Habitable zone

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"Goldilocks planet" redirects here. For the planet initially nicknamed "Goldilocks", see 70 Virginis b. For other uses, see Goldilocks (disambiguation).
"Goldilocks Zone" and "Comfort zone (astronomy)" redirect here. For other uses, see Comfort zone (disambiguation).
An example of a system to predict the location of the habitable zone around types of stars

In astronomy, the habitable zone (HZ) is the distance from a star where an Earth-like planet can maintain liquid water on its surface[1] and potentially therefore 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 in theory 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 interchanged.

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".[2]

"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

A "Goldilocks planet" is a planet that falls within a star's habitable zone, and the name is often specifically used for planets close to the size of Earth.[3][4] The name comes from the story of Goldilocks and the Three Bears, in which a little girl chooses from sets of three items, ignoring the ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is "just right". Likewise, a planet following this Goldilocks Principle is one that is neither too close nor too far from a star to rule out liquid water on its surface and thus life (as humans understand it) on the planet. However, planets within a habitable zone that are unlikely to host life (e.g., gas giants) may also be called Goldilocks planets. The best example of a Goldilocks planet is the Earth itself.

The concept "Habitable zone" is used also at a selection of target stars during transmission of interstellar radio messages (IRMs) under projects METI. For example, IRMs A Message From Earth and Hello From Earth have been sent to the star Gliese 581 because this star has its own planetary system and one of its planets is situated in a habitable zone.

Contents

[edit] 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 (CHZ) or ecosphere is the spherical shell of space surrounding a star where such planets might exist. Liquid water is considered important because 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% of the luminosity of the Sun will have a CHZ centered at about 0.50 AU, while a star with 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 in order to receive the right amount of energy from the star to maintain liquid water.

[edit] Further requirements for habitability

[edit] Flare radiation

Small stars such as red dwarfs produce much more dangerous stellar flare activity than a star the size of the Sun. The flares would blast planets in the liquid-water-zone of red dwarfs with radiation. See Habitability of red dwarf systems#Variability.

[edit] Tides

Stars smaller than the Sun have liquid-water-zones much closer to the star so planets would experience larger tides which could remove axial tilt resulting in a lack of seasons resulting in much colder poles and much hotter equator which over time would boil away all the water. Also the planet's day could be synchronized with its year causing one-half of the planet to permanently face the star and the other half to be permanently frozen.[5]

[edit] Potential examples

Gliese 581 g, currently believed to be the fourth 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 circumstellar habitable zone of space surrounding its star,[6] however its status is still officially unconfirmed.[7]

55 Cancri f, though a Jupiter like gas giant exoplanet, orbits and also resides within the yellow dwarf star companion of 55 Cancri binary star systems habitable zone.[8] While conditions upon this massive and dense planet are not conducive to the formation of water or for that matter biological life as we know it, the potential exists for a system of satellite moons to be orbiting the planet and thus transiting through this zone and being conducive for biological development.

GJ 1214 b, though just outside of the habitable zone, does provide indications of being an ocean planet, meaning it is believed to be an extrasolar planet of the superearth variety, surrounded by a deep liquid ocean of water, similar to some of the Jovian Moons of the Sun's solar system, only with much warmer temperatures than these ice covered water worlds.

HD 28185 b takes 1.04 years to orbit its parent star. Unlike most known long-period planets, the orbit of HD 28185 b has a low eccentricity, comparable to that of Mars in our solar system.[9] The orbit lies entirely within its star's habitable zone.[10] Since HD 28185 b orbits in its star's habitable zone,[11] some have speculated on the possibility of life on worlds in the HD 28185 system. While it is unknown whether gas giants can support life, simulations of tidal interactions suggest that HD 28185 b could harbor Earth-mass satellites in orbit around it for many billions of years.[12] Such moons, if they exist, may be able to provide a habitable environment, though it is unclear whether such satellites would form in the first place.[13]

On February 2, 2011, the Kepler Space Observatory Mission team released a list of 1,235 extrasolar planet candidates, including 54 that may be in the "habitable zone."[14][15][16] Based on these latest Kepler findings, astronomer Seth Shostak estimates that "within a thousand light-years of Earth" there are "at least 30,000 of these habitable worlds."[17]

