In search for alien life, purple may be the new green

From house plants and gardens to fields and forests, green is the color we most associate with surface life on Earth, where conditions favored the evolution of organisms that perform oxygen-producing photosynthesis using the green pigment chlorophyll a.

But an Earth-like planet orbiting another star might look very different, potentially covered by bacteria that receive little or no visible light or oxygen, as in some environments on Earth, and instead use invisible infrared radiation to power photosynthesis. Instead of green, many such bacteria on Earth contain purple pigments, and purple worlds on which they are dominant would produce a distinctive "light fingerprint" detectable by next-generation ground- and space-based telescopes, Cornell scientists report in new research.

"Purple bacteria can thrive under a wide range of conditions, making it one of the primary contenders for life that could dominate a variety of worlds," said Lígia Fonseca Coelho, a postdoctoral associate at the Carl Sagan Institute (CSI) and first author of "Purple is the New Green: Biopigments and Spectra of Earth-like Purple Worlds," published April 16 in Monthly Notices of the Royal Astronomical Society: Letters.

"We need to create a database for signs of life to make sure our telescopes don't miss life if it happens not to look exactly like what we encounter around us every day," added co-author Lisa Kaltenegger, CSI director and associate professor of astronomy in the College of Arts and Sciences.