What happens when we look at Earth as an exoplanet?

About 30 years ago, Carl Sagan pointed two different cameras toward the Earth, to see what we could see. On a close flyby, a different probe was able to detect the presence of water as gas, liquid, and ice; enough oxygen in the atmosphere that life was quite plausible; and, the loudest signal of all, modulated radio signals that could only conceivably be “generated by an intelligent form of life.” That life, of course, was us, as that planet was Earth, observed during a flyby of the Galileo probe on its way to Jupiter. Astronomer Enric Pallé, a research scientist at the Canaries Institute for Astrophysics, told me, “What we’re trying to understand from looking at the Earth as a planet is to know which are the observables that we can search for in exoplanets, we see on the Earth that can give us clues about what these planets are made of, and what are their atmospheres made of, and even try to infer if there is life on the surface.” This was the gist of Sagan’s 1993 Galileo paper. But research that makes Earth an exoplanet does more than let us imagine ourselves someday inhabiting a distant planet—it evokes a distant planet that’s inhabited by someone else. “Earth is my favorite planet,” Kaltenegger said, “but maybe we’re not that interesting yet.” She still feels the imagined kinship of extraterrestrial observers, though.