Earth plants could thrive under the light of an alien sun, according to a recent experiment.
A team of astrobiologists planted garden cress seeds and cultured dishes of photosynthetic bacteria under lights designed to simulate the light of a type of small, dim star called a K-dwarf. Seedlings and bacteria both grew well under the alien light — and the bacteria actually seemed to prefer alien starlight to the Earthly sunlight they evolved under. The results could eventually help astrobiologists figure out exactly what the chemical fingerprints of life in an alien atmosphere should look like.
Technical University of Berlin astrobiologist Iva Vilovic and her colleagues published their work in the International Journal of Astrobiology.
Life in an Alien Greenhouse
Vilovic and her colleagues grew their garden cress seedlings and cyanobacteria cultures under lamps designed to simulate the light from a small, dim star called a K-dwarf. These stars are slightly larger and brighter than the stars’ better known cousins, red dwarfs, but they’re still only about 45 to 80 percent as massive as our Sun. Because they burn cooler (the nuclear reactions happening in their cores produce less heat than our Sun), their light is redder than the sunlight we’re used to. K-dwarfs may be excellent places to look for alien life, so Vilovic and her colleagues wanted to see how some Earth life might do on the surface of an alien planet orbiting one.
Experiments like this one may bring astrobiologists one step closer to understanding how life — taking in carbon dioxide and exhaling oxygen — might change the makeup of a planet’s atmosphere, creating a chemical fingerprint we can use to spot a lived-on world from light years away.
Compared to seedlings and bacteria grown under solar lamps, the simulated alien samples fared surprisingly well. The cress plants’ leaves and stems turned a slightly deeper shade of green under the light of the K-dwarf, possibly because they produced a different amount of chlorophyll in response to the redder light. But the cress seedlings grew just as well under K-dwarf light as under sunlight, and even sprouted a little sooner.
To the researchers, that raises an intriguing possibility right out of science fiction: “If garden cress, which has evolved under the light of our Sun, can demonstrate growth when exposed to K-dwarf radiation, it raises the possibility that garden cress [that] evolved on an exoplanet orbit a K-dwarf star could potentially outperform Earth’s garden cress in terms of biomass production,” write Vilovic and her colleagues in their paper.
Cyanobacteria — the microbes that invented photosynthesis 2.5 billion years ago — absolutely thrived under the alien light, with colonies growing faster and thicker than their Earthling counterparts. Vilovic and her colleagues point out that the cyanobacteria species living on Earth today tend to grow in places where sunlight gets filtered through sand, or even layers of other bacteria, so longer wavelengths of light tend to be more likely to actually reach them. These bacteria may have evolved to make good use of those longer wavelengths.
Plants and photosynthetic bacteria are like little chemical factories, using energy from sunlight to power the reactions that turn carbon dioxide and water into oxygen and sugar. Vilovic and her colleagues weren’t sure whether those factories would run as efficiently under light made up mostly of shorter wavelengths than sunlight – but it turns out that cress is just as efficient at making oxygen and sugar under K-dwarf light as solar light, and cyanobacteria is even more efficient.
Red(dish) Dwarf Stars
According to Vilovic and her colleagues, K-dwarf stars are extremely underrated as potential homes for alien life.
The search for habitable worlds beyond our Solar System has focused, so far, mostly on the smallest of stars: red dwarfs, also called M-dwarfs. That’s partly because there are simply so many red dwarfs out there that the odds are in favor of at least some of them having habitable planets (about 75 percent of the stars in our galaxy are red dwarfs). It’s also easier to spot an Earth-sized planet orbiting in the habitable zone of a red dwarf than in the more distant habitable zone of a larger, brighter star like our Sun. But red dwarfs pose some challenges for life; for one thing, their habitable zone is so close to the star that we’re still not sure if it’s even possible for potentially-habitable planets in these star systems to hold onto their atmospheres. They’re also prone to dramatic flares of ultraviolet and x-rays, which could scour a nearby planet clean of life.
K-dwarfs might be the perfect compromise. They’re still small and dim enough to have the same advantages as red dwarfs when it comes to the search for planets, but they’re hot and bright enough that their habitable zones are set slightly farther away — where planets’ atmospheres are less likely to get scraped away by stellar winds. And K-dwarfs are less prone to the flares that could imperil life around a red dwarf.
What’s Next?
Vilovic and her colleagues hope other astrobiologists will run experiments similar to this one, testing different plant species from different ecosystems here on Earth: aquatic plants, woodland plants, and desert plants, for example. They also hope eventually to monitor the complex chemical makeup of the air around their test subjects, to understand whether photosynthesizing plants might exhale a slightly different mix of chemicals depending on the type of light they’re growing under.
Eventually, all of this information could be used to build computer models that predict what a planet’s atmosphere might look like if there’s plant or bacterial life growing under a particular type of alien sun, converting sunlight and CO2 into oxygen. Those models could help spot likely places to look for life, identifying the best targets for telescopes like the James Webb Space Telescope.
