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How do astronomers search for evidence of life in an exoplanet’s atmosphere?

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Habitable zone The habitable zones (green) for stars that are like the Sun (middle), hotter than the Sun (top), and cooler than the Sun (bottom). The red areas are those in which liquid surface water would be lost as a result of a runaway greenhouse effect, and the blue areas are those in which liquid surface water would be completely frozen.

Astronomers search for evidence of life on exoplanets primarily through remote-sensing methods that analyze the light reflected by or emitted from a planet's atmosphere. By using space-based telescopes to perform spectral analysis, scientists look for chemical signatures that indicate a state of thermodynamic disequilibrium.12

On Earth, the simultaneous presence of gases like oxygen and methane is a strong indicator of life, as these gases would otherwise react and disappear if not continuously replenished by biological processes. Because these substances are not in chemical equilibrium, their detection in an exoplanet's atmosphere could serve as a potential biosignature.12

In addition to searching for these gases, researchers may look for other complex organic molecules, such as terpenes, which are hydrocarbons produced by plants. While these remote-sensing techniques are essential for studying worlds within or near a star's habitable zone, scientists remain cautious, as many chemical signals can potentially be explained by non-biological processes. Consequently, the search involves analyzing an array of data to distinguish between biological activity and geological or photochemical phenomena.13