With the search for extraterrestrial intelligence in an excited buzz about the – still unconfirmed – possibility of a signal from our nearest neighbor, the timing is right for a paper indicating life might stand a chance there. The publication date is sheer coincidence, but the work calls into question theories that stars like Proxima Centauri emit too many flares to support life on nearby planets.
It’s easy to assume the conditions that make Earth so well-suited to us are essential for life, but that’s not necessarily the case. The Sun is a very quiet star, its storms of high energy radiation are small compared to the galactic average. Perhaps this is why we could evolve here; larger flares and coronal mass ejections might have stripped the atmosphere of its protective ozone shield, astronomers have reasoned.
If so, that would dramatically cut the number of star systems that could host life, most notably ruling out Proxima Centauri based on a recent study of that star's activity.
Northwestern University PhD candidate Howard Chen has challenged the idea intense flare activity equals no life. In Nature Astronomy, Chen and coauthors have published modeling that predicts atmospheres of worlds exposed to frequent flares will be different from our own, but perhaps just as suited to life. Better still, they report that if life does exist around high-flare stars, it may be easier for us to detect with technology anticipated in the near future.
Using observations from nearby M-type stars (red dwarfs) such as Proxima and TRAPPIST-1, the paper models the effect of flares on the chemistry of planets in those stars' habitable zones.
"We compared the atmospheric chemistry of planets experiencing frequent flares with planets experiencing no flares. The long-term atmospheric chemistry is very different," Chen said in a statement. "Continuous flares actually drive a planet's atmospheric composition into a new chemical equilibrium." Nevertheless, this new chemical balance could still support life, even if it would poison lifeforms adapted to Earth conditions."
Flares hitting atmospheres composed like that of ancient Earth cause temporary surges in concentrations of certain gases, such as nitrogen oxide. Some of these are much easier for telescopes to detect than oxygen, and others considered top priorities to look out for. If a planet has enough life to alter its atmosphere, we may be able to see that by checking shortly after its star flares.
With red dwarfs making up the majority of stars that are close enough we may be able to study their atmospheres in the near future, Chen's work could matter far beyond Proxima Centauri. If he's right, we could have lots of places to look for atmospheres modified by living things. If he's wrong, there may be very few.