The Sun’s closest neighboring star, Proxima Centauri, might not be as welcoming as we believed — a team of astronomers have detected a flare so powerful from the star that it throws the habitability of its system into serious doubt.

Proxima Flaresauri.

Artist’s impression of a flare from Proxima Centauri.
Image credits Roberto Molar Candanosa / Carnegie Institution for, NASA/SDO, NASA/JPL.

A team of astronomers led by Meredith MacGregor from the Carnegie Institution for Science discovered the flare while reanalyzing recordings taken last year by the Atacama Large Millimeter/submillimeter Array (ALMA), an array of 66 radiotelescope antennas nestled in the Atacama desert.

Now, by their very nature, solar flares are some of the most violent and energetic events we know of. Think of them as magnetic short-circuits in a star. What happens during a flare is that ebbs and flows in a star’s magnetic field start accelerating electrons (negatively-charged particles) close to the speed of light. Enough of these build up that they start interacting with stellar plasma (highly electrically charged atoms), ripping it out of the star, causing it to erupt. This eruption can be seen across the electromagnetic spectrum.

And that’s where the bad news starts: even by solar flare standards, what the team discovered was humblingly violent. The flare they detected from Proxima Centauri was over 10 times brighter, at its peak, than the largest flares that we’ve ever recorded from the Sun at similar wavelengths.

“March 24, 2017 was no ordinary day for Proxima Cen,” said MacGregor.

The flare increased Proxima Centauri’s brightness by a factor of 1,000 over 10 seconds. It was also preceded by a smaller flare. Taken together, the event lasted for under two minutes — which would explain why nobody noticed them in the first place. For context, ALMA observed the star for over 10 hours between January and March of last year, when the flares erupted.

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We knew from previous observation that Proxima Centauri was prone to regular bouts of flares, although they were much smaller and emitted chiefly in the x-ray spectrum. However, the findings now cast a lot of doubt on the habitability of the exoplanet Proxima b, which up to now raised a lot of interest as a potentially habitable planet. Proxima b orbits its star around 20 times closer than the Earth orbits the Sun, so flares of this magnitude are a huge problem. The team estimates that a flare 10 times larger than a major solar flare would drench the planet with 4,000 times more radiation that Earth gets from a solar flare. That’s enough to raise literal hell on the planet, the team explains:

“It’s likely that Proxima b was blasted by high energy radiation during this flare,” says MacGregor.

“Over the billions of years since Proxima b formed, flares like this one could have evaporated any atmosphere or ocean and sterilized the surface, suggesting that habitability may involve more than just being the right distance from the host star to have liquid water.”

So it might be healthier to steer away from Proxima b until we find a way to accurately predict, and then successfully weather, these flares.

The findings also allowed the team to get a better image of the Proxima Centauri system, and infirm previous estimation that it contains large bodies of dust and larger particles, similar to our asteroid belt.

The paper “Detection of a Millimeter Flare From Proxima Centauri” has been published in the journal Astrophysical Journal Letters.

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