Beside us

How might we detect such distant life? Not likely with fleets of starships. The energies required for star travel (barring a science-fiction style warp drive) are simply too great and travel times too long. Instead, sophisticated ground and space-based telescopes are being used to detect planets of nearby star systems. How planets block the light of their star as detected from Earth can reveal the chemical composition of the planet's atmosphere. Some scientists believe that the chemical composition of the planet's atmosphere will provide evidence of the presence of life. How is this possible? How can simple gases tell us about the presence or absence of life as we know it? To answer that, we must look at our own planet.
Earth has a radius of some 6400 km. Ninety-nine percent of the earth's atmosphere is contained within a layer approximately 50 km thick. Life on earth inhabits a layer no more than 9 km thick, extending from a bare few kilometers above sea level (airborne organisms and life on mountains) to a few kilometers below (deep ocean basin creatures and subterranean microbial communities).