What is best way to find alien civilizations on planets outside our solar system? TV type transmissions?
Leaving A Mark
Astrophysicists at the Harvard‑Smithsonian Center for Astrophysics suggest that we might be able to locate advanced civilizations by detecting their air pollution. Yes. That's right. Air pollution.
By measuring the light reflecting off planet atmospheres as they orbit their host stars, we can already measure gases like oxygen and methane. These could be signs of life or the result of natural geological processes. Even if they come from living organisms, that doesn't tell us if that life is a mat of microbes floating in an ocean, a forest of alien plants, of philosophers gathering on a mountain top.
Pollution's Outer Limits
The scientists said the James Webb Space Telescope should be able to detect two types of an industrial pollutant that humans have already created: chlorofluorocarbon or CFC. CFCs are ozone destroying chemicals used in solvents and aerosols.
Unfortunately, there are a couple of limitations for this detection idea to work. First, the CFC levels would need to be ten times those on Earth. Secondly, the planet would have to orbit a fairly dim, dying white draft star. Neither of those conditions would be healthy for an alien civilization.
Not to be deterred, the scientists suggest that an advanced civilization might intentionally pollute the atmosphere to create global warming on an otherwise cold planet. Some pollutants can last up to fifty thousand years, while others last only ten. Detecting long‑lived pollutants, but not short‑lived ones, could hint that the civilization has succumbed to its rampant pollution and is already gone.