A little over two years ago, in March 2016, a telescope array called Evryscope observed a “superflare,” or extremely strong stellar eruption, on the nearby star Proxima Centauri. The flare was so bright that it was visible with the naked eye on Earth, the first time that’s ever happened. Now an analysis by Ward Howard from the University of North Carolina at Chapel Hill and colleagues shows the devastating consequences such an event would have on the Earthlike planet circling our stellar neighbor.
The terrestrial planet Proxima b was discovered in the Proxima Centauri system less than two years ago, and speculations have abounded since that the planet might be habitable. The new study puts a damper on that idea, and may mean that life on Proxima b is all but impossible. Assuming an Earth-type atmosphere, and assuming that flares like the one seen in 2016 are common, 90 percent of the ozone layer would be destroyed within five years, and all of it would be gone within a few hundred thousand years. Without the protective ozone layer, ultraviolet light produced by such a superﬂare would reach the surface with 100 times the intensity needed to kill even UV-hardy microbes. Thus, it is safe to assume that Proxima b is not a second Earth with a large biosphere and complex life. Microbial life—or its remnants—may still be present below ground, but the surface of Proxima b is not a place for life to thrive.
Proxima Centauri, unfortunately, is expected to have produced superflares for much of its life cycle, and so is not the type of location my colleague William Bains and I envision for our Cosmic Zoo scenario—a planet or moon that remains habitable long enough for complex life to evolve. Instead, this nearby solar system appears to be the kind of galactic real estate that’s not suitable for life.
That doesn’t mean Proxima b and planets like it are not worth studying. We need to learn more about worlds that are geophysically similar to Earth, but lack life, just as Laneuville Matthieu from the Tokyo Institute of Technology in Japan and colleagues have done in a recent paper published in the journal Astrobiology. We need to understand the different evolutionary paths a planet can take—with life, and without life. How would Earth look today if life never evolved here? These kinds of questions will help us come up with irrefutable signatures of life elsewhere in our galactic neighborhood.