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How does the Sun’s continuously habitable zone differ from its present-day habitable zone?
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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 Sun’s habitable zone is the orbital region where a planet can maintain liquid water on its surface. Because a star’s luminosity increases over time, the boundaries of its habitable zone shift outward as it ages.12
The differences between the two zones are as follows:
- Present-day Habitable Zone: Currently, the Sun's habitable zone is estimated to extend from approximately 0.9 to 1.5 astronomical units from the Sun.1
- Continuously Habitable Zone: This refers to the region where liquid water could have been present consistently from early in the Sun’s life up to the current epoch. Spanning the period from four billion years ago to the present, this zone is narrower, extending from about 0.9 to 1.2 astronomical units.1
As the Sun grows brighter, regions that were once too cold may eventually become habitable, while regions that were previously habitable may become too hot, potentially leading to the loss of surface water through a runaway greenhouse effect.1