Have Carl Sagan and astrobiologists been too hard on stellar flares? A new study imagines an alternative scenario where they power photosynthesis around some of our nearest exoplanet neighbors.
An artist’s impression of a habitable exoplanet orbiting a red dwarf. Image credit: Sci-News.com / NASA.
Stellar flares are brief, enormous outbursts of electromagnetic radiation from stars that can last anything from a few minutes to hours.
They occur when energy, stored in twisted magnetic fields, usually above sunspots, is suddenly released.
Our Sun’s largest flares are the most powerful magnetic events in the Solar System, emitting enhanced levels of radiation across the electromagnetic spectrum, from harmless radio waves, to higher energy gamma rays, the penetrating radiation used to kill bacteria on medical equipment.
It is the amount of this later radiation thrown out by flares that worries astrobiologists, especially those looking for life around cooler M-dwarf stars, where habitable zone planets orbit far closer in than the Earth circles our Sun.
“Flares have previously been regarded only as destroying life. No-one had anything positive to say about them,” says University of Delaware’s Professor Dermott Mullan.
However, flare’s life ending reputation has recently been questioned. Last year, Sukrit Ranjen at Harvard saw their heightened UV emissions as a useful spark in the origin of life and the formation of DNA. It was this work that inspired Mullan, an astrobiology lecturer, to look more closely at flares in light of another bugbear of his concerning life around M-dwarfs.
“All photons from M-dwarf stars are in the infrared where the energies are not enough to drive photosynthesis,” says Mullan.
However, flares produce a significant increase in higher energy visible light photons, which are the most effective in driving photosynthesis.
“If they could use UV photons to aid the formation of life, could I use the visible photons also associated with flares to also deliver a positive benefit for life?” asked Mullan.
To find out, he modeled M-dwarf flare output against the fundamental energy requirements of the most common and successful photosynthesis reactions on Earth — those within chlorophyll. He looked at the energies of light that can be absorbed by chlorophyll, and used to strip electrons from water, the first step in the reaction.
Their results, which will be published in the Astronomical Journal, suggest that the typical effectiveness of photosynthesis on an M-dwarf exoplanet can be as low as 1% of our own planet’s ability to harvest the energy of the Sun. This should call into question the ability of M-dwarfs to support biospheres, however during a flare, this value increases up to as much as 50-60% of Earth values.
In contrast to what Mullan describes as the pessimistic conclusion of Sagan regarding the negative effects of stellar flares, and in particular their UV flux, on the chance for life, Mullan suggests that the opposite conclusion may be worth exploring considering their valuable visible light output.
“It is almost as if the optical spectra of M-dwarf flares are (in a sense) tuned to exploit optimally the wavelength-dependent absorbances,” writes Mullan in his paper of the chlorophyll photosynthesis reactions.
Mullan’s calculations are based on individual flares lasting up to 10s of hours. However evidence from the Kepler satellite suggests that periods of increased stellar activity and flares, akin to the 11 year cycle of increasing and decreasing activity from our own Sun, are common for M-dwarf stars.
Whilst individual flares might not provide sufficient time for any exoplanet biosphere to take advantage, Mullan believes these cycles could provide a summer like peak for alien biospheres, driving ‘seasonal’ cycle of death and rebirth that would replace our annual calendar of winter and summer.
Owen Lehmer from the University of Washington, who wasn’t involved in the paper, points out Mullan’s work does not address the original concerns of Sagan and flares ability to rapidly degrade planetary atmospheres.
“However, if a planet was able to remain habitable under such conditions it seems plausible that the flaring events could play a role in driving biological cycles,” he notes.
“A future study looking at how photosynthetic organisms could remain dormant for extended periods of time then rapidly adjust to the flux from flares would be interesting,” adds Lehmer, who also wonders how an organism could be tuned to operate optimally across several orders of magnitude of flare fluxes.
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D.J. Mullan & H.P. Bais. 2018. Photosynthesis on a planet orbiting an M dwarf: enhanced effectiveness during flares. AJ, in press; arXiv: 1807.05267
