You know life when you see it. Rocks are not alive, but trees are. Buildings and cars are not alive. But what about viruses? They can reproduce. What about a fire? It grows and uses energy. The dividing line can seem hazy at times. If we do find extraterrestrial life, will we recognize it?
Biologists agree that to be considered alive, a thing has to extract energy from its environment to maintain itself; carry out some kind of metabolism and get rid of wastes; have a boundary to separate its parts from the environment; and be able to reproduce or replicate itself. Viruses exhibit only some of these characteristics and could be generously considered half-alive at best. Fires can replicate themselves but don't have a boundary—a skin or membrane; they're simply unregulated chemical reactions that produce a lot of heat.
The next generation of telescopes—the James Webb Space Telescope, for example—will be able to detect the waste gases of life on exoplanets. Biosignature gases such as oxygen and methane don't normally stick around long. Without life to consistently produce the gases, geological and chemical processes would turn them into other compounds. Finding signatures of key ingredients for life—water and organic compounds—along with their waste products would be compelling evidence that life exists outside our solar system.
From National Geographic's "Are We Alone?" special magazine:
The next generation of telescopes—the James Webb Space Telescope, for example—will be able to detect the waste gases of life on exoplanets. Biosignature gases such as oxygen and methane don't normally stick around long. Without life to consistently produce the gases, geological and chemical processes would turn them into other compounds. Finding signatures of key ingredients for life—water and organic compounds—along with their waste products would be compelling evidence that life exists outside our solar system.
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