Insane X-ray ‘superflares’ may have made Proxima b uninhabitable long ago

By Jessica Hal

The hype train for Proxima b is impressive. Pale Red Dot, the Project Blue space telescope, and the recent collaboration between the ESO and Breakthrough Starshot represent just some of the money that people have been willing to invest in the idea that there’s a livable planet in the Centauri system, or at least something that we can make livable.

But there might be some hiccups before we can pull up our space boots and march off to do some good old-fashioned colonizing. Proxima b is in orbit around Proxima Centauri, which is a red dwarf. While there’s been some encouraging news lately about how red dwarf stars might host habitable exoplanets (increasing our total pool of Goldilocks-zone contenders), NASA scientists have some cautionary words. Newly published research from the Goddard Space Center suggests that Proxima b might not be as habitable as we think. Space weather may have driven off its atmosphere long ago.
Red dwarf stars are fairly quiet neighbors, but only after a certain point in their life cycle. When they’re young, they’re much louder and wilder, and they frequently give off major CMEs and stellar flares that we can pick up here on Earth or with space telescopes. Vladimir Airapetian, a solar scientist at Goddard and lead author of the report, believes Proxima b is no exception to this rule of violent stellar youth. “When we look at young red dwarfs in our galaxy, we see they’re much less luminous than our sun today,” Airapetian said in a statement. “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 [young] 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.”
Extreme UV radiation (XUV), on the cusp between UV and X-rays, means business. The difference between UV and X-rays is supposed to be that UV radiation isn’t really ionizing, while X-rays are. But XUV rays are capable of ionizing light elements like hydrogen. And also some heavier elements: important stuff for life, like carbon, nitrogen and oxygen. Red dwarf stars put out XUV “superflares” early in their lives, and that’s bad news for the atmosphere of any exoplanet within the reach of such an event. Getting hit by a superflare can ionize an atom, breaking it up into charged particles which get flung off into space.
“During ionization, radiation strikes the atoms and knocks off electrons. Electrons are much lighter than the newly formed ions, so they escape gravity’s pull far more readily and race out into space,” NASA officials explained in the same statement. “Opposites attract, so as more and more negatively charged electrons are generated, they create a powerful charge separation that lures positively charged ions out of the atmosphere in a process called ion escape.”

This extreme space weather may have affected the habitable zone around Proxima Centauri, rendering Proxima b inhospitable to developing life and stripping it of its atmosphere. This could ensure that no extraterrestrial life had developed there, and make it very difficult for us to establish a human presence. Then again, having been thusly sterilized might just make Proxima b an aseptic, comfortable little exoplanet free of living organisms and ready for human terraforming. Potayto, potahto. It’ll probably depend on our level of technological sophistication. If we’re ready to terraform, we may not need to worry about the presence of oceans; we’ll just make our own.

Even if the XUV bombardment makes the odds a little steeper for Proxima b,  Airapetian isn’t worried. “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,” he said.

“If we want to find an exoplanet that can develop and sustain life, we must figure out which stars make the best parents,” said Airapetian. “We’re coming closer to understanding what kind of parent stars we need.”