An alien astronomer, viewing Earth from a great distance, could detect the presence of life on the planet with comparative ease. This could not be done directly, by imaging, but rather indirectly, by spectral analysis of the composition of the atmosphere. Even with the most grandiose alien telescopes, it is doubtful that extraterrestrial astronomers could directly detect organisms, groups of organisms, or even the most immense structures life has formed: coral reefs, forests, forest fires, red tides, city lights, the Great Wall of China, freeways, and dams. At best, images of Earth would only slightly resolve what Carl Sagan referred to as the "pale blue dot." The principal clue alerting distant astronomers to the presence of life would be a spectacular and unmistakable signature. Spectral analysis of infrared light would reveal that life plays such a major role on the planet that it controls the composition of the atmosphere.
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