After a week of rampant speculation, astronomers have officially announced the discovery of Proxima b, a potentially habitable world circling our nearest neighbouring star. But even as engineers prepare for an interstellar voyage to scope out Proxima b for signs of life, some experts warn that M dwarf systems like Proxima Centauri may be unable to support life at all. Concept of a planet glowing in response to a solar flare from an M dwarf star. Image: Jack O'Malley James
For all those wondering what exactly we'll discover on the surface of Proxima b — a barren wasteland, or a vibrant, alien biosphere? — a new scientific paper offers optimism. According to astrobiologists at Cornell University, life could in theory evolve to survive in the high-radiation environment of an active M dwarf, by converting the star's most damaging rays into harmless visible light. We know this is possible, because the mechanism — called biofluorescence — has evolved numerous times already, right here on Earth.
"The key question that will come out of [the Proxima b] discovery is how you could ever envision life on that planet," study co-author Lisa Kaltenegger told Gizmodo. "I think it's fair to use Earth as an inspiration."
Before we get ahead of ourselves, a little background on M dwarfs. Smaller, cooler and dimmer than the sun, M dwarfs are the most abundant stars in the galaxy. They're extremely long lived — Proxima Centauri already has 250 million years on our sun, and astronomers estimate it will outshine us by a few trillion more. Best of all, according to recent exoplanet surveys, M dwarfs appear to be hotbeds for small, rocky exoplanets in the habitable zone where liquid water can form. All of these factors make the galaxy's dimmest bulbs exciting places to consider in the search for alien life.
Here's the problem: Because M dwarfs are so faint, the habitable zone is very, very close to the star itself. Proxima b sits a mere 7.5 million kilometres from Proxima Centauri — nearly 10 times closer than Mercury orbits the Sun.
In such a tight orbit, a few things can happen. For one, the planet can become tidally locked, with a permanent day and night side, which could trigger strong atmospheric winds and some wild climate dynamics. ***
“What I find so amazing [about Breakthrough Starshot] is that is has become OK to talk about interstellar travel. People are seriously considering it.”
*** What's more, many M dwarf stars, including Proxima Centauri, produce powerful flares that spew bursts of harmful radiation into space daily. As the authors of the new exoplanet discovery paper note, Proxima b suffers X-ray fluxes approximately 400 times greater than what we experience here on Earth. According to Kaltenegger, Proxima Centauri's energetic outbursts are likely to deliver a lot of DNA-damaging UV radiation to the surface, as well.
Could life survive on such a world? If it lived deep underground, maybe. Unfortunately for us, that would make the chances of detecting its fingerprints from afar quite slim.
But as Kaltenegger and Jack O'Malley James argue in their new paper, which has been submitted to The Astrophysical Journal, there may be another way. Certain reef-building corals contain proteins capable of absorbing UV radiation and reemitting its energy at a longer, safer wavelengths; a process known as biofluorescence. By "downshifting" the sun's harshest rays, it is believed that corals are able to protect their symbiotic algal partners from UV damage.
Biofluorescence is not unique to coral — it's evolved independently across the tree of life on several occasions, suggesting that its adaptive advantages are pervasive.
Inspired by this observation, O'Malley James and Kaltenegger set about to determine whether a biofluorescent life form could produce a remotely detectable trace on an alien world orbiting an active M dwarf star. Using fluorescent coral proteins, they modelled the spectral signature of a planet with an Earth-like atmosphere in the habitable zone.
They found that if biofluoresence were present across the planet's surface, either in land-based life forms or shallow ocean basins, the planet would produce a distinct, short-lived biosignature during a UV-flare. Essentially, life would cause the planet to glow. "You can envision an ocean world that gets struck by a UV flare, and all of a sudden it lights up," Kaltenegger said.
It's certainly a beautiful idea — but would biofluoresence offer our radiation-hardy aliens enough protection on a world like Proxima b? Nobody has the answer, in part because we don't yet know how much UV radiation hits the planet's surface. But in laboratory experiments, bioengineers have succeeded in pushing the efficiency of UV fluorescent proteins to 100 per cent. Given a high-radiation environment, it stands to reason that natural selection might do the same.
Coral biologist and biofluorescence expert Charles Mazel emphasised that while the mechanism Kaltenegger and O'Malley James propose is plausible, an organism might also cope with high UV by dissipating the energy as heat, or by putting it to work in its cell, similar to how plants harness visible light for photosynthesis. "The fluorescence idea is one of several possible strategies," he told Gizmodo in an email. "Probable? Not possible for me to say. Possible? I suppose possible."
What's most exciting about this idea is that astronomers might be able to detect the glow of alien life on a nearby exoplanet within a few years, thanks to the next generation of extremely large telescopes. And if we do wind up catching some tantalising glimmers of activity on Proxima b? All the more reason for Breakthrough Starshot to re-route its interstellar voyage.
First announced by Stephen Hawking and Yurni Milner earlier this year, the effort to send a fleet of nanocraft to Alpha Centauri within a generation could be in for a slight course correction, depending on what we learn about the Proxima Centauri system in the years to come.
"What I find so amazing [about Breakthrough Starshot] is that is has become OK to talk about interstellar travel," Kaltenegger said. "People are now seriously considering it. I think the inspiration that comes from talking about this, and putting the technology within reach, will be an amazing thing."