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How can oxygen and methane in a planet’s atmosphere be possible signs of life?

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Planet TOI 700e — extraterrestrial life An illustration of the Earth-size planet TOI 700e orbiting within the habitable zone of its star. The habitable zone is the area around a star where liquid water, and thus possibly life, can survive on a planet's surface. Its Earth-size sibling, TOI 700d, can be seen in the distance.

In astrobiology, the simultaneous presence of oxygen and methane in a planet’s atmosphere is considered a potential sign of life because these gases exist in a state of thermodynamic disequilibrium.1

On Earth, methane is highly reactive and would quickly oxidize into water and carbon dioxide if not continuously replenished. The fact that methane persists at detectable levels implies a constant, rapid source of production. While non-biological processes can generate methane, its presence alongside oxygen—which is also maintained by biological photosynthesis—strongly suggests active biological cycles.213

Because these gases react with one another, their coexistence requires a continuous supply from the surface to maintain their atmospheric concentrations. Detecting such a chemical imbalance from a distance serves as a "preliminary positive test" for life, as it indicates that some process is actively working against chemical equilibrium to replenish these gases. However, scientists must exclude non-biological explanations, such as the photodissociation of water, before confirming a biological origin.21