Some parts of nature seem almost to be defined by their color—leaves are green, blood is red, and the sky is blue—but NASA scientists say the leaves on extra-terrestrial plants could be nearly any color in the rainbow, even black. And they’ve developed a model to predict plant color on other planets, which could also help us in the search for life outside our solar system.
The familiar color of our plants is not a universal property but an accident of our particular sun and atmosphere, says astrobiologist Nancy Kiang of NASA’s Goddard Institute for Space Studies. Although the sun emits visible light of every color, oxygen and ozone in our atmosphere absorb some of the blue and green light, leaving more red shining through to the surface. With so much red light bouncing around, plants evolved chlorophyll to efficiently absorb red and also blue light. “Photosynthesis is constrained by the number of photons, and there are more photons of red light reaching Earth than any other color,” Kiang says. Chlorophyll can't absorb every color, and it reflects most of the green photons, or packets of light, and those reflected photons give leaves their hue.
From the surface of Earth it's obvious enough that sunlight is rich with red, the most plentiful energy source for plants. Strong telescopes could also determine the color of sunlight reaching a planet like Earth even up to 45 light years distant, says NASA astrobiologist Victoria Meadows. By comparing light emitted directly by the sun and sunlight reflected from Earth, scientists could tell that Earth’s atmosphere has substantial ozone and oxygen, which absorb blue and ultraviolet light, but let more red light pass through. (They would also see other absorption "signatures" from atmospheric chemicals like water and carbon dioxide which are less consequential from plants' perspective.) Looking at Earth from afar Kiang says she’d predict that plants here use red light, because it's so abundant, and would pin their color as “anywhere from green to orange.”
Kiang and her colleagues have used hypothetical star-planet combinations to predict extraterrestrial plant color with some intuitively jarring results. With a cooler star than our sun, little blue light would filter through an oxygen-rich atmosphere, so plants might have to rely on invisible infrared light for photosynthesis. Because they would reflect most visible light, they might appear white to our eyes. A hotter star could instead yield red or orange plants, though blue plants would be unlikely under any scenario, Kiang says.
Extrapolating this thinking to extra-solar planets is tantalizing, but it will have to wait for powerful telescopes that can distinguish the light reflecting off a distant planet from the much brighter light of the nearby star. The European Space Agency has plans to build a such a telescope, but it won’t be ready until 2015. In the meantime, we'll have to content ourselves with dreams of fuchsia-colored palm trees in fields of orange grass.