Definitions
Nearby Words

# Habitable zone

In astronomy a habitable zone (HZ) is a region of space where conditions are favorable for life as it may be found on Earth. There are two regions that must be favorable, one within a planetary 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", "Comfort Zone", "Green Belt" or "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 d, the third planet 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 habitable zone of space surrounding its star.

### Circumstellar habitable zone

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. 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:

$d_\left\{AU\right\} = sqrt \left\{L_\left\{star\right\}/L_\left\{sun\right\}\right\}$

where
$d_\left\{AU\right\} ,$ is the mean radius of the HZ in astronomical units,
$L_\left\{star\right\} ,$ is the bolometric luminosity of the star, and
$L_\left\{sun\right\} ,$ 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 a star evolves it becomes brighter, increasing its luminosity. This moves the CHZ further away from the star over time. To maximise the time during which life can exist, 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 temperature of a planet is affected by its atmospheric concentration of greenhouse gases.

### 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) , although the concept has recently been challenged

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 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 planetary 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 so 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. 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 Galactic Habitable Zone or GHZ is also known as the Goldilocks Zone. The term derives from Goldilocks, the fairytale of a girl who prefers porridge which is "not too hot, and not too cold". Astronomer James Lovelock, proponent of the Gaia hypothesis, is credited with coining the term. The concept could be said to be flawed in that life exists at temperatures between −15 °C(5 °F) (Cryptoendoliths in Antarctica), and 120 °C(250 °F) (thermophilic bacteria in deep sea thermal vents), however this absolute temperature range is 258 to 394 kelvin - less than an order of magnitude.

### 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, 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. 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. 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".
• The Earth, which is at the assumed center of the habitable zone, is at the same distance from the sun as the moon, which is in all ways uninhabitable.
• 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.
• Differing levels of volcanic activity, lunar effects or planetary mass 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.
• Scientists describe extensive computer simulations in the journal Astrophysical Journal Letters (September 10, 2008) 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.