Al Gore is constantly reminding us (for the better!) to watch humanity’s output of chlorofluorocarbons, aka CFCs, but it could be these CFC’s that show us a planet harboring complex alien life much like our own out in the universe.
CFC’s are ozone-depleteing pollutants that both stick around for tens of thousands of years and is almost entirely man-made. Therefore if there is life out there that is anything like us they would be likely be creating pollutants like us.
In a few years, NASA is launching the James Webb Space Telescope (JWST). This amazing new telescope will be able to capture starlight filtering through atmospheres of exoplanets. Using this ability, scientists can compare the spectrum of light from the planets before and after it goes through the atmosphere therefor detecting the type of gases being admitted and whether there is pollution like we have on earth.
Typically, scientists search for signs of oxygen when they are looking for planets with life, but by searching for CFC’s and signs of pollution they could find something closer to intelligent life and not just planets with planets and bacteria like organisms.
However, this is all based on assuming that aliens would be as destructive on their planet as we have been on ours.