concept

Biosignature

Any sign - a substance, a combination of gases, a pattern - that can only be produced or sustained by life, and so serves as evidence of a biosphere from a distance. The strongest atmospheric biosignature is not a single gas but the coexistence of gases that should react and destroy each other (such as O₂ and CH₄) - they can persist together only if something continually replenishes them. An astronomer reading that combination in a distant planet's air can conclude it is alive without ever landing.