Remember chlorofluorocarbons, aka CFCs? The big, bad ozone-depleting pollutant 1) sticks around for tens of thousands of years and 2) is almost entirely man-made. That means if extraterrestrial life are anything like us, according to astrophysicists at Harvard, CFCs could be a key to finding aliens.
If we can't get in contact directly with any of the aliens that might live on the Milky Way's billions and billions of planets, then the next best option is to look for evidence that they have reshaped their worlds. And silly as it might sound, our best hope is that aliens use hairspray and central air.
This video—taken by the Ozone Monitoring Instrument onboard NASA's Aura satellite—shows the ozone hole as it reaches its yearly maximum, 23 years after the ban of chlorofluorocarbons, once the heart of all fridges and hair spray cans worldwide.