[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:

INNER edge OUTER edge References Notes
0.725 AU 1.24 AU Dole 1964 [18] Used optically thin atmospheres and fixed albedos.
0.95 AU 1.01 AU Hart et al. 1978, 1979 [19] stars K0 or later cannot have HZs
0.95 AU 3.0 AU Fogg 1992 [20] Used Carbon cycles.
0.95 AU 1.37 AU Kasting et al. 1993 [21]
– 1%–2% farther out Budyko 1969 [22] ... and Earth would have global glaciation.
– 1%–2% farther out Sellers 1969 [23] ... and Earth would have global glaciation.
– 1%–2% farther out North 1975 [24] ... and Earth would have global glaciation.
4%–7% closer – Rasool & DeBurgh 1970 [25] ... and oceans would never have condensed.
– – Schneider and Thompson 1980 [26] disagreed with Hart.
– – Kasting 1991 [27]
– – Kasting 1988 [28] Water clouds can shrink HZ as they counter GHG effect with higher albedos.
– – Ramanathan and Collins 1991 [29] GHG effect IR trapping is greater than water cloud albedo cooling, and Venus would have to have started "Dry."
– – Lovelock 1991 [30]
– – Whitemire et al. 1991 [31]

[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),[32][33] although the concept has been challenged.[34]

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).[35] 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 damage 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.[36] Some types of spiral galaxies in later time periods have been depleted of gas in dust in regions near to the galactic center, resulting in minimal new star formation in those parts of the galaxy. 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] 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. 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".[37]
  • 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.[38]
  • Scientists describe extensive computer simulations in the Astrophysical Journal[39] 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.[40]

[edit] Searching for planets in the zone

Goldilocks planets are of key interest to researchers looking either for existing (and possibly intelligent) life or for future homes for the human race.[41]

The Drake equation, which attempts to estimate the likelihood of non-terrestrial intelligent life, incorporates a factor (ne) for the average number of life-supporting planets in a star system with planets. The discovery of extrasolar Goldilocks planets helps to refine estimates for this figure. Very low estimates would contribute to the Rare Earth hypothesis, which posits that a series of extremely unlikely events and conditions led to the rise of life on Earth. High estimates would reinforce the Copernican mediocrity principle, in that large numbers of Goldilocks planets would imply that Earth is not especially exceptional.

Finding Earth-sized Goldilocks planets is a key part of the Kepler Mission, which uses a space telescope (launched on 7 March 2009 UTC) to survey and compile the characteristics of habitable-zone planets.[42] As of April 2011, Kepler has discovered 1,235 possible planets, with 54 of those candidates located within the Goldilocks zone.[43]

[edit] Potential examples

Although the extrasolar planet 70 Virginis b was initially nicknamed "Goldilocks" because it was thought to be within the star's habitable zone, it is now believed to be closer to its sun making it far too warm to be "just right" for life, thus it is not a Goldilocks planet.[44]

The Gliese 581 system has a set of slightly oversized terrestrial planets mirroring our own solar system's. It is currently believed that the third planet, planet c, is analogous to Venus's position (slightly too close), the fourth planet g (unconfirmed as of Oct. 2010) to the Earth/Goldilocks position, and the fifth planet d to the Mars position. Planet d may be too cold, but unlike Mars, it is several times more massive than Earth and may have a dense atmosphere to retain heat. One caveat with this system is that it orbits a red dwarf, probably resulting in most of the issues regarding habitability of red dwarf systems, such as all the planets likely being tidally locked to the star.

