On Earth, green is a symbol of life and growth, but a recent study found that photosynthesizing organisms on other planets could have color signatures like red, orange, yellow, or black—but probably not blue.
During photosynthesis, plants use the sun’s light energy to build useful organic molecules. On Earth, the plant pigment chlorophyll absorbs the most abundant and highest-energy colors that reach the planet’s surface—red and blue, respectively—while reflecting green, giving vegetation its color. But the distribution of colors reaching planets orbiting other suns could be different.
“Pigments must adapt to the spectrum of available light,” explains biometeorologist Nancy Kiang of NASA’s Goddard Institute for Space Studies.
Kiang and her colleagues surveyed diverse photosynthetic Earth organisms to learn how they take advantage of the sun’s particular blend of light. They then predicted what colors would be most useful for photosynthesis on planets surrounding a variety of star types. Stars hotter than our sun give off more blue light. On nearby planets, plants might take on a range of colors, including red, orange, and yellow, in addition to familiar green. Cooler stars—like red dwarfs, the most common stars in the universe—give off less visible light. “Because visible light is scarce but desirable, maybe those plants would be black,” Kiang says, since black objects absorb light of all colors.
Blue may be the least likely plant color on any planet, as it would mean passing up high-energy blue light, but under certain conditions, it could be possible. “Looking at how pigments might be adapted on planets around other stars makes us really appreciate how life is specially adapted to our planet and our sun,” Kiang notes. “We might not be alone, but we have to treasure the planet we have.”