The search for extraterrestrial intelligence moves up a gear

IN 1990, A year into the journey to Jupiter of an American spacecraft called , Carl Sagan, a well-known astronomer, turned the probe’s instruments back towards Earth. He wanted to find out whether it was possible to detect evidence of life on the planet from a distance. took spectrographic measurements of sunlight streaming through Earth’s atmosphere and found methane and oxygen, both indicators of living processes. The probe also took photographs of Earth at different wavelengths and uncovered something called the “red edge”—a sharp change in the reflectance of the planet at red wavelengths, which Sagan ascribed to the presence of photosynthetic plant life on the surface.