How far from the Sun does the Sun’s habitable zone extend?

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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.

At present, the Sun’s habitable zone is estimated to extend from approximately 0.9 to 1.5 astronomical units (the distance between Earth and the Sun).1

The inner boundary is defined by the point where a runaway greenhouse effect would cause a planet's water to boil away, while the outer boundary marks the limit where greenhouse warming is insufficient to maintain surface temperatures above freezing. Because a star’s luminosity increases over time, these boundaries shift outward as the star ages. Consequently, the "continuously habitable zone"—the region where liquid water could have been present from four billion years ago to the present—is narrower, extending from about 0.9 to 1.2 astronomical units.1