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52 The Circumstellar Habitable Zone
The area around a star within which a planet or planets with sufficient mass and atmospheric pressure can support liquid water at the planet’s surface is called the circumstellar habitable zone ; also called the Goldilocks Zone .First theorized in 1953, numerous exoplanets have been discovered within this Zone. Not all biologists and astronomers agree that a planet must exist in the circumstellar habitable zone to support life or even intelligent life; many point to extremophiles found on Earth; organisms which flourish in extreme Earth conditions that would be harmful to most life.
It appears that one in five stars have an Earth-sized planet in a circumstellar habitable zone. There could be as many as 40 billion circumstellar habitable zone planets in the Milky Way Galaxy.