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.
