Extraterrestrial: The First Sign of Intelligent Life Beyond Earth

by Avi Loeb

On September 14, 2020, scientists on Earth announced the first report of a possible biosignature in the atmosphere of another planet. This new potential evidence of extraterrestrial life had not been discovered near some far-off star. Rather, much like ‘Oumuamua, it had been found right next to Earth, in our own solar system. A team led by Jane Greaves from Cardiff University in the UK had tentatively discovered a chemical compound called phosphine (PH₃) in the clouds of our neighboring planet, Venus. Searching for its spectral fingerprint in absorption of light at millimeter wavelengths, they had detected signs of the gas at an elevation of about 35 miles above the planet’s surface. The surface of Venus is currently too hot for liquid water to exist there, and so its rocky terrain is, as far as we know, inhospitable to life. But at such a height, the temperature and pressure resemble the conditions in Earth’s lower atmosphere—raising the distinct possibility that microbes could live inside liquid droplets suspended in the atmosphere of Venus. On Earth, phosphine is a product of life. And as of this writing, no alternative chemical pathways have been identified for producing phosphine at the detected levels in the Venusian atmosphere. This potential discovery galvanized the astronomy community much as did the sighting of ‘Oumuamua almost exactly three years earlier. As then, a flurry of calculations was inspired in my research group by the initial announcement. For instanc...