Consider the possibility of alien plants. After all, plenty of exoplanets likely have conditions friendly to the development of plants, even if evolution there never makes it as far as complex organisms and animals. But if moss, algae, and lichen envelop lush exoplanets in the faraway realms of the Milky Way, those worlds and the stars they circle could be completely different than our own. Extraterrestrial flora could be nothing like we’ve ever seen before.
Most of the rocky exoplanets discovered so far orbit red dwarf stars, the most abundant type of star in the galaxy. They give off fainter, redder light than the sun. “It’s natural to ask, if photosynthesis happens in a range of visible light— 400 to 700 nanometers—and you take a star that’s fainter, cooler, and redder, is there enough light to support photosynthesis?” says Thomas Haworth, a physicist at the Queen Mary University of London. His tentative answer to that question, recently published in the Monthly Notices of the Royal Astronomical Society, is a “yes, sometimes.” His team’s conclusion, that conditions around red dwarf stars aren’t a deal breaker for life, is encouraging. But life might have adapted very differently to the light of redder suns.
Most plants on Earth, including leafy vegetation, mosses, and cyanobacteria, use photosynthesis to turn sunlight and carbon dioxide into energy and oxygen. Plants use chlorophyll pigments to transform that solar energy into chemical energy. Chlorophyll gives plants their green color, and it’s tuned to absorb sunlight in the part of the spectrum that goes from violet-blue to orange-red. But astrobiologists have noted that there’s a “red edge” for vegetation, meaning that chlorophyll doesn’t absorb many photons at longer, redder wavelengths beyond 700 nanometers. Those are precisely the wavelengths at which these small red dwarf stars give off most of their light. That seems to pose a problem for photosynthetic species.
So along with his colleague, biologist Christopher Duffy, Haworth tried to envision how extraterrestrial photosynthesis might work, even under unusual conditions. “We wanted to develop a general model of photosynthesis that wasn’t tied to any particular species,” Duffy says. In particular, they modeled light-harvesting antennae—pigment-protein complexes that all photosynthetic organisms have—which collect photons and channel the light energy down to a reaction center that carries out the photochemistry needed to turn it into chemical energy.
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