Potentially habitable exoplanets may not be so habitable after all
Last year, astronomers announced the discovery of Proxima b, a potentially habitable planet orbiting Proxima Centauri, which at just four light years away is the closest star to our solar system. But a team of NASA scientists is looking to change the way habitable zones are defined by taking into account the impacts of space weather. The proposed change would likely mean Proxima b, and other exoplanets orbiting red dwarf stars, probably aren't habitable after all.
The habitable zone is the region surrounding a star in which a planetary surface can support liquid water. This is traditionally calculated based on the how much light and heat the star emits. Stars more massive than our sun that produce more light and heat have a habitable zone that is further away from the star than the Earth is from our sun, and vice versa.
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But as the interdisciplinary team of NASA scientists points out, this doesn't take into account emissions, such as X-rays and ultraviolet radiation, or stellar activity, such as flares and coronal mass ejections. The scientists say such space weather can erode the exoplanet's atmosphere, stripping it of hydrogen and oxygen molecules, the two ingredients of water.
Such effects are particularly pronounced for exoplanets orbiting red dwarfs, which are often prime targets for potentially habitable exoplanets because they are the coolest, smallest and most numerous stars in the universe. Because they are cooler than yellow dwarfs like our sun, traditional thinking puts the habitable zone much closer to the star than in our solar system. But red dwarfs are also more active than our sun, with stellar eruptions that are more frequent and more powerful than we experience on Earth.
"When we look at young red dwarfs in our galaxy, we see they're much less luminous than our sun today," says Vladimir Airapetian, lead author of the paper and a solar scientist at NASA's Goddard Space Flight Center. "By the classical definition, the habitable zone around red dwarfs must be 10 to 20 times closer-in than Earth is to the sun. Now we know these red dwarf stars generate a lot of X-ray and extreme ultraviolet emissions at the habitable zones of exoplanets through frequent flares and stellar storms."
When high-energy X-ray and extreme UV emissions from these flares hit a planet's atmosphere, they break molecules into atoms then ionize atmospheric gases, which knocks electrons free of the atoms and forms ions. Because the negatively-charged electrons are significantly lighter than the newly formed ions, they are able to escape the planet's gravity more easily and leak out into space. Along the way they attract the positively charged ions out of the atmosphere in a process known as ion escape.
"We know oxygen ion escape happens on Earth at a smaller scale since the sun exhibits only a fraction of the activity of younger stars," said Alex Glocer, a Goddard astrophysicist and co-author of the paper. "To see how this effect scales when you get more high-energy input like you'd see from young stars, we developed a model."
The team's model estimates that young red dwarf stars generate enough high-energy radiation to allow elements such as oxygen and nitrogen, which are heavier then hydrogen (the lightest element), to escape a planet's atmosphere.
"The more X-ray and extreme ultraviolet energy there is, the more electrons are generated and the stronger the ion escape effect becomes," explains Glocer. "This effect is very sensitive to the amount of energy the star emits, which means it must play a strong role in determining what is and is not a habitable planet."
According to the model, a young red dwarf would render a close-in exoplanet uninhabitable within a few tens to a hundred million years, as the loss of hydrogen and oxygen from the atmosphere would result in the planet's water supply being wiped out before the seeds of life have had a chance to sprout.
The team says the classical definition of habitability would still apply for exoplanets orbiting mature stars with a mild space weather environment, but for stars with X-ray and extreme UV emissions seven to 10 times greater than the average produced by our sun, then the new definition would apply.
Because Proxima b orbits its red dwarf host star, Proxima Centauri, 20 times closer than the Earth is to our sun, the scientists expect that the exoplanet is most likely not habitable. Rather, it is subjected to high doses of X-rays and extreme UV radiation from superflares that occur on average every two hours, which the team estimates would see the planet's atmosphere stripped of any oxygen within 10 million years. Space weather conditions would also be exacerbated by intense magnetic activity and stellar wind.
"We have pessimistic results for planets around young red dwarfs in this study, but we also have a better understanding of which stars have good prospects for habitability," says Airapetian. "As we learn more about what we need from a host star, it seems more and more that our sun is just one of those perfect parent stars, to have supported life on Earth."