In the search for life beyond Earth, astronomers should look for signs of pollution in the atmospheres of alien planets outside the Earth's solar system, a new study says.

The next-generation James Webb Space Telescope, which is set to launch in 2018, could hunt for worlds harboring alien life by sniffing their atmospheres for chlorofluorocarbons (CFCs), greenhouse gases that destroy ozone in the Earth's atmosphere. These chemicals could be detected on planets with atmospheres that are 10 times thicker than Earth's, the researchers said.

Scientists already scan the atmospheres of alien worlds for traces of oxygen and methane, gases that typically coexist in the presence of life. But searching for signs of pollution elsewhere in the universe could yield clues about more-advanced alien civilizations, the researchers said. [7 Huge Misconceptions About Aliens]

Of course, to very advanced civilizations, Earth's own greenhouse gases might signal a primitive world, the scientists said.

"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," study leader Henry Lin, a student at Harvard University, said in a statement.

It's also possible that this type of search could uncover civilizations that long ago died. Some pollutants on Earth can remain in the atmosphere for up to 50,000 years. Others are more short-lived, however, lingering for only 10 years in the atmosphere. By looking for both types of pollutants, astronomers could find dead alien civilizations; if a planet has traces of long-lived pollution in its atmosphere, but signs of shorter-lived chemicals are missing, that could signal that a civilization lived there long ago.

"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," study co-author Avi Loeb, a professor in the department of astronomy at Harvard, said in a statement.

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Pollutants such as chlorofluorocarbons would likely only be visible on an Earth-like planet near a white dwarf, which is the burned-out remains of a sun-like star. Otherwise, the star's light would make it too difficult to tease out the "signal" of CFCs in the atmosphere.

Even if telescopes detect CFCs in an alien atmosphere, it's unclear if that would mean the planet itself is habitable. Still, advanced extraterrestrials may deliberately introduce CFCs or similar pollution into the atmosphere to warm the air of a planet that would otherwise be too cold for life, the researchers said.

"People often refer to E.T.'s as 'little green men,' but the E.T.'s detectable by this method should not be labeled 'green' since they are environmentally unfriendly," Loeb said.

The research is available online, and has been accepted for publication in the Astrophysical Journal.

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