When looking for life on other planets, we do not look for little green men, or cities, or canals. Telescopes are nowhere near powerful enough to see that kind of detail. Our best satellite-based telescopes hope to see a dot, if we are lucky, if we know where and when to look.
The way we look for life is to look at the color of that dot. The precise spectrum will indicate the chemicals that are present. Certain chemicals are a sign of life: molecular oxygen, for example. Oxygen bonds really well to very many chemicals. It is unlikely to see much of it unbonded. We only have unbonded oxygen on earth because we have plants that make it. Oxygen molecules are a sure sign of life. Methane is also a likely indicator. This makes oxygen molecules a biosignature: a sign of life.
Professor Adam Frank is looking for technosignatures: a light spectrum that indicate the presence of a technology-using society. Solar panels and pollutants are two types of technosignature.