On February 2, 2011, the Kepler Space Observatory Mission team released a list of 1235 extrasolar planet candidates, including 54 that may be in the "Habitable Zone."[14][15][16][45] Six candidates (KOI 326.01, KOI 701.03, KOI 268.01, KOI 1026.01, KOI 854.01, KOI 70.03) in the "Habitable Zone" are listed as smaller than twice the size of Earth,[45] although the one which got the most attention as "Earth-size" (KOI 326.01) turns out to be in fact much larger.[46] A September 2011 study by Muirhead et al reports that a re-calibration of estimated radii and effective temperatures of several dwarf stars in the Kepler sample yields six additional Earth-sized candidates within the habitable zones of their stars: KOI 463.01, KOI 1422.02, KOI 947.01, KOI 812.03, KOI 448.02, KOI 1361.01.[2] Based on these latest Kepler findings, astronomer Seth Shostak estimates that "within a thousand light-years of Earth" there are "at least 30,000 of these habitable worlds."[17] Also based on the findings, the Kepler Team estimates "at least 50 billion planets in the Milky Way" of which "at least 500 million" are in the habitable zone.[47]

Exoplanet Year of discovery Notes
70 Virginis b (unconfirmed) 1996 Later confirmed not to be a Goldilocks planet.
Gliese 581 d 2007 Found to be in its star's habitable zone in 2009.
Gliese 581 g (unconfirmed) 2010 Most Earth-like Goldilocks planet found to date; probably tidally locked.{Unconfirmed}
HD 85512 b 2011 Earth-like planet in the HD 85512 system

