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In astronomy, the habitable zone (HZ) is the region in a stellar-centered orbit 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 concept generally does not include moons, because there is insufficient evidence and theory to speculate what moons might be habitable on account of their proximity to a planet.

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 interchanged.

It stands to reason 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").

Contents

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.

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.[2]

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:

INNER edge OUTER edge References Notes
0.725 AU 1.24 AU Dole 1964 [3] Used optically thin atmospheres and fixed albedos.
0.95 AU 1.01 AU Hart et al 1978, 1979 [4] stars K0 or later can not have HZs
0.95 AU 3.0 AU Fogg 1992 [5] Used Carbon cycles.
0.95 AU 1.37 AU Kasting et al 1993 [6]
– 1%–2% farther out Budyko 1969 [7] ... and Earth would have global glaciation.
– 1%–2% farther out Sellers 1969 [8] ... and Earth would have global glaciation.
– 1%–2% farther out North 1975 [9] ... and Earth would have global glaciation.
4%–7% closer – Rasool & DeBurgh 1970 [10] ... and Oceans would never have condensed.
– – Schneider and Thompson 1980 [11] disagreed with Hart.
– – Kasting 1991 [12]
– – Kasting 1988 [13] Water clouds can shrink HZ
as they counter GHG effect with higher albedos.
– – Ramanathan and Collins 1991 [14] GHG effect IR trapping is greater than water cloud albedo cooling,
and Venus would have to have started "Dry."
– – Lovelock 1991 [15]
– – Whitemire et al 1991 [16]

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).

Carbon-based life

Main article: 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.

'Goldilocks zone'

"This porridge is too hot," Goldilocks exclaimed.

So she tasted the porridge from the second bowl.
"This porridge is too cold."
So she tasted the last bowl of porridge.
"Ahhh, this porridge is just right!" she said happily.
And she ate it all up.

—Goldilocks and the three bears
Main article: Goldilocks planet

The term 'Goldilocks zone' is often used in popular writing as a nickname for the Habitable zone.[22] The term comes from the children's fairy tale Goldilocks to describe conditions that are not too hot or too cold for life as we know it.

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".[23]
  • 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.[24]
  • 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.[24]
  • Scientists describe extensive computer simulations in the Astrophysical Journal[25] 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.[26]

See also

Astronomy portal
Space portal

References

  1. ^ "VPL Glossary". http://vpl.astro.washington.edu/epo/glossary.html.  
  2. ^ Mayor et al. (2009). "The HARPS search for southern extra-solar planets,XVIII. An Earth-mass planet in the GJ 581 planetary system". Astronomy and Astrophysics. http://obswww.unige.ch/~udry/Gl581_preprint.pdf.  
  3. ^ Planets for Man, Dole & Asimov 1964
  4. ^ Hart et al 1978, 1979 Icarus vol.37, 351–35
  5. ^ Fogg 1992
  6. ^ Kasting et al 1993, Icarus 101, 108–128
  7. ^ Budyko 1969
  8. ^ Sellers 1969
  9. ^ North 1975
  10. ^ Rasool & DeBurgh 1970
  11. ^ Schneider and Thompson 1980
  12. ^ Kasting 1991
  13. ^ Kasting 1988
  14. ^ Ramanathan and Collins 1991
  15. ^ Lovelock 1991
  16. ^ Whitemire et al 1991
  17. ^ Charles H. Lineweaver, Yeshe Fenner 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. doi:10.1126/science.1092322. PMID 14704421.  
  18. ^ On the "Galactic Habitable Zone"
  19. ^ Is Iron an Energy Source for Early Life on Anoxic Earth?
  20. ^ Solstation>Habitable
  21. ^ Ask an astronomer: How many stars are there in our Galaxy (Milky Way)?
  22. ^ The Goldilocks Zone – NASA
  23. ^ Evolving the Alien by Ian Stewart and Jack Cohen
  24. ^ a b Planets for Man
  25. ^ Rok Roškar, Victor P. Debattista, Thomas R. Quinn, Gregory S. Stinson, and James Wadsley, Riding the Spiral Waves: Implications of Stellar Migration for the Properties of Galactic Disks, Astrophysical Journal Letters, Volume 684, Number 2, 2008 September 10 [1]
  26. ^ Immigrant Sun: Our Star Could be Far from Where It Started in Milky Way Newswise, Retrieved on September 15, 2008.

External links


Simple English


In astronomy a habitable zone (HZ) is a region of space where conditions are best for life to form as it is 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 areas are the most likely to have extraterrestrial life.

The area in a solar system where life is most likely is called the circumstellar habitable zone (CHZ). The places in a galaxy where life is most likely is called the galactic habitable zone (GHZ). The habitable zone may also be called the "life zone", "comfort zone", "green belt" or "Goldilocks Zone" (because it's neither too hot nor too cold, but "just right").

Circumstellar habitable zone

In a solar system, astronomers believe a planet must be in the habitable zone to have life. The circumstellar habitable zone is an area around a star where a planet would be able to have liquid water. Liquid water is believed to be necessary for all forms of life.

Bigger stars are generally hotter, so the circumstellar habitable zone would be farther away from the star than for the Sun. Smaller stars are cooler, so the circumstellar habitable zone would be closer to the star than for the Sun. The size and brightness of a star determine where the circumstellar habitable zone is found around the star.

Galactic habitable zone

A solar system must also be located in an ideal place inside a galaxy for life to form. This idea is called the galactic habitable zone. To have life, the solar system must be close enough to the center of the galaxy to have the right elements to support life. Areas far away from the center of the galaxy don't have enough heavy elements for life to form. However, the solar system can't be too close to the center of the galaxy. The center of the galaxy has higher levels of radiation which would prevent life from forming.

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