The habitable zone (HZ) in astronomy is a region of space where an Earth-like planet can maintain liquid water on its surface^[1] and Earth-like life. The habitable zone is the intersection of two regions that must both be favorable to life; one within a planetary system, and the other within a 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.

The habitable zone is not to be confused with the planetary habitability. While planetary habitability deals solely with the planetary conditions required to maintain carbon-based life, the habitable zone deals with the stellar conditions required to maintain carbon-based life, and these two factors are not meant to be juxtaposed.

Astronomers^[who?] believe^{[citation needed]} 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 in its infancy). The HZ may also be referred to as the "life zone", "Comfort Zone", "Green Belt" or "Goldilocks Zone" (because it's neither too hot nor too cold, but "just right").

[edit] Habitable Zone Edge predictions for our solar system.

In our own solar system, the CHZ is thought to extend from a distance of 0.725 to 3.0 astronomical units, based on various scientific models:

[edit] Circumstellar habitable zone

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

Within a planetary 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. Water is a desirable solvent for life because it is the solvent for carbon-based life, or life on Earth.^{[citation needed]}

For example, a star with 25% the luminosity of the Sun will have a CHZ centered at about 0.50 AU and a star twice the Sun's luminosity will have a CHZ 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 receive the right amount of energy from the star to maintain liquid water.

Gliese 581 d, the outermost of the four planets of the red dwarf star Gliese 581 (approximately 20 light years distance from Earth), appears to be the best example which has been found so far of an extrasolar planet which orbits in the theoretical circumstellar habitable zone of space surrounding its star.^[16]

[edit] Galactic habitable zone

The location of a planetary system within a galaxy must also be favorable to the development of life, and this has led to the concept of a galactic habitable zone (GHZ),^[17] although the concept has recently been challenged.^[18]

To harbor life, a system must be close enough to the galactic center that a sufficiently high level of heavy elements exist to favor the formation of rocky, or terrestrial, planets, which are needed to support life (see: planetary habitability). Heavier elements also need to be present, as they are the basis of the complex molecules of life. While any specific example of a heavier element may not be necessary for all life, heavier elements in general become increasingly necessary for complex life on Earth (both as complex molecules and as sources of energy)^[19]. It is assumed they would also be necessary for simpler and especially more complex life on other planets.

On the other hand, the planetary system must be far enough from the galactic center that it would not be affected by dangerous high-frequency radiation, which would cause damage and harmful alterations to the DNA of any carbon-based life. Also, most of the stars in the galactic center are old, unstable, dying stars, meaning that few or no stars form in the galactic center^[20]. Because terrestrial planets form from the same types of nebulae as stars, it can be reasoned that if stars cannot form in the galactic center, then terrestrial planets cannot, either.

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 and some 6,000 light years in width (2 kiloparsecs), 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 (see: elliptical galaxy).

[edit] Carbon-based life

Although astronomers are often criticized for assuming that all life is like life on Earth, or carbon-based life, and that all life has the same needs as carbon-based life, there is no other type of life of which humans have so precise an understanding. While astronomers understand this, there are 100 billion stars in the galaxy^[21] and it would take millennia to image every star for planets, and even longer if scientists tried to send a radio signal to every planet in every extrasolar system. So by assuming that extraterrestrial life is carbon-based, scientists who are in SETI can greatly narrow down the number of star systems in which they believe life is plausible.

[edit] 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 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 is thought to have a subsurface ocean with an environment similar to the deep oceans of Earth. The existence of extremophiles (such as the tardigrades) on Earth makes life on Europa seem more plausible, despite the fact that Europa is not in the presumed CHZ. Astronomer Carl Sagan believed that life was also possible on the gas giants, such as Jupiter itself; Iain M. Banks's novel The Algebraist is based on the same idea. A discovery of any form of life in such an environment would expose these hypothetical restrictions as too conservative. Life can evolve to tolerate extreme conditions when the relevant selection pressures dictate, and thus it is not necessary for them to be "just right".^[22]

• The primordial atmosphere of Earth was not habitable as it was mainly carbon dioxide (much like Venus sans heat), but was transformed by early primitive plant life into a breathable atmosphere, so that any definition of a habitable zone for people has to include the presence of plant life and the possibility of photosynthesis.^[23]

• Differing levels of volcanic activity, lunar effects, planetary mass, and even radioactive decay may affect the radiation and heat levels acting on a planet to modify conditions supporting life. And while it is likely that Earth life could adapt to an environment like Europa's, it is far less likely for life to develop there in the first place, or to move there and adapt without advanced technology. Therefore, a planet that has moved away from a habitable zone is more likely to have life than one that has moved into it.^[23]

• Scientists describe extensive computer simulations in the Astrophysical Journal^[24] that show that, at least in galaxies similar to our own Milky Way, stars such as the sun can migrate great distances, thus challenging the notion that parts of these galaxies are more conducive to supporting life than other areas.^[25]

[edit] See also

	Astronomy portal
	Space portal

• Alternative biochemistry
• Extraterrestrial liquid water
• Goldilocks planet
• Rare Earth hypothesis

[edit] References

[edit] External links

• The Encyclopedia of Astrobiology, Astronomy and Spaceflight (Habitable Zone)
• The Encyclopedia of Astrobiology, Astronomy and Spaceflight (Galactic Habitable Zone)
• "Stars and Habitable Planets" at SolStation
• On the Galactic Habitable Zone
• Swiss Scientist: Search for Life Next
• Stephen H. Dole and Issac Asimov's Planets for Man Defining what environment is habitable for people. (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: exoplanets in terms of their historical chances for residing in the habitable zone". www.planetarybiology.com. http://www.planetarybiology.com/hz_candidates/. 
• "Exoplanets in relation to host star's current habitable zone". www.planetarybiology.com. http://www.planetarybiology.com/exoexplorer_planets/. 
• "exoExplorer: a free Windows application for visualizing exoplanet environments in 3D". www.planetarybiology.com. http://www.planetarybiology.com/exoexplorer/. 
• "Calculating the location of habitable zone at zero age main sequence (ZAMS)". www.planetarybiology.com. http://www.planetarybiology.com/downloads/astronomical_circumstances_06.pdf. Retrieved 2008-11-10. 
• Shiga, David (2008-11-19). "Why the universe may be teeming with aliens". Space. NewScientist. http://www.newscientist.com/article/mg20026831.600-why-the-universe-may-be-teeming-with-aliens.html?full=true. Retrieved 2009-11-19.