The area suitable for life in the planetary system
Habitable zone (English: Habitable zone)spaceThe name of the refers toPlanetary systemThe area suitable for life.The situation in the habitable zone is favorablelifeAnd higher life may appear like the earth.There are two possible regions, one in the planetary system and the other in the galaxy.staycompatibleIntraregionalplanetandNatural satelliteIs the best candidate, theseExtraterrestrial lifeBe able to live in an environment similar to ours.Astronomers believe that life is most likely to occur in the periplanetary habitable zone (CHZ) like the solar system and the galactic habitable zone (GHZ) of large galaxies (although astronomers have just begun to study the latter).The habitable belt may refer to "life belt", "green belt" orGoodylock belt（Goldilocks）。In our solar system, the habitable zone is 0.99 to 1.70 away from starsAstronomical unitArea between.
Gliese 581gIt's humans in the red dwarfGliese 581(about 20 from the earthLight year）The sixth planet found nearby.Gliese 581g was discovered by astronomersExoplanetThe famous example of orbits theoretically located in habitable zones.Astronomers have only found a dozen planets in habitable zones, whileKepler space telescope It is confirmed that 54 planets are located in the habitable zone.Astronomers estimateGalaxyAt least 500 million planets are located in habitable zones.
In aPlanetary systemIt is believed that the planets must be in the habitable zone for life to occur.Conceptually, the periplanetary habitation zone isfixed starIn the spherical shell like space around, the surface temperature of all planets within the range should be able towaterKeep liquid.Liquid water is considered vital to life because its role is as a solvent for biochemical reactions.In 1959, physicistsPhillip Morrison andGiuseppe KokonistaySearch for extraterrestrial civilization planThis area is mentioned in the research paper of.Frank DrakeIn 1961, he used this conceptDrake equation Make it widely known.
The distance between the habitable zone and the star can be calculated by the luminosity of the star.For the selected star, the range of distance can be determined by the following equation:
For example, a star whose luminosity is 25% of the sun has a habitable zone distance of about 0.5 AU, while a star whose luminosity is twice of the sun has a habitable zone distance of about 1.4 AU.This is because the luminosity followsInverse square law, assuming (especially) that it has atmospheric structure and thickness similar to that of the earth,ExoplanetThe distance between the center of the habitable zone and the parent star must have a global average temperature environment similar to that of the Earth.
When a star becomes brighter and its luminosity increases due to its evolution, the habitable zone around the star will move outward over time.The maximum time that living things can exist is that the longer a planet's orbit is maintained in the habitable zone, the better it will be.
The composition of the atmosphere also has an important impact.The temperature of the planet will be affected bygreenhouse gasesThe content of.
Habitable zone of the solar system
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Up to now, there is still no unified statement on the range of habitable zones of the solar system in the astronomical community. These statements advocate habitable zones between 0.725 and 3.0Astronomical unitBut they were all questioned by some astronomers.For example, Venus's orbit is near the inner edge of the habitable zone of the solar system, but it is too thickcarbon dioxideThe atmosphere makes its surface temperature as high as 462°C(864 degrees Fahrenheit), completely uninhabitable.The whole moon, Mars and dwarf planetCeresThey are all located in the habitable zone advocated by some people, and there are seasonal changes on Mars, but they cannot retain liquid water on their surface because their atmosphere is too weak.
Therefore, most of them point out that the thickness of a planet's atmosphere will affect its habitable zone position.According to the theory of extended habitable zone, aterrestrial planetIt needs to be farther from the star than the earth to retain liquid water on its surface.Gliese 667CdandGliese 581dIs a typical example of an exoplanet that can retain liquid water on its surface.
About the habitable zone of the solar system
Inner edge (AU)
Outer edge (AU)
particular year
notes
zero point seven two five
one point two four
1964
It is obtained by measuring optical thin atmosphere and fixed albedo.The aphelion of Venus is just within the habitable zone.
-
1.385–1.398
1969
It is obtained by measuring the ice albedo when global glaciers appear.This statement was supported by astronomers Cyrus and Nov.
0.88–0.912
-
1970
According to the measurement of Venus atmosphere.Rasul and Deberg pointed out that only in this habitable zone can the earth form stable oceans.
zero point nine five
one point zero one
1979
It is obtained by measuring the composition of the earth's atmosphere and surface temperature.This statement is sometimes quoted in publications.
-
three
1992
According to measurementcarbon cycleGet the outer edge of habitable zone.
-
one point three seven
1993
According to the cooling effect of measured cloud albedo.
-
two
2010
According to the measurement of high rail inclination andEccentricity of trackobtain.Seasonal liquid water can only appear in this habitable zone.
zero point seven five
-
2011
According to estimatesDesert planetThe minimum distance between the two polar ice caps is obtained.
