It has been suggested that Goldilocks phenomenon be merged into this article or section. (Discuss)

In astronomy a habitable zone (HZ) is a region of space where conditions are favorable for life as it can be found on Earth. There are two regions that must be favorable, one within a solar system and the other within the galaxy. Planets and moons in these regions are the likeliest candidates to be habitable and thus capable of bearing extraterrestrial life similar to our own. Astronomers believe that life is most likely to form within the circumstellar habitable zone (CHZ) within a solar system, and the galactic habitable zone (GHZ) of the larger galaxy (though research on the latter point remains nascent). The HZ may also be referred to as the "life zone", "Green Belt" or the "Goldilocks Zone" (because it's neither too hot nor too cold, but "just right"). In our own solar system, the HZ is thought to extend from a distance of 0.95 to 1.37 astronomical units. Image File history File links Please see the file description page for further information. ... It has been suggested that this article or section be merged into Habitable zone. ... A giant Hubble mosaic of the Crab Nebula, a supernova remnant Astronomy (also frequently referred to as astrophysics) is the scientific study of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earths atmosphere (such as the cosmic background radiation). ... For other uses, see Life (disambiguation). ... The eight planets and three dwarf planets of the Solar System. ... A natural satellite is an object that orbits a planet or other body larger than itself and which is not man-made. ... Understanding planetary habitability is partly an extrapolation of the Earths conditions, as it is the only planet currently known to support life. ... “Green people” redirects here. ... Major features of the Solar System (not to scale; from left to right): Pluto, Neptune, Uranus, Saturn, Jupiter, the asteroid belt, the Sun, Mercury, Venus, Earth and its Moon, and Mars. ... NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 17,000 parsecs in diameter and approximately 20 million parsecs distant. ... It has been suggested that this article or section be merged into Habitable zone. ... The astronomical unit (AU or au or a. ...

Gliese 581 c, the second planet of the red dwarf star Gliese 581 (approximately 20 light years distance from earth), appears to be the best example of an extrasolar planet which orbits in the theoretical habitable zone of space surrounding its star. Gliese 581 c (IPA: ) is a super-earth extrasolar planet orbiting the red dwarf star Gliese 581. ... Gliese 581 is an M2. ... A light-year or lightyear (symbol: ly) is a unit of measurement of length, specifically the distance light travels in vacuum in one year. ... An extrasolar planet, or exoplanet, is a planet beyond the Solar System. ...

Circumstellar habitable zone

A range of theoretical habitable zones with stars of different mass (our solar system at centre). Not to scale.

Within a solar system, it is believed a planet must lie within the habitable zone in order to sustain life. The circumstellar habitable zone (or ecosphere) is a notional spherical shell of space surrounding stars where the surface temperatures of any planets present might maintain liquid water. Liquid water may be vital for life because of its role as the solvent needed for biochemical reactions. In 1959, physicists Philip Morrison and Giuseppe Cocconi described the zone in a SETI research paper. In 1961, Frank Drake popularized the concept in his Drake equation. Image File history File links Habitable_zone-en. ... Image File history File links Habitable_zone-en. ... Major features of the Solar System (not to scale; from left to right): Pluto, Neptune, Uranus, Saturn, Jupiter, the asteroid belt, the Sun, Mercury, Venus, Earth and its Moon, and Mars. ... STARS can mean: Shock Trauma Air Rescue Society Special Tactics And Rescue Service, a fictional task force that appears in Capcoms Resident Evil video game franchise. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... Philip Morrison, (November 7, 1915 – April 22, 2005), was institute Professor, Emeritus and Professor of Physics, Emeritus at the Massachusetts Institute of Technology (MIT). ... Giuseppe Cocconi (born 1914) is a physicist who first proposed using microwaves to search for interstellar communication. ... This article is about the search for extra-terrestrial intelligence. ... Professor Frank Drake Frank Drake (born May 28, 1930, Chicago, Illinois) is an American astronomer and astrophysicist. ... The Drake equation (also known as the Green Bank equation or the Sagan equation) is a famous result in the speculative fields of xenobiology, astrosociobiology and the search for extraterrestrial intelligence. ...

The distance from a star where this can take place can be calculated from star size and luminosity. The CHZ of a particular star is "centered" on a distance determined by the equation:

where

is the mean radius of the HZ in astronomical units,

is the bolometric luminosity of the star, and

is the bolometric luminosity of the Sun.

For example, a star with 25% the luminosity of the Sun will have an HZ centered at about 0.50 AU and a star twice the Sun's luminosity will have an HZ centered at about 1.4 AU. This is a consequence of the inverse square law of luminous intensity. The "center" of the HZ is defined as the distance that an exoplanet would have to be from its parent star to have an approximately Earth-like global average temperature, assuming (among other things) that it has a similar atmospheric composition and thickness. The astronomical unit (AU or au or a. ... Luminosity has different meanings in several different fields of science. ... The Sun (Latin: Sol) is the star at the center of the Solar System. ... In physics, an inverse-square law is any physical law stating that some quantity is inversely proportional to the square of the distance from a point. ... An extrasolar planet, or exoplanet, is a planet beyond the Solar System. ...

As stars evolve they become brighter and hotter. This moves the CHZ further away from the star over time. To maximise the potential for life, a planet would ideally be in an orbit which is kept in the HZ for as long as possible.

Atmospheric composition also has an important effect.

The galactic habitable zone

The location of a solar system within the galaxy must also be favorable to the development of life, and this leads to the concept of a galactic habitable zone.

