywhere where there is a source of energy (and maybe liquid water) we should at least entertain the notion that there could be life. This can be from tidal heating or radioactive decay from within a rocky planet as well as due to trapped heat owing to a Venus-style atmosphere (though probably a bit less corrosive than on Venus would be preferable). Suns are not the only source of available energy in the universe.

I would not be totally shocked if we found life beneath the ice crusts of some of the moons within our solar system (way outside the 'habitable zone')
1 / 5 (1) Apr 04, 2018
Unless the atmosphere is some sort of gaseous lead? The x-rays projected by red dwarf stars would sterilize the surface of any close orbiting planets.

An improbable possibility of micro-organisms surviving deep underground. But in such an enclosed environment? I doubt if it would ever evolve beyond the simplest slime.

As for life existing on a deep ocean floor? It is true that there are micro-organisms that feed upon the output of Earth's ocean volcanic vents.

However, that Earth deep ocean ecology thrives because of a constant rain of nutrients from the upper ocean levels.

For red dwarf planets? The lack would certainly effect how complex such ocean dwellers could get.

As with the deep ground life, deep ocean life would lack a practical reason to evolve beyond slime. Without survival pressure from competing lifeforms.

It seems impractical to waste limited public funding & resources exploring those worlds. That results in predictably repetitious results.

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