Habitable zone
In astronomy, the habitable zone (HZ) is the region in a star-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").
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Circumstellar habitable zone
Within a planetary system, a planet must lie within the habitable zone in order to sustain liquid water on its surface. Beyond the outer edge, a planet will not receive enough solar radiation to make up for radiative losses, leaving water to freeze. A planet closer than the inner edge of this zone will absorb too much radiation, boiling away surface water. The circumstellar habitable zone (or ecosphere) is this notional spherical shell of space surrounding a star where such planets might exist. Liquid water is considered important for two reasons. Carbon compounds dissolved in water form the basis of all Earthly life, so watery planets are good candidates to support similar carbon-based biochemistries. Even on a "dead" world, the presence of liquid water would greatly simplify the colonization of the planet.
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 cannot 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][18] although the concept has recently been challenged.[19]
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).[20] 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.[21] 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).
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.
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 and the Three Bears 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 a cooler version of present-day Venus), 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
- Alternative biochemistry
- Extraterrestrial liquid water
- Goldilocks planet
- Rare Earth hypothesis
References
- ↑ "VPL Glossary". http://vpl.astro.washington.edu/epo/glossary.html.
- ↑ 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.
- ↑ Planets for Man, Dole & Asimov 1964
- ↑ Hart et al 1978, 1979 Icarus vol.37, 351–35
- ↑ Fogg 1992
- ↑ Kasting et al 1993, Icarus 101, 108–128
- ↑ Budyko 1969
- ↑ Sellers 1969
- ↑ North 1975
- ↑ Rasool & DeBurgh 1970
- ↑ Schneider and Thompson 1980
- ↑ Kasting 1991
- ↑ Kasting 1988
- ↑ Ramanathan and Collins 1991
- ↑ Lovelock 1991
- ↑ Whitemire et al 1991
- ↑ Guillermo Gonzalez, Donald Brownlee, Peter Ward, The Galactic Habitable Zone I. Galactic Chemical Evolution, 12 Mar 2001
- ↑ 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.
- ↑ On the "Galactic Habitable Zone"
- ↑ Is Iron an Energy Source for Early Life on Anoxic Earth?
- ↑ Solstation - Habitable
- ↑ The Goldilocks Zone – NASA
- ↑ Evolving the Alien by Ian Stewart and Jack Cohen
- ↑ 24.0 24.1 Planets for Man
- ↑ 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]
- ↑ Immigrant Sun: Our Star Could be Far from Where It Started in Milky Way Newswise, Retrieved on September 15, 2008.
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.
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