Worlds around red dwarf stars might build an ozone “shield” in response to stellar flares.
In the hunt for habitable exoplanets, red dwarf stars — also known as M dwarfs — make tempting targets. Not only are these stars extremely common, it’s also easier to observe terrestrial planets in their habitable zones, where they can potentially host water on their surfaces, than it is to find rocky worlds orbiting stars like our Sun. But M dwarfs are also incredibly volatile, blasting any would-be biospheres on their habitable zone planets with high-energy radiation.
However, new findings suggest that habitable worlds orbiting red dwarfs might not be completely defenseless against their systems’ turbulent stellar weather. Simulation results, to be published in the Monthly Notices of the Royal Astronomical Society, show that repeated stellar flares could build up a “shielding layer” of ozone on such planets, offering some protection from future flares.
“Planets orbiting M dwarfs are our best chance in the next 20 years for finding signs of life outside the solar system,” says exoplanet researcher Ian Crossfield (University of Kansas). While today’s telescopes aren’t powerful enough to glimpse the atmospheres of rocky worlds around stars like our Sun, he adds, “there’s a small number of planets orbiting the smallest and coolest of the M stars that we might be able to make some very interesting atmospheric measurements for.”
The question, then, is whether these worlds really could be habitable — or if there’s something about M stars that's fundamentally incompatible with supporting life. And there are reasons to wonder.
Red dwarfs are much smaller and cooler than the Sun, so their planets must orbit much closer to fall within the habitable zone. This forces those planets into a tidal lock, which keeps one side facing star-wards, sweltering, while the dark side freezes.
M dwarfs are also far more active than Sun-like stars. As frequently as once a month, they can fling out flares and particles in events as powerful as the most powerful in all the Sun’s recorded history. It's even possible that these flares, as well as violent eruptions of charged particles known as coronal mass ejections, could strip planets their atmospheres altogether.
Previous studies leave room for hope, though, since red dwarfs tend to flare from their poles, which might spare planets the worst of their blasts. And assuming that planets around M dwarfs can hold onto their atmospheres, flares would still leave a mark on their chemical composition.
To understand how flares might meddle with the atmospheres of potentially habitable M dwarf planets, researchers created a computer model of such a planet and subjected it to simulated flares. The results showed that flares could increase the amount of UV-blocking ozone in the atmosphere 20-fold.
“The surprising thing in our result was that actually ozone grows quite rapidly once we actually ‘turn on’ the flares,” says study lead Robert Ridgway (University of Exeter, UK).
In the simulations, this shielding layer cuts UV radiation from subsequent flayers by 85%, although even the reduced UV index of 55 would still be high by terrestrial standards. (Typical UV indices on Earth range from 0 to 10.)
To simplify matters, the simulations assume the planet starts with an Earth-like atmosphere full of oxygen, Ridgway said, and Earth didn’t have such an oxygen-rich atmosphere until life was already well-established. “Something we plan to look at with future work is what happens in a more anoxic environment,” which might be more realistic, he continued.
Coronal mass ejections in the simulations also built up nitrous oxide in the atmosphere. This gas, as well as ozone, is considered a possible indicator of life, or biosignature. As astronomers search for such life signs in exoplanet atmospheres, they’ll need to know how likely it is that they could be made by abiotic processes, like stellar flares, instead. Simulations like this one can help provide that information.
“The detection of life outside the solar system, if it happens someday, it’s going to be probably the single most consequential result in all of the field of exoplanets,” said Crossfield. “So it really pays to be extremely careful.”