Years ago I wrote a story called ‘Rembrandt’s Eye,’ using as background a planet whose foliage was predominantly red. The story, which ran in a short-lived semi-pro magazine called Just Pulp, came back to mind when the news from Caltech arrived. Researchers at the Virtual Planetary Laboratory there now believe that Earth-type worlds may have foliage that is largely yellow, orange or, as in the case of my planet, red. The green of Earth’s plant life is anything but a universal standard.
This interesting conclusion emerges from computer models designed to provide pointers for the future search for plant life on exoplanets. After all, astronomers will need to know what they might see in the spectra we’ll one day be able to harvest from space-borne observatories. Ponder everything that’s involved, from the color of the main sequence primary star to the aquatic habitats of aqueous plants. The search involves the way photosynthesis might occur under varying conditions, with the filtering effect of planetary atmospheres as a major player.
Image (click to enlarge): This graph shows the intensity of light by color (wavelength) that reaches the surface of Earth-like planets orbiting different types of stars. From hotter to cooler, the star types are F, G, K, and M. Our Sun is a G2 star (yellow line). A planet orbiting an F2 star (red line) has more blue light at the surface, whereas Earth and the K2 star planet receive more red light. Planets around M stars receive much less visible light but much more infrared light. Atmospheric gases such as ozone (O3), oxygen (O2), water vapor (H2O), and carbon dioxide (CO2) absorb light at specific wavelengths, producing the pronounced dips that astronomers might someday detect. Then in the diagram’s horizontal axis, mark the wavelengths from 0 to 0.4 microns as UV, 0.4 to 0.7 as visible, and longer than 0.7 as infrared. Credit: NASA.
Also affecting foliage color are factors like stellar flare activity, the chemical reactions stellar radiation causes in the atmosphere, the role of ozone, carbon dioxide and water vapor, the amount of water available and the quantity of light that reaches the surface. As the simulations ran, a wide variety of habitable scenarios came into play, including one that removed most of the ozone that shields against surface radiation.
Surprisingly, survivable habitats may occur even in places like this in a ‘sweet spot’ below the surface of the water, says Victoria Meadows (Caltech VPL):
“We found that the sweet spot could be up to nine meters underwater for a planet orbiting a star significantly cooler than our sun, and photosynthesis could still take place. Something with a floatation capability could be protected from solar flares and still get enough photons to carry on.”
On Earth, plants absorb blue and red light while reflecting away large amounts of green. But the dominant color on other planets depends on so many different factors in the atmosphere and the light emitted by the planet’s star that not even infrared can be ruled out for photosynthesis. Some of the more exotic landscapes of science fiction authors may yet be realized, though doubtless in ways that will continue to surprise us.
The papers are Kiang et al., “Spectral signatures of photosynthesis I: Review of Earth organisms” (abstract here) and “Spectral signatures of photosynthesis II: coevolution with other stars and the atmosphere on extrasolar worlds” (abstract), both slated to appear in a forthcoming issue of Astrobiology. Also see this NASA backgrounder.