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How does thermodynamic disequilibrium help scientists search for life on distant planets?

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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 the field of astrobiology, scientists search for life on distant planets by using remote-sensing methods to detect thermodynamic disequilibrium, particularly within a planet's atmosphere and hydrosphere.1

In a state of thermodynamic equilibrium, a system’s chemical composition remains stable and predictable based on physical laws. However, life acts as a process that continuously generates or consumes substances, creating chemical imbalances that would not exist otherwise.21

For example, Earth’s atmosphere contains significant amounts of methane and oxygen. In true thermodynamic equilibrium, the concentration of methane should be nearly nonexistent because it would be rapidly oxidized. The presence of these gases in large, steady-state abundances indicates that an active process—such as biological metabolism—is constantly replenishing them. By identifying such chemical, mechanical, or spectral discrepancies, researchers can flag a planet as a potential candidate for hosting life, as these imbalances suggest that something is actively working against the natural tendency toward equilibrium.1