0.77—0.87
1.02—1.18
2013
According to the measurement of high pressure, the pressure of habitable planets must be at least 15Millibar。
zero point nine nine
one point six eight eight
2013
According to the out of control greenhouse effect and water loss algorithm.It is only applicable to planets with atmospheric pressure and atmospheric composition similar to that of the Earth.
zero point five
-
2013
Derived from estimated atmospheric composition, pressure, and relative humidity.
zero point nine seven
-
2013
It is only applicable to planets with atmospheric pressure and atmospheric composition similar to that of the Earth.
Habitable zone of galaxy
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The position of the planetary system in the galaxy is also a factor determining the development of life, which leads to the concept of the habitable zone of galaxies. However, this concept has recently been questioned.
To shield life, a solar system must be close enough to the core of the galaxy to have enough heavy elements so that the planets can become rocky planets when they form.Heavy elements must exist, because they are necessary for the formation of complex life molecules, such as ironhemoglobinIodine is a componentthyroid hormoneIt is assumed that iron is the material needed by all living things.
Extrasolar satellites in the habitable zone described by artists
On the other hand, the solar system must be far enough away from the center of the galaxy to avoid asteroids andcometCollision with other stars, as well as supernova explosion and galaxy centerblack holeAnd so on.The impact of the radiation from supernovae on the origin of life is still unclear. It is speculated that a large amount of radiation from the galactic core will make it more difficult to form molecules with complex structures.Similarly, in many largerspiral galaxyandElliptical galaxyThe interstellar gas and dust in the central region of the star has been exhausted. Those regions are no longer the main places for star birth, and the star birth rate is far lower than other regions around.
Research shows that the content of heavy elements, orAmount of metalIt also has an effect that the higher the region is, the more likely it is that there will be giant planets orbiting close to the parent star.The gravitational tidal force caused by such a planet will lead any planet with mass like the Earth to change its orbit and surface shape, and destroy it before life is generated.For these reasons, it is difficult to confirm the habitability of galaxies with many uncertain factors.Moreover, the atmosphere of a planet cannot be too thick or too thin: too thick will form a Venus like high surface temperature, while too thin will make the planet become Mars likeDesert planet。
In ourGalaxy, the habitable zone of galaxies is considered to be slowly expanding to the distanceGalactic core25,000Light year（8,000Second gap）It includes stars between 4 billion and 8 billion years old.Other galaxies, because of their different structures, may have larger or smaller habitable zones, or even none at all.
Galaxy habitable zone or GHZ is like Goldilocks Zone. The term Goldilocks Zone comes from fairy talesGoodylockThe girl in the fairy tale likes porridge which is neither too cold nor too hot.astronomerJames Lovelock In assertingGaia hypothesisTurn this noun into a term when you use the.This concept means that life exists at - 15 ℃ (5 ℉) (an Antarctic alga, Cryptoendalith) and 121 ℃ (250 ℉) (active around deep-sea hot poresThermophilic bacteria）In the temperature section of, the absolute temperature converted is 258 to 394K, less than an order of magnitude.
Future technology may allow us to measure the location and number of planets suitable for the Earth in the Milky Way, which will enable us to know more about the habitability of galaxies.
critique
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The concept of habitable zone isIan StewartandJack CohenCoauthor《Evolution of aliens》(Evolving the Alien).Based on the following two reasons: First, it is assumed that aliens need the same living conditions as the Earth;Second, even with such a hypothesis, other environments may also create areas suitable for life outside the aforementioned habitable zone.Europa, for example, is believed to have an ocean similar to the deep sea environment of the Earth beneath its surface.Although Europa is not in the habitable zone around the planetExtremophile, which makes Europa's hypothesis of being alive sound.Planetary biologistCarl Sagan Believe ingas giant Like Jupiter itself, there may also be creatures living in it.Different forms of life can be found in different environments, and it is too conservative to expose these limited hypotheses.
Different degrees of volcanic activity, the role of satellites or the quality of planets may affect the level of radiation and heat of planets, and thus change the conditions supporting life.At the same time, if Earth like creatures can be transplanted and adapted to Europa's environment in places where it is almost impossible to be the origin of life, or use advanced technology.Then, a planet moving out of the habitable zone is very like moving life to a place that is not suitable for survival.
Some theories suggest that life has already existed underground shortly after the birth of the earth to avoid the harsh environment on the surface.And scientists have also proved that life can survive underground.In fact, biologists have found life more than 10 kilometers underground.Therefore, they believe that planets not located in habitable zones are suitable for life.
