What conditions define a star’s habitable zone?

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Extrasolar planet Artist's conception of the extrasolar planet HD 209458b, some 150 light-years from Earth.

A star’s habitable zone is defined as the orbital region where an Earth-like planet can maintain liquid water on its surface, a condition hypothesized to be essential for life.

The boundaries of this zone are determined primarily by the star's luminosity:

Inner Boundary: The point where a runaway greenhouse effect would cause surface water to boil away.
Outer Boundary: The distance at which greenhouse warming is insufficient to keep surface temperatures above freezing.

Because a star’s luminosity increases over time, these boundaries shift outward as the star ages. Consequently, astronomers often look for a "continuously habitable zone," where conditions remain suitable for liquid water over long periods.

Additionally, a star's mass influences the zone's viability. High-mass stars may not remain stable long enough for life to evolve, while planets orbiting very faint, low-mass stars must be so close that they may become tidally locked, potentially threatening their atmospheric stability.1