Amid much excitement in 2016, astronomers revealed the discovery of an Earth-sized planet around Proxima Centauri, the star closest to our Sun. This exoplanet, just 4.2 light years from Earth, was close enough to its red dwarf star that water might well exist on its surface.

Alas, now we know that life probably does not live on the planet, at least not on the surface. In March 2016, astronomers using an array of telescopes known as Evryscope observed a "superflare" 10 times larger than any previous one detected from the red dwarf star. Based on these observations and those of other instruments with spectrographs, the astronomers determined that about five of these superflares occur in a given year.

In an unpublished paper that describes their use of a model for interactions between the flares and a planetary atmosphere, the astronomers suggest such extreme solar activity would reduce the ozone of an Earth-like atmosphere by 90 percent within just five years, with complete depletion occurring within a few hundred thousand years. This means that ultraviolet light observed in the recent superflare reached the surface with 100 times the intensity needed to kill even microbic life that is resistant to UV light.

"Recent results have suggested that some more complex life such as lichens evolved for extreme environments and with adaptations such as UV-screening pigments may survive these radiation levels," the astronomers write in a journal article preprint. "This suggests that life on Proxima b will have to undergo complex adaptations to survive, even if the planetary atmosphere survives the long-term impact of the stellar activity."

That's too bad, because Proxima b has some intriguing characteristics. The planet is probably rocky, rather than a gas giant, and it's tidally locked to its star, meaning one side is always sunny while the other is perpetually gloomy. Therefore, if the planet does have an atmosphere, the surface temperature may vary from +30 degrees Celsius on the light side to -30 degrees Celsius on the dark side. These conditions would allow water, if the planet has any, to exist on the surface of the world as a liquid at some locations.

The arXiv. Abstract number: 1804.02001 (About the arXiv). Submitted to AAS Journals.