Looking for life? Look for pollution.

Four-thousand years on Earth and we’ve a lot of dirt to show for it. Why would an advanced alien civilization be any different?

That’s the motivation that three astrophysicists from Harvard University have used to determine that powerful telescopes could look for signs of chlorofluorocarbons (CFCs) in alien atmospheres as signs of alien civilization.

“If the civilization reaches an industrial revolution similar to ours, then the chances are high” of finding CFCs in their atmosphere, Avi Loeb, a member of the study from the Harvard-Smithsonian Center for Astrophysics, told Is Nerd. “In our paper, we are demonstrating the detectability of the related signal if industrial pollution exists in the atmosphere of a planet.”

The team have estimated that NASA’s upcoming James Webb Space Telescope could look for signs of tetrafluoromethane (CF4) and trichlorofluoromethane (CCl3F) in alien atmospheres with a few days’ exposure. However, this is a high opportunity cost for such a powerful telescope, so the trio propose looking for these CFCs if biomarkers like molecular oxygen are found first.

While oxygen, alongside methane and nitrous oxide, points to the possible existence of primitive life, CFCs are almost exclusively anthropogenic.

Absorption spectroscopy

The space telescope will be studying starlight that has passed through an exoplanet’s atmosphere. Molecules of CF4 and CCl3F will absorb photons of light of specific wavelengths, casting a shadow. Astronomers then match the shadows with the molecules.

The team’s pre-print paper says their technique would be suitable for Earth-like exoplanets orbiting white-dwarfs. This is because photons of the wavelengths absorbed by CF4 and CCl3F are available in sufficient quantities from the star. On the downside, methane and nitrous oxide also absorb light along similar wavelengths as CF4, and oxygen and water along similar wavelengths as CCl3F.

Nevertheless, they find that the James Webb Space Telescope could detect the presence of high CF4 and CCl3F concentrations in 3 and 1.5 days respectively. An advantage of looking for CFCs like CF4 is, according to their pre-print paper, its longevity. “[Given] the half-life of CF4 in the atmosphere is [about] 50,000 years … it is not inconceivable that an alien civilization which industrialized many millennia ago might have detectable levels of CF4,” they write.

NASA plans to launch the space telescope, successor to the Hubble, in 2018. By then, it will be one of the next generation of telescopes (diameter in the range of 24-40 meters), each of which could look for signs of alien civilization using the Harvard team’s technique. “They include the Giant Magellan Telescope, the European Extremely Large Telescope, and the Thirty Meter telescope,” Dr. Loeb said.

Of them, the Giant Magellan is planned to a have a dedicated instrument called G-CLEF with exceptional spectroscopic capabilities, he added. Construction for the Extremely Large Telescope began last week in Chile.