The search for extraterrestrial intelligence moves up a gear

Feb 19, 2020

IN 1990, A year into the journey to Jupiter of an American spacecraft called Galileo, 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.

Galileo 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.

There was, however, a third clue—and an indicator that life not only exists on Earth, but has also developed intelligence. This came from the narrowband electromagnetic radiation that was streaming from Earth’s surface—in other words television and radio channels leaking into space. “That, as far as we know, is an unmistakable indicator of technology and an unmistakable indicator of life,” says Andrew Siemion, an astrophysicist at the Berkeley SETI Research Centre in California. “And indeed, it is the most detectable signature of life on this planet as viewed from...


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