Plants do not have to be green. To be sure, the vast majority of vascular plants on Earth are green because during photosynthesis (the conversion of photons of light into stored chemical energy) they absorb more of the red and blue wavelength light emitted by the sun. But in the murky depths of Earth's waters lurk photosynthetic bacteria that appear purple to the human eye, employing light in the infrared spectrum to store energy; more archaic plants—such as lichens and moss—utilize more of the blue spectrum in visible light. There are even red, shade-dwelling vascular plants. "We did a broad survey of organisms that perform photosynthesis in order to understand how light selects for photosynthesis pigments given different types of environments," says biometeorologist Nancy Kiang of NASA's Goddard Institute for Space Studies at Columbia University. "The photon flux spectrum peaks in the red, which is where chlorophyll has peak absorption."
In fact, the photosynthetic organisms of Earth are exquisitely tuned to take full advantage of the specific sunlight that filters down to the surface (and below the sea). By understanding how photosynthesis works on Earth, Kiang and her colleagues aim to predict how it might occur in alien atmospheres, potentially enabling scientists to identify planets that support life with future telescopes. "Photosynthesis is a very widespread, very successful process. It is the dominant form of life," Kiang says. "It is detectable at the global scale and you can see it from really big distances from the planet."
"It's much easier to see the effects of bacteria at a distance of 10 parsecs [1.9 x 1014 miles, or 32.6 light-years], than the effects of giraffes," adds Victoria Meadows, lead scientist at the Virtual Planetary Laboratory at the California Institute of Technology, who helped coordinate the effort.
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Based on this modeling and analysis, scientists could detect alien photosynthesis simply by measuring the light reflected from suitable alien worlds. So, for example, a cooler, dimmer but far more abundant M-type star emits photons that peak in the infrared range. If scientists detect significant absorption around that particular wavelength on a planet in its orbit, they might reasonably assume that some form of photosynthesis was taking place, Kiang argues in a paper in the current Astrobiology. Plants on that dark world might predominantly be black to absorb as much of the available light as possible, Kiang says.
Anyone training an advanced telescope on Earth would detect such a signature of life: Earth seems to suck up light in the red wavelength while sending back a large portion of the sun's infrared energy. "Plants are a lot less green than they are infrared," Kiang notes. "When satellites measure vegetative cover they look at the infrared signal of plants, which is a strong reflectance signal. They are not absorbing infrared."
It remains a mystery why most plants do not take advantage of this infrared light by absorbing it (snow algae and lichens seem to make use of it, for unknown reasons), but it does provide a clear signal to anyone watching that there is something going on that is more than what would be expected from its various constituent elements. And when sufficiently specialized telescopes reach Earth orbit in a decade or so, researchers may be able to detect similar signatures of photosynthesis on an Earth-size worlds orbiting an F type star (larger and hotter than our own G-type sun), for example.
Plants on that newly discovered planet would be predominantly orange, not unlike the artist's rendering of the grass above, Kiang says. But given the conditions here on Earth, plants find it easiest to be green. As Kiang adds: "Life here is very intimately adapted to the qualities of our home planet and the sun."
