Habitable Zone

In astronomy and astrobiology, habitable zone (more accurately, circumstellar habitable zone or CHZ) is the scientific term for the region around a star within which it is theoretically possible for a planet with sufficient atmospheric pressure to maintain liquid water on its surface.1

The significance of the concept is in its inference of conditions favorable for life on Earth – since liquid water is essential for all known forms of life, planets in this zone are considered the most promising sites to host extraterrestrial life. The terms "ecosphere" and "Liquid Water Belt" were introduced by Hubertus Strughold and Harlow Shapley respectively in 1953. Contemporary alternatives include "HZ", "life zone", and "Goldilocks Zone".

"Habitable zone" is sometimes used more generally to denote various regions that are considered favorable to life in some way. One prominent example is the Galactic Habitable Zone, coined by Guillermo Gonzalez in 1995 (representing the distance of a planet from the galactic centre), based on the position of the Earth in the Milky Way. If different kinds of habitable zones are considered, their intersection is the region considered most likely to contain life.

The location of planets and natural satellites (moons) within its parent star's habitable zone (and a near circular orbit) is but one of many criteria for planetary habitability and it is theoretically possible for habitable planets to exist outside the habitable zone. The term "Goldilocks planet" is used for any planet that is located within the circumstellar habitable zone (CHZ) although when used in the context of planetary habitability the term implies terrestrial planets with conditions roughly comparable to those of Earth (i.e. an Earth analog). The name originates from the story of Goldilocks and the Three Bears, in which a little girl chooses from sets of three items, ignoring the ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is "just right". Likewise, a planet following this Goldilocks Principle is one neither too close nor too far from a star to rule out liquid water on its surface.

Dozens of planets have been confirmed in the habitable zone, though most found to date are significantly larger than the Earth, possibly due to sampling bias due to larger planets currently being more easily observed. The Kepler spacecraft has identified a further 54 candidates and current estimates indicate "at least 500 million" such planets in the Milky Way.

Habitable zones, however, are not stable. Over the life of a star, the nature of the zone moves and changes. Astronomical objects located in the zone are typically close in proximity to their parent star and as such are more exposed to adverse effects such as damaging tidal forces and solar flares. Combined with galactic habitability, these and many other exclusionary factors reinforce a contrasting theory of interstellar "dead zones" where life cannot exist, supporting the Rare Earth hypothesis.

Some planetary scientists have suggested habitable zone theory may prove limiting in scope and overly simplistic. There is growing support for equivalent zones around stars where other solvent compounds (such as ammonia and methane) could exist in stable liquid forms. Astrobiologists theorise these environments could be conducive to alternative biochemistry. Additionally there is probably an abundance of potential habitats outside of the habitable zone within subsurface oceans of extraterrestrial liquid water. It may follow for oceans consisting of ammonia or methane.

Habitable zones are used in the Search for Extra-Terrestrial Intelligence and is based on the assumption, should intelligent life exist elsewhere in the Universe, it would most likely be found there.

Read more about Habitable Zone:  History, Circumstellar Habitable Zone, Galactic Habitable Zone, Finding Habitable Zone Planets and Moons, Criticism

Other articles related to "habitable zone, habitable":

Habitable Zone - Criticism
... The concept of a habitable zone is criticized by Ian Stewart and Jack Cohen in their book Evolving the Alien, for two reasons the first is that the ... Therefore, a planet that has moved away from a habitable zone is more likely to have life than one that has moved into it ...
Super-Earths - Discoveries - First Super-Earth in Habitable Zone
... around Gliese 581, both on the edge of the habitable zone around the star where liquid water may be possible on the surface. 6.8 million mi, 11 million km), it is on the "warm" edge of the habitable zone around Gliese 581 with an estimated mean temperature (without taking into ... like Venus, but that its sister planet, Gliese 581 d, does in fact lie within the star's habitable zone, with an orbit at 0.22 AU and a mass of 7.7 Earths ...
Gliese 581 C - Habitability and Climate
... "The super-Earth Gl 581c is clearly outside the habitable zone, since it is too close to the star." The study by Selsis et al ... claims even "a planet in the habitable zone is not necessarily habitable" itself, and this planet "is outside what can be considered the conservative habitable zone" of the parent star, and further ...
Planetary Habitability - Other Considerations - "Good Jupiters"
... their stars in circular orbits far enough away from the habitable zone not to disturb it but close enough to "protect" terrestrial planets in closer orbit in two critical ways ... In contrast, Jupiter-sized bodies that orbit too close to the habitable zone but not in it (as in 47 Ursae Majoris), or have a highly elliptical orbit ... See the discussion of a stable habitable zone above ...
Gliese 581
... been the subject of a "huge amount of attention" in the quest to discover the first habitable extrasolar planet first for c, and then d and g ... after Gliese 581 c, the first low-mass extrasolar planet found near a habitable zone, was discovered in April 2007 ... is likely to have a runaway greenhouse effect, and hence is probably too hot to be habitable, analogous to Venus ...

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