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Pollution may be a clear sign of life - maybe not-so intelligent - out there

In this artist’s conception, the atmosphere of an Earth-like planet displays a brownish haze - the result of widespread pollution.Christine Pulliam/Harvard-Smithsonian Center for Astrophysics

On the off chance aliens in a distant solar system are wrecking their home planet and spewing industrial pollutants into the atmosphere, we might be able to catch them in the act, according to a team of Harvard and Smithsonian scientists.

In the search for habitable worlds, scientists have been cataloging planets circling faraway stars, in search of ones in the "Goldilocks zone" that is neither too hot nor too cold for life. They also have been working to analyze the atmospheres of those planets, searching for basic chemical signatures of life such as methane or oxygen, a capability that will be greatly increased with NASA's pending James Webb Space Telescope.


But with a little bit of tongue-in-cheek, scientists have described in the Astrophysical Journal how that telescope could also be used to look for signs of "truly'' intelligent life. That means keeping an eye out for signs that aliens are shaping their environment by doing more than just exhaling and excreting; scientists can also search for byproducts of aluminum production, silicon manufacturing, refrigeration, and even hair spray.

The lead author of the paper, a rising Harvard University sophomore named Henry Lin, is quick to crack jokes about the research.

"The ironic thing, I guess, is that even though we call them a biomarker for intelligent life, I like to think of it as a biomarker for unintelligent life," Lin said. "I imagine alien civilizations, if they're trying to measure our atmosphere, will see industrial pollution and say, 'No intelligent life there.' "

The endeavor is also a serious one that he undertook in part to see if detecting the complex industrial molecules would be possible, without having to build a whole new instrument. The researchers found that the James Webb telescope is up to the job, but only for earth-like planets that are in orbit around white dwarf stars, which are the remnants of stars like our sun, and only at concentrations of the pollutants that are about 10 times greater than on earth.


To do the project, Lin had to settle on which industrial pollutants were measurable and interesting signatures of intelligence at work. Working with Gonzalo Gonzalez Abad, a Smithsonian scientist knowledgeable about industrial pollution in the earth's atmosphere, he settled on chlorofluorocarbons, or CFCs, best known because they deplete the ozone layer.

One of the CFCs that they determined can be detected sticks around in the atmosphere for a long time, making it a good marker of whether aliens have been busy building things and ruining the environment. Another type of CFC they determined is detectable only lingers for about 10 years, so finding it would be a sign that aliens were active right now.

It's easy to be dismissive of the technique, but Lin said that he has learned from his mentor, Harvard astronomer Avi Loeb, to never assume you know the answer before the experiment has been done.

"You should balance conservative science with less conservative, more risky science just because the payoffs from doing riskier science are much larger," Lin said. "Yes, it's fun. But I think it's also something that is missing now in a lot of the work we do. We're too caught up in getting funding, so we are just doing conservative things. We should also spend time doing things that are 'crazy' to some people."


Carolyn Y. Johnson can be reached at cjohnson@globe.com. Follow her on Twitter @carolynyjohnson.