The "habitable zone" actually refers to a certain distance around a starInternal waterIt can exist in liquid form becauseLiquid waterScientists believe that it is an indispensable element for the survival of life. Therefore, if a planet happens to fall within this range, it is considered that it has a greater chance to have life or at least an environment where life can exist.
The "livable belt" is also called "livable belt".The discussion is divided into a variety of life objects, and the general research is the "livable zone of animals".For any animal, plant or even microorganism, there will be a zone most suitable for its survival.If we summarize all living things on the earth and the different conditions they need in different stages of evolution, we can expand these conditions to the conditions of the planet, the sun and evenStellar systemThe concept of "livable zone of the universe" came into being.
Most of the space in the universe is a dangerous place for life. Only a few places can become an oasis of life and provide suitable atmosphere, temperatureLiquid water。Empty space, hotfixed starIt is difficult for gaseous planet surfaces to host life.It seems that it is difficult to clearly give what life needsenvironment condition。After providing the most basic environment for life, the earth has provided a perfect habitat for life in all aspects, and the earth is the only planet with life residence discovered so far.The discussion of the "livable zone of the universe" needs to focus on space, timebio-diversityAnd other aspects.
For astronomers, "potentially livable" planets are those that can sustain life, but not necessarily allow human survival.Habitability depends on many factors, but liquid water and atmosphere are the two most important factors.The gravity of a planet will causeradial velocity Regular changes occur, and multiple planets will make their mainStellar motionComplex rocking activities occur.[1]
As an ideal place for all kinds of life, one of the most important characteristics of the earth is its location, that is, the distance from the sun.The most suitable distance from the sun is the basis for defining the "habitable zone", where planets similar to the Earth may exist.The planets in the habitable zone will not receive less heat from the starsWater freezesThere is not much to make the water boil, just to maintain a liquid ocean.
The range of livable zone should be limited by the low temperature at the outer edge and the high temperature at the inner edge.solar systemTwo neighbors of——VenusAnd Mars give a livable zoneOuter boundaryExample of.If it is close to the sun, there will be no liquid ocean on the surface of the planet, like Venus;If it is too far away, it will be frozen again.If the earth moves outward for some distance, or weakenssolar radiationWill make the earth'ssurface temperature Lower until covered with ice and snow.Gradually, carbon dioxide is also frozen, formingdry iceParticle compositionAnd finally they are frozen at the polesice sheet。The width of the habitable zone depends on the similarities and differences between the planet and the earth, as well as the size and age of the stars.
On the surface of a rocky planet with a certain carbon content, if there is a stable liquid ocean, it will have the conditions to generate and retain life.In relation to the Earth's orbit, Venus andsunlightThe distance between Venus and Mars is 30% and 50% respectively. From the perspective of receiving sunlight, Venus needs to be reduced by half, while Mars needs to be doubled.
Biology
aboutterrestrial planetFor example, the habitable zone of animals is such a range that the planet can maintain a liquid ocean on the surface and can maintain the global average temperature below 50 ℃.This temperature is basically animal lifeLiving conditionsThe upper limit of.
If we consider modern humans（Smart life）A narrower "habitable zone" will be obtained if the living environment is less than 50%, that is, a planet that can grow enough crops for human consumption.On the contrary, a wider and easier to define livable zone is the "livable zone" for microbial survival: it almost covers the wholesolar system, and the solar system has been formed until now.For other types of "livable zones", they are wider than higher organisms but narrower than microorganisms.
time
Take the solar system as an example. Because the sun is brightening, the habitable zone of the solar system is also moving outward. As the temperature increases, the Earth will gradually become Venus. However, this situation will occur after 2-3 billion years, and the Earth will be in the habitable zone for 6-8 billion years.Of course, with the increasing brightness of the sun, a frozen planet may become suitable for the generation and reproduction of life.
Main scope
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In 1978, celestial bodiesphysical scientistMichael Hart made a simulation calculation, and the results were surprising.He took into account in his calculationsSolar luminosityChange over time.Four billion years ago, the sun's luminosity was only 70% of what it is today.With the increase of solar luminosity, the livable zone moves outward.Hart called the narrow belt where the earth is located the "sustainable habitable belt", which moves outward with the evolution of the entire solar system.
His calculation shows that if the distance between the earth and the sun is 1% further away, there will be an irreversibleGlaciationIf the distance is 5% closer, it may also be in an irreversible greenhouse state.If the Earth's orbit were a bit more flat, the above distance limit would be more stringent.
