When looking for aliens, try finding their pollution

Fluorinated gases in exoplanet atmospheres might be detectable by future telescopes

SMOG ALERT  A brown haze envelops a rocky planet orbiting a white dwarf star, in an artist’s illustration. Future telescopes might be able to detect such pollution from afar.

Christine Pulliam/CfA

In the search for “little green men,” astronomers may want to focus on extraterrestrials that aren’t too green. A team suggests looking for industrial pollution in the atmospheres of distant planets as a signpost for intelligent life – assuming, of course, that pollution is a hallmark of intelligence.

As the tally of known planets beyond our solar system grows –it’s up to about 1,800 – the search for life beyond Earth drifts away from science fiction and toward the realm of possibility. Radio astronomers routinely scope out newfound solar systems for hints of engineered signals. Observations at other wavelengths of light are revealing the environments on these distant worlds.

NASA’s James Webb Space Telescope, scheduled to launch in 2018, will be able to sniff out in detail the chemical makeup of exoplanet atmospheres. The space observatory will do this by looking for planets that cross in front of, or transit, their stars as seen from Earth. During a transit, some of the light from the host star grazes the planet’s atmosphere, where molecules imprint their signature on the light’s spectrum.

Finding the signatures of chemicals such as oxygen and methane in alien skies would flag a planet as potentially life-bearing. These molecules don’t last long and need something (or someone) to constantly churn them out. And plants on Earth leave a distinct mark on infrared light that could be detected from many light-years away. But just because an organism breathes doesn’t mean it’s intelligent.

Henry Lin, an undergraduate at Harvard, calculated that the Webb telescope might be able to detect other chemicals that could point to the presence of not just life, but technologically savvy life. He suggests the infrared telescope could look for fluorinated gases, some of which are byproducts of aluminum and semiconductor processing on Earth. These chemicals also absorb certain frequencies of infrared light, which makes them potent insulators that contribute to global warming. For Lin’s purposes, that also makes them detectable from afar. “The molecules that are worst for our own atmosphere,” says Lin, “are easier to detect.”

Lin and astrophysicists Gonzalo Gonzalez Abad and Abraham Loeb, also of Harvard, wondered whether the instruments and techniques that are used to hunt for biomarkers could instead “target not just things that breathe, but more sophisticated life forms … like polluters.” Because these observations take a long time – a few dozen hours – Lin envisions looking for industrial waste only once a planet’s atmosphere is confirmed to contain molecules such as water and oxygen. Their research, posted July 21 at arXiv.org, will appear in Astrophysical Journal Letters.

The notion is pretty speculative and there are some caveats. First, the technique is best suited to Earth-sized planets cuddled up close to the cores of long-dead stars, called white dwarfs, which are very compact and have cooled to roughly the same temperature as the sun. Because white dwarfs are roughly the same size as Earth, a transiting Earth-sized planet will block a large fraction of the starlight, making the planet easier to detect.

While a dead star might not sound like ideal real estate, white dwarfs are remarkably stable and can provide a steady environment in which life might thrive for many billions of years, providing plenty of time for a civilization to industrialize.

Second, the technique works only if the concentration of fluorinated gases is about 10 times as high as it is on Earth. Because some molecules hang around in the atmosphere longer than others, Lin speculates that researchers might be able to distinguish between a society that is actively polluting and one that has learned the error of its ways. On a more somber note, finding fluorinated gases on a world without oxygen – a world where no one is breathing – could be a warning: Maybe some species never learn.

“It would be very interesting to find pollutants on other planets,” says Lisa Kaltenegger, an astronomer at Cornell University, “and see if all intelligent civilizations make the same mistakes.” However, using Earth’s atmosphere to interpret the environment of planets around white dwarfs is challenging, she adds. These planets have survived the death of their sun, which may irrevocably tarnish their environments. The only certainty is that such alien worlds have innumerable surprises in store.   

Christopher Crockett is an Associate News Editor. He was formerly the astronomy writer from 2014 to 2017, and he has a Ph.D. in astronomy from the University of California, Los Angeles.

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