Hopes that life might exist on a rocky planet orbiting the nearest star to the Sun have taken a knock after the star fired off a deadly superflare.
A telescope monitoring southern stars saw Proxima Centauri become around 70 times brighter than usual, making it briefly visible in binoculars.
Proxima is a red-dwarf (M-dwarf) companion of the bright star Alpha Centauri, just 4.2 light-years away. It normally shines at only 11th magnitude itself, needing a small telescope to see it.
The superflare, announced this week in a draft paper published by arxiv.org, was a particularly violent example of many which the star unleashes. It was observed in March 2016.
The discoverers say the star’s extreme behaviour casts doubt on whether life can exist on Proxima b, a rocky planet discovered in the so-called habitable zone around the star where surface water could exist as a liquid.
The study of Proxima Centauri’s flares was led by Ward Howard, of the University of North Carolina. His team observed with an unusual telescope called the Evryscope which is made up of an array of 7cm instruments fitted with camera lenses. The Evryscope is a UNC project led by Nicholas Law and funded by the National Science Foundation.
The Evryscope used for the study is based at the Cerro Tololo Inter-American Observatory, in Chile. It is able to observe the entire sky above the horizon simultaneously and continuously. Its designers say it can be used to search for transiting exoplanets around bright stars, M-dwarfs and white dwarfs. It can also detect microlensing events, nearby supernovae, and gamma-ray burst afterglows.
Professor Brian Cox commented on the discovery in a tweet:
Our nearest neighbouring star increased in brightness by a factor of 68 in March 2016, likely bathing its nearby rocky planet in a lethal (at least to earth-based life) dose of UV radiation. Makes you think about how fortunate we are in our gentle little corner of the galaxy! https://t.co/c9ilq0K45t
— Brian Cox (@ProfBrianCox) April 9, 2018
After measuring how often Proxima Centauri fires off flares, the team say that any Earth-like atmosphere would be stripped of 90 per cent of its ozone within five years, so that it would probably be destroyed completely within a short geological timescale.
A superflare like that observed would have probably reached the surface of Proxima b with around 100 times the intensity required to kill simple microorganisms hardy to ultraviolet light.
The team say that over the last two years, the Evryscope has recorded 23 other large Proxima flares. Coupling those rates with the single superflare detection, they predict at least five superflares occur each year.
Any life that was not protected by an ocean or by living underground would therefore struggle to survive if exposed to such superflares.
The team conclude in their paper, The First Naked-Eye Superflare Detected from Proxima Centauri, that superflares will significantly impact the habitability of planets orbiting red dwarf stars. That’s because two thirds of M-dwarfs are similarly active, and they make up the majority of stars in the Milky Way galaxy.
Incidentally, the term “naked-eye” in the paper’s title may be rather optimistic as the flare reached the limit of what only those with exceptional sight might see under pristine dark skies.
The research paper holds a glimmer of hope for life on Proxima b. It says: “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 . . . 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.”
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