Habitable zone

Encyclopedia : H : HA : HAB : Habitable zone

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In astronomy a habitable zone (HZ) is a region of space where conditions are favorable for the creation of life. Planets and moons in these regions are the likeliest candidates to be habitable and thus capable of bearing extraterrestrial life. 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").

The Circumstellar habitable zone

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 in which life could potentially survive. 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 CHZ of a particular star is "centered" on a distance determined by the equation:

:[d_ = \sqrt /L_}]

where

:[d_ \,] is the mean radius of the HZ in astronomical units,

:[L_ \,] is the bolometric luminosity of the star, and

:[L_ \,] 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 sun 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.

The Galactic habitable zone

The Galactic habitable zone is the region of a galaxy that is close enough to the galactic center that a sufficiently high level of heavy elements exist to favor the formation of rocky planets, but is far enough from the 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 well understood. 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. These giant planets may destroy Earth-mass planets before life has a chance to form. For these reasons, there are a lot of 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.

See also

• Planetary habitability

References

 

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