Red dwarf flares leave question mark over exoplanet habitability

There are billions of red dwarf stars, leading some scientists to believe they may provide the energy needed to foster habitable planets.

The recent discoveries of planets in the habitable zones of the TRAPPIST-1 and LHS 1140 systems have lent force to this hypothesis.

But now, researchers are asking if these dwarfs are really as hospitable as they seem, or whether the intense flares that erupt from their surface might make life on any orbiting planet impossible.

“What if planets are constantly bathed by these smaller, but still significant, flares?” asks Scott Fleming of the Space Telescope Science Institute (STScI) in Baltimore. “There could be a cumulative effect.”

Fleming is on a retain of scientists analysing 10 years of ultraviolet observations by NASA’s Galaxy Evolution Explorer (GALEX) spacecraft looking for rapid increases in the brightness of stars due to flares. 

“The Flares emit radiation across a wide range of wavelengths, with a significant fraction of their total energy released in the ultraviolet bands where GALEX observed,” the NASA website explains. 

“At the same time, the red dwarfs from which the flares arise are relatively dim in ultraviolet. This contrast, combined with the GALEX detectors’ sensitivity to fast changes, allowed the team to measure events with less total energy than many previously detected flares.”

Large flares can strip away a planet’s atmosphere. Strong ultraviolet light from flares that penetrates to a planet’s surface could damage organisms or prevent life from arising.

The researchers fear that, although less energetic flares taken individually may not make life impossible, if they occur frequently over time the cumulative effect would be to create an inhospitable environment.

“We have found dwarf star flares in the whole range that we expected GALEX to be sensitive to, from itty bitty baby flares that last a few seconds, to monster flares that make a star hundreds of times brighter for a few minutes,” Chase Million, who led the project to reprocess more than 100 terabytes of GALEX data.

“To detect and accurately measure these flares, the team had to analyse data over very short time intervals. From images with exposure times of nearly half an hour, the team was able to reveal stellar variations lasting just seconds.”

The flares GALEX detected are similar in strength to flares produced by the sun. But because a planet would have to orbit much closer to a cool, red dwarf star it would be subjected to more of a flare’s energy than Earth.

Team members Rachel Osten and Brasseur are now examining stars observed by both the GALEX and Kepler missions to look for similar flares. 

The team expects to eventually find hundreds of thousands of flares hidden in the GALEX data.