For decades, if not centuries, stargazers of every stripe have wondered how we could possibly spot intelligent life elsewhere in the universe. After all, it’s not easy to see the telltale signs of a technological civilization from cosmic distances. Until very recently, this was a speculative question, mostly entertained by astronomers over drinks, or in their spare time after other more “serious” research was complete. But in the wake of the exoplanet revolution, this research has gone mainstream.

At a panel discussion co-hosted by the Aspen Institute and The Atlantic, several astronomers chimed in with their favorite intelligent-life-finding methods. Lisa Kaltenegger, a professor of astronomy at Cornell, and the Director of that institution’s Carl Sagan Institute, put forth the most intriguing possibility. Kaltenegger suggested that astronomers could look for Freon in the atmosphere of a distant planet. Freon is an artificial gas used in refrigeration, among other applications. An excess of Freon could indicate that the planet is home to an intelligent species, one that prefers certain foods, drinks, and industrial materials cooled.

Kaltenegger went on to say that the presence of Freon might not indicate a maximally intelligent species. Overuse of chemicals of this sort could, on the contrary, suggest that a species was needlessly destructive of its home planet, or even one that had gone extinct, prematurely. “You have to know what level to look for,” she said.

Indeed, other astronomers have suggested scanning exoplanets for “extinction signatures,” including signs of runaway nuclear wars, or rotting corpses from artificial pathogens gone amok. The search for extraterrestrial life is often depicted as a mystical, New Agey enterprise, but it certainly has its dark side.