Alien Foliage Could Be Yellow, Red

April 12, 2007 — If you want to detect life on a planet orbiting another star, you first need to know what it looks like from afar. That's what a interdisciplinary team of researchers is trying to figure out, by comparing the light-spectrum signatures of plants here on Earth to those that might be found on various kinds of exoplanets.

"Photosynthesis creates global-scale evidence of life," said Nancy Kiang, a bio-meteorologist at NASA's Goddard Space Flight Center. "Vegetation has a different spectra from, say, bare rock."

But it's not as easy as looking for green, she told Discovery News. Even the plants of Earth come in a wide variety of colors, which represents the colors of sunlight they are reflecting, or not using and throwing away. What's more telling is what wavelengths of light plants are absorbing to perform photosynthesis.

For Earth plants, the most useful colors of sunlight are in the red and blue ends of the spectrum, said Kiang. The greens, yellows and oranges are reflected. The reason is the wealth of red light that reaches Earth's surface.

Blue light, on the other hand, is less abundant, but bluer photons pack more wallop than any other visible light, so they are more effective at driving photosynthesis.

One side benefit of this work, which has been written up in two papers in the latest issue of the journal Astrobiology, is that we finally have the answer to the question "Why are plants green?" said Kiang.

If we an assume foliage on other planets evolved as efficiently as Earth plants, it's pretty unlikely that they would reflect blue light, said Kiang, since that's the most valuable and powerful visible light.

On the other hand, planets around bluer or redder stars than the sun may have a smaller range of light available to them for photosynthesis, she said.

"We wanted to see what are the plausible wavelengths of light for photosynthesis," said Kiang.

To do this they took the spectral patterns of a variety of bluer or redder stars, and ran them in a computer simulation of an Earth-like exoplanet. After the starlight had modified the virtual atmospheres of the virtual exoplanets, the researchers looked at what starlight could reach the surface for use in photosynthesis.

"You have to get the right mix of photons" to get enough energy for photosynthesis, said photosynthesis researcher John Raven at the University of Dundee in the U.K. Raven and another colleague took the initial foray in the exo-foliage question a few years ago. The NASA team's efforts, however took the matter much further, he said.

"I was quite pleased with it," Raven said.

The NASA team concluded that the most likely colors reflected by alien plants would be yellow, orange or perhaps red, depending on the star they orbit. Green is another possibility, of course.