The habitable zone
The orbital region where liquid water may exist on a planet's surface.
The habitable zone (HZ), also known as the circumstellar habitable zone (CHZ) or the Goldilocks zone, is the range of orbits around a star within which a planetary surface could potentially support liquid water. It is a key concept in astronomy and astrobiology, used to guide the search for potentially habitable exoplanets.
- field
- Astronomy, astrobiology
- known_for
- Defining the orbital region where liquid water can exist on a planet's surface
- modern_concept_developed_by
- Su-Shu Huang (1950s–60s)
- key_model_developed_by
- James Kasting and colleagues (1993)
Lore & Background
The concept of a habitable zone arose in geography in the late 19th century. American astrophysicist Su-Shu Huang developed the modern concept of circumstellar habitable zones in a series of papers in the 1950s and 60s. The term 'Goldilocks zone' emerged in the 1970s. In 1993, James Kasting and colleagues developed a model for habitable zones across stars with different brightness.
Reader's Guide
The habitable zone is central to the search for habitable planets because discoveries of exoplanets yield approximate orbital radii. Since the concept was first presented, many stars have been confirmed to possess an HZ planet, including systems with multiple HZ planets. On November 4, 2013, astronomers reported, based on Kepler data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way. Proxima Centauri b, about 4.2 light-years away, is the nearest known exoplanet and orbits in its star's habitable zone. The HZ is also of interest to the study of habitability of natural satellites, as planetary mass moons in the HZ might outnumber planets. However, a planet in a habitable zone may still be uninhabitable, as surface conditions depend on many individual properties beyond the zone definition. Some astronomers have suggested the term 'habitability' should not even be used. Extensions of the concept include the galactic habitable zone and circum-planetary habitable zones.
Did You Know?
- The term 'Goldilocks zone' references the fairy tale 'Goldilocks and the Three Bears', where the girl chooses the item that is 'just right'.
- As of 2013, Kepler data suggested there could be as many as 40 billion Earth-sized planets in habitable zones of Sun-like stars and red dwarfs in the Milky Way.
- Proxima Centauri b, about 4.2 light-years away, is the nearest known exoplanet and orbits in its star's habitable zone.
- The concept of a 'galactic habitable zone' was introduced by Guillermo Gonzalez and others in 1999.
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