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.