To harbor life, a solar system must be close enough to the galactic center that a sufficiently high level of heavy elements exist to favor the formation of rocky planets. Heavier elements must be present, since they form complex molecules of life, such as iron as the foundation for hemoglobin and iodine for the thyroid gland. General Name, Symbol, Number iron, Fe, 26 Chemical series transition metals Group, Period, Block 8, 4, d Appearance lustrous metallic with a grayish tinge Standard atomic weight 55. ... Structure of hemoglobin. ... General Name, Symbol, Number iodine, I, 53 Chemical series halogens Group, Period, Block 17, 5, p Appearance violet-dark gray, lustrous Standard atomic weight 126. ...

On the other hand, the solar system must be far enough from the galaxy center to avoid hazards such as impacts from comets and asteroids, close encounters with passing stars, and outbursts of radiation from supernovae and from the black hole at the center of the galaxy. The effect of radiation from supernovae on living organisms is not clear. Presumably, large amounts of radiation, which occur near the center of a galaxy, make formation of complex molecules more difficult. Comet Hale-Bopp, showing a white dust tail and blue gas tail (February 1997) A comet is a small astronomical object similar to an asteroid but composed largely of ice. ... 253 Mathilde, a C-type asteroid. ... Remnant of Keplers Supernova, SN 1604. ... Simulated view of a black hole in front of the Milky Way. ...

Also, studies have shown that regions in which the level of heavy elements, or metallicity, is very high seem to be more likely to harbor massive planets orbiting close to their star. The gravitational tidal forces induced by such planets would distort the orbit and surface shape of any Earth-mass planets, which could destroy them before life has a chance to form.^{[citation needed]} For these reasons, there are many uncertainties in determining where the habitable zone in a galaxy may lie. The globular cluster M80. ...

In our galaxy (the Milky Way), the GHZ is currently believed to be a slowly expanding region approximately 25,000 light years (8 kiloparsecs) from the galactic core, containing stars roughly 4 billion to 8 billion years old. Other galaxies differ in their compositions, and may have a larger or smaller GHZ – or none at all. Future technologies may enable us to determine the number and location of Earth-type planets in the Milky Way, greatly refining our understanding of the Galactic habitable zone. The Milky Way as seen from Death Valley The Milky Way is the galaxy where the Solar System (and the Earth) is located. ... A light year, abbreviated ly, is the distance light travels in one year: roughly 9. ... A parsec is the distance from the Earth to an astronomical object which has a parallax angle of one arcsecond. ...

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 hypothesis assumes alien life has exactly the same requirements as terrestrial life; the second is that, even assuming this, other circumstances may result in suitable planets outside the "habitable zone", for instance the Jovian satellite Europa, which is thought to have a subsurface ocean with an environment similar to the deep oceans of Earth. The existence of extremophiles on Earth makes life on Europa seem more plausible, despite the fact that Europa is not in the presumed CHZ. Planetary biologist Carl Sagan believed that life was also possible on the gas giants, such as Jupiter itself. A discovery of any form of life in such an environment would expose these hypothetical restrictions as too conservative. Ian Stewart, FRS (b. ... Jack Cohen is a reproductive biologist at the University of Warwick, England. ... Evolving the Alien: The Science of Extraterrestrial Life (2002, second edition published as What Does a Martian Look Like? The Science of Extraterrestrial Life) is a book about xenobiology by biologist Jack Cohen and mathematician Ian Stewart. ... Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... Apparent magnitude: 5. ... An extremophile is an organism, usually unicellular, which thrives in or requires extreme conditions that would exceed optimal conditions for growth and reproduction in the majority of mesophilic terrestrial organisms. ... Insert non-formatted text here Carl Edward Sagan (November 9, 1934 – December 20, 1996) was an American astronomer and astrobiologist and a highly successful popularizer of astronomy, astrophysics, and other natural sciences. ... Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ...

Differing levels of volcanic activity, lunar effects, or planetary mass may all affect the radiation and heat levels acting on a planet to modify conditions supporting life. However, while it is relatively certain that life on Earth would be able to adapt to an environment like Europa's, it's far less likely for life to have developed there in the first place, or for it to somehow move there and adapt afterwards without advanced technology (or possibly capabilities equivalent to). A planet that used to be in the habitable zone is more likely to have life than one that has moved into it.^{[citation needed]}

See also

• Planetary habitability
• Rare Earth hypothesis
• Gliese 581 c
• Water on other planets

Understanding planetary habitability is partly an extrapolation of the Earths conditions, as it is the only planet currently known to support life. ... The Rare Earth hypothesis is a hypothesis in planetary astronomy and astrobiology which argues that the emergence of complex multicellular life (metazoa) on Earth required an extremely unlikely combination of astrophysical and geological events and circumstances. ... Gliese 581 c (IPA: ) is a super-earth extrasolar planet orbiting the red dwarf star Gliese 581. ... This article or section does not cite any references or sources. ...

References

• Charles H. Lineweaver, Yeshe Finner and Brad K. Gibson (January 2004). "The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way". Science 303 (5654): 59–62. 

External links

• More references are at the end of the article "Habitable Zone" at The Encyclopedia of Astrobiology, Astronomy and Spaceflight
• "Stars and Habitable Planets" at SolStation
• On the Galactic Habitable Zone
• Swiss Scientist: Search for Life Next
• Stephen H. Dole's Habitable Planets for Man – a complete study on the planetary characteristics required to develop a complex lifeform (copyright 1964)