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Two words on this next story - polluting aliens. We know air pollution is clogging the skies of planet Earth, but one scientist thinks Earth may be one of many polluted worlds. Alien civilizations, if they're out there, could be messing up their environment, too. And their pollution could be a way to find them. NPR's Geoff Brumfiel reports on the search for extraterrestrial smog.
GEOFF BRUMFIEL, BYLINE: Nobody knows if there's intelligent life out there in the universe. Over the years there've been plenty of ideas about how to spot little, green men.
AVI LOEB: People refer to little, green men. But ETs that are detected by this method should not be labeled as green.
BRUMFIEL: That's Avi Loeb, an astronomer at Harvard University. He's got a new suggestion based on his experience here on Earth. Scan the skies for little, brown men - the chronic polluters. Astronomers have been able to glimpse the atmospheres of planets outside our solar system for a while now. And there's a new space telescope scheduled for launch in 2018 that Loeb says, he could use.
LOEB: The idea would be that when a planet like the Earth is passing in front of its host star a small fraction of the light from the star would pass through the atmosphere and show - potentially - evidence for these pollutants.
BRUMFIEL: Certain pollutants don't occur naturally. So if astronomers saw them that would point to industrial activity on the planet. And that would indicate intelligence. Loeb has published some calculations in the exciting, September issue of the Astrophysical Journal Letters. They showed that if the new telescope looks at the right kind of star...
LOEB: The pollution will be detectable if it's 10 times bigger than in the Earth's atmosphere.
BRUMFIEL: Of course if intelligent life really was intelligent than you might expect they'd pollute less, not more. But, they might have a reason - for example they might be colonizing a cold planet and deliberately creating a greenhouse effect to warm it up. Or high levels of pollution could show that the aliens spoiled their world.
LOEB: It may indicate that we are looking at the ruins of a civilization that destroyed itself. And that would serve as an alarm signal about the risks of not being environmentally friendly.
BRUMFIEL: Either way it would prove that when it comes to making a mess we are not alone in the universe. Geoff Brumfiel, NPR News. Transcript provided by NPR, Copyright NPR.