[edit] See also

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[edit] References

  1. ^ "VPL Glossary". http://vpl.astro.washington.edu/epo/glossary.html. 
  2. ^ The Goldilocks Zone – NASA
  3. ^ Muir, Hazel (25 April 2007). "'Goldilocks' planet may be just right for life". New Scientist. http://www.newscientist.com/article/dn11710. Retrieved 2009-04-02. 
  4. ^ "The Goldilocks Planet". BBC Radio 4. 31 August 2005. http://www.bbc.co.uk/radio4/science/goldilocksplanet.shtml. Retrieved 2009-04-02. 
  5. ^ New Conditions for Life on Other Planets: Tidal Effects Change 'Habitable Zone' Concept, ScienceDaily (Feb. 24, 2011)
  6. ^ 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. 
  7. ^ Michael D. Lemonick (2010-11-08). "Is Anybody Out There?". Time. http://www.time.com/time/magazine/article/0,9171,2028077,00.html. Retrieved 2011-04-27. 
  8. ^ http://www.scientificamerican.com/ " Exoplanets in the habitable zone"
  9. ^ Butler et al.; Wright, J. T.; Marcy, G. W.; Fischer, D. A.; Vogt, S. S.; Tinney, C. G.; Jones, H. R. A.; Carter, B. D. et al. (2006). "Catalog of Nearby Exoplanets". The Astrophysical Journal 646 (1): 505–522. doi:10.1086/504701. http://www.iop.org/EJ/article/0004-637X/646/1/505/64046.html. (web version)
  10. ^ a b Jones, Barrie W.; Sleep, P. Nick; Underwood, David R. (2006). "Habitability of Known Exoplanetary Systems Based on Measured Stellar Properties". The Astrophysical Journal 649 (2): 1010–1019. doi:10.1086/506557. http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?2006ApJ...649.1010J&db_key=AST&nosetcookie=1.
  11. ^ Mullen, L. (2001). "Extrasolar Planets with Earth-like Orbits". http://nai.nasa.gov/news_stories/news_detail.cfm?ID=126. Retrieved 22 July 2006
  12. ^ Barnes, J., O'Brien, D. (2002). "Stability of Satellites around Close-in Extrasolar Giant Planets". Astrophysical Journal 575 (2): 1087–1093. doi:10.1086/341477. http://adsabs.harvard.edu/abs/2002ApJ...575.1087B.
  13. ^ Canup, R., Ward, W. (2006). "A common mass scaling for satellite systems of gaseous planets". Nature 441 (7095): 834 – 839. doi:10.1038/nature04860. PMID 16778883. http://www.nature.com/nature/journal/v441/n7095/abs/nature04860.html.
  14. ^ a b "NASA Finds Earth-size Planet Candidates in Habitable Zone, Six Planet System". NASA. 2011-02-02. http://kepler.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=98. Retrieved 2011-02-02. 
  15. ^ a b Borenstein, Seth (2 February 2011). "NASA spots scores of potentially livable worlds". MSNBC News. http://msnbc.msn.com/id/41387915/ns/technology_and_science-space. Retrieved 2011-02-02. 
  16. ^ a b Overbye, Dennis (2 February 2011). "Kepler Planet Hunter Finds 1,200 Possibilities". New York Times. http://www.nytimes.com/2011/02/03/science/03planet.html. Retrieved 2011-02-02. 
  17. ^ a b Shostak, Seth (3 February 2011). "A Bucketful of Worlds". Huffington Post. http://www.huffingtonpost.com/seth-shostak/a-bucketful-of-worlds_b_817921.html. Retrieved 2011-02-03. 
  18. ^ Planets for Man, Dole & Asimov 1964
  19. ^ Hart et al 1978, 1979 Icarus vol.37, 351–35
  20. ^ Fogg 1992
  21. ^ Kasting et al 1993, Icarus 101, 108–128
  22. ^ Budyko 1969
  23. ^ Sellers 1969
  24. ^ North 1975
  25. ^ Rasool & DeBurgh 1970
  26. ^ Schneider and Thompson 1980
  27. ^ Kasting 1991
  28. ^ Kasting 1988
  29. ^ Ramanathan and Collins 1991
  30. ^ Lovelock 1991
  31. ^ Whitemire et al 1991
  32. ^ Guillermo Gonzalez, Donald Brownlee, Peter Ward, The Galactic Habitable Zone I. Galactic Chemical Evolution, 12 Mar 2001
  33. ^ 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. arXiv:astro-ph/0401024. Bibcode 2004Sci...303...59L. doi:10.1126/science.1092322. PMID 14704421. 
  34. ^ Nikos Prantzos (2006). "On the "Galactic Habitable Zone"". Space Science Reviews 135: 313–322. arXiv:astro-ph/0612316. Bibcode 2008SSRv..135..313P. doi:10.1007/s11214-007-9236-9. 
  35. ^ Is Iron an Energy Source for Early Life on Anoxic Earth?
  36. ^ Solstation - Habitable
  37. ^ Evolving the Alien by Ian Stewart and Jack Cohen
  38. ^ Planets for Man
  39. ^ 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]
  40. ^ Immigrant Sun: Our Star Could be Far from Where It Started in Milky Way Newswise, Retrieved on September 15, 2008.
  41. ^ Joe Palca. "'Goldilocks' Planet's Temperature Just Right For Life". http://www.npr.org/templates/story/story.php?storyId=130215192. Retrieved 5 April 2011. 
  42. ^ David Koch; Alan Gould (March 2009). "Overview of the Kepler Mission". NASA. http://www.kepler.arc.nasa.gov/about. Retrieved 2009-04-02. 
  43. ^ Mike Wall (April 2011). "Image Shows 1,235 Potential Alien Homeworlds". FOX News. http://www.foxnews.com/scitech/2011/04/03/image-shows-1235-potential-alien-homeworlds/. Retrieved 2011-04-03. .
  44. ^ "70 Virginis b". Extrasolar Planet Guide. Extrasolar.net. http://www.extrasolar.net/planettour.asp?PlanetID=22. Retrieved 2009-04-02. 
  45. ^ a b Borucki, William J.; Koch, David G; Basri, Gibor; Batalha, Natalie; Brown, Timothy M.; et. al. (1 February 2011). "Characteristics of planetary candidates observed by Kepler, II: Analysis of the first four months of data". arXiv:1102.0541 [astro-ph.EP]. 
  46. ^ Grant, Andrew (8 March 2011`). "Exclusive: "Most Earth-Like" Exoplanet Gets Major Demotion—It Isn’t Habitable". 80beats. Discover Magazine. http://blogs.discovermagazine.com/80beats/2011/03/08/exclusive-most-earth-like-exoplanet-gets-major-demotion%e2%80%94it-isnt-habitable/. Retrieved 2011-03-09. 
  47. ^ Borenstein, Seth (19 February 2011). "Cosmic census finds crowd of planets in our galaxy". AP News. http://apnews.excite.com/article/20110219/D9LG45NO0.html. Retrieved 2011-02-19. 

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