If there were life forms on other planets, the aliens can detect us through our tangible presence on Earth that leaves ‘traces that could be observed from afar.’ The chemicals known as CFCs emitted from Earth will make the planet detectable to aliens because CFCs ‘strongly absorb infrared light at characteristic wavelengths, making them detectable in the atmosphere even when present at concentrations of only parts per trillion. CFCs do not form naturally, so detecting them on a world orbiting another star would be good evidence of alien technology.’

Though CFCs may indicate the presence of an advanced civilization, ‘an exceptionally sensitive telescope would be needed to pick them up.’ This type of telescope should even be more sensitive than the two most ambitious space telescopes currently being planned: NASA’s Terrestrial Planet Finder and the European Space Agency’s Darwin mission. The possibility looks real but still far into the future when humans would be capable of putting up a ‘flotilla of infrared telescopes in space.’

But the manufacture of CFCs has been under a worldwide ban after it was confirmed that the chemicals damaged the Earth’s ozone layer. CFCs are, thus, slowly disappearing from the Earth’s atmosphere. Less damaging substitutes to CFCs, in the form of industrial solvents, cleaners, and refrigerants, have since been used, and these also have ‘characteristic infrared fingerprints.’

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Via NewScientist