Since its discovery was announced in the summer of 2016, the exoplanet Proxima b has been the darling of post-Earth dreamers hoping to find new homes for humanity (within a reasonable cosmic commute). Proxima b seemed like the perfect fit—it is Earth-sized and orbits within the habitable zone of the closest star to the Sun, the M-class red dwarf Proxima Centauri, which is four light years distant. These bonuses have made it a central target of the $100 million Breakthrough Starshot mission, funded by billionaire Yuri Milner, which aims to send tiny spacecraft to other star systems.
But from the get-go, many scientists have raised red flags about the potential habitability of Proxima b, focusing in particular on the proclivities of red dwarfs. Though these stars outlive stars like the Sun for billions of years, they also commonly barf out highly energetic flares capable of irradiating nearby planets into sterility.
Now, research led by Meredith MacGregor, an astronomer at the Carnegie Institution for Science, confirms this hunch with a detection of a colossal flare from Proxima Centauri that occurred on March 24, 2017. In a paper published Monday in the Astrophysical Journal Letters, MacGregor and her co-authors present observations of the event, recorded by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.
Though this blast of radiation did not exceed two minutes, MacGregor’s team found it caused Proxima Centauri to shine 1,000 times brighter at its peak than during its periods of “quiescent emission,” meaning its normal, dormant phases.
"It's likely that Proxima b was blasted by high energy radiation during this flare," MacGregor said in a statement. "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."
Space weather in our solar system poses threats too, especially as civilization becomes more dependent on electronic infrastructure vulnerable to solar flare damage. But despite Proxima Centauri’s modest stature—its about 12 percent as massive as the Sun—the dwarf is hundreds of times more magnetically active than our star, leading to exponentially more devastating flares. The March 2017 event, for instance, is estimated to have been ten times more powerful than the biggest eruptions witnessed from the Sun.
“M dwarf stars have much stronger magnetic fields than the Sun, which likely has something to do with their strong flaring activity,” MacGregor told me in an email. “We think that stars' magnetic fields originate in what's called the 'convective zone' of the star, where ionized gas or plasma rises to the surface. These moving charged particles generate magnetic fields.”
“The convective zone occupies only a small fraction of the Sun,” she added, “but, an M dwarf like Proxima Centauri is thought to be entirely convective, which implies a much stronger magnetic field and thus much more activity.”
The new findings cast doubt on Proxima b’s nascent image as the leading candidate for the first exoplanetary human colonies, and tempers some of the high expectations about the hospitality of red dwarf systems as a whole.
“While this result doesn't entirely rule out the possibility of life in M dwarf systems (it is only one event after all), it does raise some serious questions,” MacGregor told me. “Since ALMA only observed the star for 10 hours and detected this flare, it is likely that massive flares like this could occur frequently for stars like Proxima Centauri.” She also pointed out that because Proxima b orbits 20 times closer to its star than Earth does to the Sun, damage from flares would be that much more extreme.
Still, the Proxima Centauri system remains the focus of intense research and speculation, and it may even contain planets that orbit at safer distances than Proxima b. And regardless of its capacity to bear life, it would be premature to remove our closest neighboring star from humanity’s bucket list of interstellar destinations.
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