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Why are Earth-size planets in habitable zones important in the search for life?

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artist's conception of Kepler-186f — extraterrestrial intelligence An artist's conception depicts Kepler-186f, the first Earth-sized extrasolar planet found in its star's habitable zone.

Earth-size planets located within a habitable zone are central to the search for life because this region defines the orbital distance where a planet’s surface temperature could allow for liquid water to exist. Because liquid water is a fundamental requirement for all known life on Earth, scientists hypothesize that extraterrestrial life would share this necessity.21

Searching for Earth-size worlds is a primary goal of astrobiology because these planets are considered the most promising candidates to host life similar to our own. Astronomers use space-based telescopes to identify these planets and analyze their atmospheres for chemical signatures—such as oxygen or methane—that could indicate the presence of biological activity.1

While the habitable zone is a useful, conservative definition, it is not the only place life might exist; however, focusing on Earth-like planets in these zones provides a clear, scientifically grounded starting point for detecting life beyond our solar system.21