Life is messy. So to find aliens, why not look for their pollution?
As part of its mission, NASA's upcoming James Webb Space Telescope (JWST) will be able to look at starlight filtered through the atmospheres of Earth-sized planets and search for signs of life. Most proposed plans involve hunting for highly reactive gases such as oxygen that usually need a living source to replenish them. But these methods might only hint at relatively simple life such as plants and microbes.
Henry Lin at Harvard University thinks we could find more advanced civilisations if we look instead for industrial pollution. His team calculates that JWST should be able to spot two kinds of chlorofluorocarbons (CFCs), complex carbon-based gases used in solvents and aerosols.
"Their production requires a network of chemical reactions that are only known to be produced artificially on Earth," says team member Avi Loeb at the Harvard-Smithsonian Center for Astrophysics.
Lasting impact
The team calculated that it would take JWST only a few days of observation to detect high levels of CFCs on an Earth-like planet orbiting a white dwarf star. Although such objects are the smouldering cores of dead sun-like stars, some are known to host planets, and previous work has suggested that such worlds may have the right conditions to support life.
JWST would only be able to filter out signs of CFCs from highly polluted atmospheres, the team found, but still within levels that humans could tolerate. In addition, the telescope could in principle detect the remnants of civilisations that annihilated themselves, since some CFC molecules survive for up to 100,000 years and could outlast their sources, says Loeb.
"Aliens are often referred to as little green creatures, but 'green' also means 'environmentally friendly'," says Loeb. "The latter definition implies that the detectable CFC-rich civilisations would not be 'green'."
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