When it was first discovered in 2016, Proxima b looked like a prime spot to hunt for extraterrestrial life. Just four light-years away, this Earth-sized exoplanet orbits within the habitable zone of its red dwarf parent, Proxima Centauri, leading astronomers to suspect that liquid water, and by extension life, could exist on its surface. Unfortunately, more recent studies have painted a bleaker picture, and now the final nail in the coffin could be a huge stellar flare detected last year, which would have baked the surface of the planet.

Traditionally, the habitable zone is defined as the region around a star where temperatures are just right for liquid water to form on the surface. With the highest chance of relatively Earth-like conditions, planets within these zones are the best places to start searching for potential alien life.

Since red dwarfs are smaller and cooler than the Sun, their habitable zone is much closer to the star, and that causes some problems. Red dwarfs are more active, meaning they would bombard tightly-orbiting planets with X-rays and ultraviolet radiation, stripping away any atmosphere they may otherwise have.

The new study raises more doubts about Proxima b's already-questionable habitability. The team examined data gathered last year by the Atacama Large Millimeter/submillimeter Array (ALMA), and found that on March 24, 2017, Proxima Centauri fired off a huge stellar flare. At its peak, the flare was 1,000 times brighter than the star normally is, and 10 times brighter than the largest flares our Sun is capable of. Being so close to such a storm, Proxima b may have been hit with 4,000 times more radiation than solar flares inflict on the Earth, which is shielded by its magnetic field.

"It's likely that Proxima b was blasted by high energy radiation during this flare," says Meredith MacGregor, lead author of the study. "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."

While it sounds like an event this energetic would have been easily spotted, an earlier study interpreted the data in another way. By taking the average brightness of both the star and the flare, that team reported that Proxima Centauri was encircled by one or more large, cold dust rings, like the Kuiper Belt in our solar system.

Instead of taking the average, the new study looked at the data as a function of time, allowing them to spot the sudden spike in emission. That changes our understanding of the Proxima Centauri system.

"There is now no reason to think that there is a substantial amount of dust around Proxima Cen," says Alycia Weinberger, co-lead author of the study. "Nor is there any information yet that indicates the star has a rich planetary system like ours."

The research was published in The Astrophysical Journal Letters.

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