As exoplanets transit their parent star, the light that is filtered through the atmosphere can reveal a great deal of information about its composition and thus, its habitability. While traditional chemical markers for life would include water vapor and carbon dioxide, the upcoming James Webb Space Telescope (JWST) may be able to detect an unfortunate side effect of intelligent life: pollution.
The research that would bring a new avenue in the search for extraterrestrial intelligence (SETI) was led by Avi Loeb of the Harvard-Smithsonian Center for Astrophysics (CfA). The paper has been accepted for publication in The Astrophysical Journal, but has been made available on arXiv.org ahead of print.
"We consider industrial pollution as a sign of intelligent life, but perhaps civilizations more advanced than us, with their own SETI programs, will consider pollution as a sign of unintelligent life since it's not smart to contaminate your own air," lead author Henry Lin explained in a press release.
Chlorofluorocarbons (CFCs) were widely used as refrigerants, solvents, and aerosol propellants, but have been phased out of regular use since the Montreal Protocol of 1987 because of their ozone-depleting properties. JWST, which is slated to launch in 2018, could potentially spot two types of CFCs from thousands of light-years away, assuming the atmospheric concentrations of these pollutants are ten times higher than on Earth.
"People often refer to ETs as 'little green men,' but the ETs detectable by this method should not be labeled 'green' since they are environmentally unfriendly," Loeb quipped.
In addition to needing incredibly high concentrations of CFCs in order to detect the pollution, the exoplanets will need to orbit white dwarf stars. White dwarfs occur when a star dies that isn’t massive enough to become a neutron star. Fortunately, 97% of the stars in the Milky Way fit this bill. Unfortunately, before the star becomes a white dwarf, it becomes a red giant that swells up and could very well incinerate the planet’s atmosphere.
But, assuming astronomers find a white dwarf with an inhabited planet that survived the red giant phase and is super polluted, we could very well identify a probable planet full of intelligent alien life. Maybe.
Not all CFCs are eliminated from the atmosphere at the same rate. On Earth, some pollutants are eliminated in about a decade, though others can last tens of thousands of years. If a mix of the pollutants are detected, it is likely that the source is still there. If there aren’t any short-lived molecules present, that could mean one of two things.
"In that case, we could speculate that the aliens wised up and cleaned up their act. Or in a darker scenario, it would serve as a warning sign of the dangers of not being good stewards of our own planet," says Loeb.
While the JWST has some extremely stringent requirements for spotting alien pollution, it is hoped that future telescopes will be able to detect those chemicals around stars similar to our Sun.
