Plants Could Still Grow Well under Alien Skies
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Photosynthesis changed Earth in powerful ways. When photosynthetic organisms appeared, it led to the Great Oxygenation Event. That allowed multicellular life to evolve and resulted in the ozone layer. Life could venture onto land, protected from the sun's intense ultraviolet radiation.
But Earth's photosynthetic organisms evolved under the sun's specific illumination. How would plants do under other stars?
Our sun is a G-type star, sometimes called a yellow dwarf. It seems like a normal star to us, but yellow dwarfs aren't that common. Only about 7% to 8% of stars in the Milky Way are G-type stars. When it comes to understanding habitability on exoplanets, we need to understand the more plentiful types of stars.
Some scientists propose that K-dwarf stars are the most optimal host stars for habitable exoplanets. They're between about 50% and 80% as massive as G-type stars, are more abundant and have stable luminosities for billions of years longer than sun-like stars.
The sun will be stable on the main sequence for about 10 billion years, while K-type stars can be stable for up to 70 billion years. Despite this, much exoplanet habitability research focuses on M-dwarfs, or red dwarfs, which may actually be far more inhospitable to life because of flaring and tidal locking.
In a new study, a trio of researchers simulated the light output from a K-dwarf star and grew two photosynthetic organisms in those conditions to see how they responded. The research article is "Observation of significant photosynthesis in garden cress and cyanobacteria under simulated illumination from a K dwarf star." It's published in the International Journal of Astrobiology, and the lead author is Iva Vilović, a Ph.D. student in the Astrobiology Research Group at the Technical University of Berlin.
Garden cress, whose Latin name is Lepidium sativum, is a common garden green used in salads, soups, and sandwiches. It's an adaptable plant that grows rapidly. The cyanobacterium Chroococcidiopsis is an extremophile known for lying dormant for 13 million years and remaining viable. It can resist radiation, desiccation, and extreme temperatures and is of interest in astrobiology.
We expect photosynthesis to play a role in astrobiology. Starlight provides the energy for organisms to synthesize organic compounds. In order to understand photosynthesis in astrobiology, we need to understand how other stars could power photosynthesis.
"Therefore, understanding any planet in the context of its stellar environment is an essential step in assessing its habitability," the authors write.
Astronomers search for Earth-like planets around sun-like stars because that's the only life we know of. They also pay special attention to M-dwarfs because they're so plentiful and are known to host many rocky exoplanets in their habitable zones. Scientists have demonstrated that photosynthetic organisms from Earth can grow under simulated M-dwarf light. But M-dwarf habitability faces a whole host of potential barriers.
In this work, the researchers focused on K-dwarfs. They lack the magnetic activity that appears to generate extremely powerful flaring on M-dwarfs, flaring so powerful that it could sterilize planets in their liquid-water habitable zone.
The habitable zones around K-dwarfs are also far enough away that planets wouldn't be tidally locked, another potential barrier to habitability that affects M-dwarfs. K-dwarfs also become habitable sooner in their lives than M-dwarfs due to their rapidly weakening FUV and X-ray fluxes.
"All things combined, K dwarfs can be considered the 'Goldilocks stars' in the search for potentially life-bearing planets," the authors write.