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Why do the boundaries of a star’s habitable zone move outward over time?

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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.

The boundaries of a star’s habitable zone move outward over time because a star’s luminosity increases as it ages.1

As a star consumes its hydrogen fuel, its internal temperature and energy output rise, causing the star to become brighter. Because the habitable zone—the region where a planet can maintain liquid water on its surface—is defined by the distance at which a planet receives the appropriate amount of stellar radiation, this increase in brightness forces the zone to shift further away from the star.21

Consequently, the inner boundary, where a runaway greenhouse effect would boil away a planet's water, and the outer boundary, where temperatures become too cold to maintain liquid water, both migrate outward. This evolution means that planets initially located within the habitable zone may eventually become too hot, while planets that were once too cold may later enter the habitable zone as their star brightens.1