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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.

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.

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 is believed to 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.

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.

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 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 (assuming that iron is necessary for all life).

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. Also many of the larger elliptical and spiral galaxies have had their central regions depleted of interstellar gas and dust, and thus generally no longer form stars in those areas at the same rates as they do in the periphery.

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.

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 Goldilocks phenomenon refers to the necessity that planetary conditions such as size or temperature be "just right" in order to sustain life. The term derives from Goldilocks, the story of a girl who prefers porridge which is "not too hot, and not too cold". Our solar system's habitable zone (also referred to as the "Goldilocks Zone"), which is likewise neither too hot nor too cold for life to exist, is where the Earth is found. Astronomer James Lovelock, proponent of the Gaia hypothesis, is credited with coining the term. The concept is somewhat flawed in that life exists between 5 F (-15 C) (Cryptoendoliths in Antarctica), and 250 degrees Fahrenheit (121 degrees Celsius) (thermophilic deep sea Bacteria, Thermal Vents).

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, Jupiter's moon 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.

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. 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]}

• Planetary habitability
• Rare Earth hypothesis
• Gliese 581 c
• 55 Cancri f
• Extraterrestrial liquid water

• 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. 

• 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)
• NASA: The Goldilocks Zone
• Interstellar Real Estate: Location, Location, Location - Defining the Habitable Zone
• Definition of "goldilocks" connoting "moderate characteristics" and examples referring to planets dating to 1935.
• Exoplanet Habitable Zone Candidates
• Exoplanet Habitable Zone Residents