If ET is anything like us, scientists may be able to ferret out advanced civilizations beyond Earth by scanning planets' atmospheres for telltale industrial pollutants.
Researchers looked at two ozone-eating chlorofluorocarbons (CFCs) that would be easy to detect with the infrared light-splitting spectrometer being built for the James Webb Space Telescope, a follow-on to the Hubble observatory that is due to launch in 2018.
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The targets would be white dwarf stars, which are much smaller than the sun. An Earth-sized orbiting planet passing by, relative to the telescope's line of sight, would block a significant amount of the parent star's light, allowing for relatively quick scans.
"We consider industrial pollution a biomarker for intelligent life," Henry Lin, a Harvard University undergraduate and Intel Foundation Young Scientist Award recipient, told Discovery News.
"However, maybe this super-sophisticated alien civilization ... would consider industrial pollution a sign of unintelligent life. After all, it doesn't seem very intelligent to pollute your own atmosphere with things that can make life difficult," he said.
Lin's advisor and research collaborator Avi Loeb, also at Harvard, offers a caveat: Perhaps ET purposely engineered or terraformed its atmosphere to make a cold planet like Mars habitable.
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"In principle, there might be a reason for why pollution is a good thing," Loeb told Discovery News.
Lin, Loeb and atmospheric chemist Gonzalo Gonzalez Abad, with the Harvard-Smithsonian Center for Astrophysics, studied two types of easily detectable CFCs -- tetrafluoromethane (CF4) and trichlorofluoromethane (CCl3F), both of which are produced by industrial processes.
Based on computer models, they estimate that planets circling in the so-called "habitable zone" of parent white dwarf stars would have a two-minute transit, relative to Earth's line of sight, every 10 hours. The habitable zones are temperature regions where water can exist as a liquid on a planet's surface. Water is believed to be necessary for life.
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The next step would be to find transiting planets and use the eclipses to capture light filtering through the planets' atmospheres. Chemicals in the atmosphere leave telltale signatures that can be analyzed for oxygen and other molecules associated with life, as well as for industrial pollutants, the scientists say.
"The advantage of this approach is that we don't have to build a new instrument to search for intelligent life," Lin said.
The research will be published in The Astrophysical Journal and appears in the online archive arXiv.org.