Now, some people think that the habitable zone calculated by Hart is too narrow, because there are several effectsHe did not have.Considering that, one of the most important things is“carbon dioxide—silicateThe function of "circulation" is like an air conditioner to adjust the temperature of the earth in a proper range.Carbon dioxide contentNow only accounts forEarth's atmosphere0.03%, but it is agreenhouse gases: itsinfraredThe absorption property can prevent the heat radiated to the surface of the earth from re spreading into space.
carbon dioxide—silicateCyclic regulation starts from the surfaceWeathering, when the planet is warm, sufficient light and precipitationScouring actionWill accelerate the weathering speed, and at this time, themetal ionPrecipitate carbon dioxide in the atmosphere intocarbonate, so as to reduce the atmospheric temperature;When the surface is too cold, weathering and carbon dioxide precipitation rate will decline, and the carbon dioxide released by volcanoesgreenhouse effectAnd then increase the atmospheric temperature again.
This negative feedback effect widens the "sustainable livable zone" and makes the conditions for determining the livable zone more complex.
Accordingly, the United StatesBinzhouState universityCelestial bodies ofbiologistJames Castin and his colleagues define the "sustainable habitable zone" as: an earth like planet around a star has an atmosphere containing nitrogen, water and carbon dioxide, and the climate formed can sustain water based life.In 1993, they estimated that the width of the solar system's sustainable habitable zones ranged from 0.95 to 1.15Astronomical unit。This width is much wider than that calculated by Hart, but it is still very narrow in fact.
Stellar type
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Livable zone
Astronomers classify stars into O, B, A, F, G, K and M according to their colors.Their color ranges from blue to red, temperature from high to low, and quality from high to low.Our sun belongs to G type star.
Stellar massThe larger it is, the faster it burns and the shorter its life.The age of the sun is 5 billion years, and the age of the earth is 4.6 billion years. However, simple life has only appeared about 1.2 billion years since the birth of the earth, and complex life has only appeared after 3 billion years. Therefore, the life of stars must be long enough to breed life.In addition, the temperature of the star should not be too high, otherwiseultraviolet raysYou can go directly into deep water, which is impossible to evolve life.O. The three types of stars, B and A, have short lives and strong ultraviolet rays. They do not meet this condition. Only the four types of stars, F, G, K and M, meet this condition.
M star: deep sea life (very close to stars)
K-star: marine life and polar life (near stars)
G star: all life (habitable zone like Earth)
F star: all life (farther than the habitable zone of the earth)
Livable type
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Habitable zone of single star system
The luminosity of a star determines the position of its habitable zone, while the luminosity is determined by the size, type and age of the star.For stars larger than the sun, the habitable zone moves outward faster and lasts shorter.
The sun will be quite stable within 10 billion years after its birth, but its mass is 1.5 timesSolar massOur star will enter the red giant stage 2 billion years after its birth.When a star becomesRed GiantIts luminosity will increase thousands of times, and the livable belt will recede far away.The habitable zone of a star with a mass of more than 1.5 times the mass of the sun will not last long enough to evolve advanced life.becauseUltraviolet radiationAs the number of stars increases, the larger habitable zone of stars will be farther away or even there will be no habitable zone.
People often say that the sun is a typical star, but this is not the case because 95% of stars are smaller than the sun.For these stars smaller than the sun, their habitable zones are closer to the stars.But if the distance is very close, there will be greater danger: when the planet is very close to the sun, the tidal effect of the star will easilyTidal locking , causing only one side of the planet to face the sun.This synchronous rotation will result in extremely low temperature on the dark side of the planet and extremely high temperature during the day.
Two thirds of similar stars around the solar system arebinary star Or a star gathering system.Two situations can be considered: one is one or morecompanionThe distance between them is too close, and the planets revolve around all the stars;One is that the companion stars are far away, and the planets only revolve around one of them.
Some recent studies have shown that only when the distance between two stars is at least 50 astronomical units can planets be formed.Researcher Alan Hale believes that to form a stable planetary orbit, the distance between two stars is either less than 30 million kilometers or more than 1.5 billion kilometers.The problem is, that isStellar systemIf a planet has been formed in, can it have a stable orbit?
The evolution and growth of life at least requires a long-term stable condition on the earth, which requires a stable orbit.OneEccentricity of trackVery high planetRuntime, sometimes entering the livable zone, sometimes outside the livable zone, which may be viable for microorganisms, but it is fatal for advanced life.Moreover, even if a planet is formed, its orbit will be perturbed by the gravity of multiple celestial bodies, which will eventually cause the planet to fall into one of the stars or be ejected from the system.
Another problem in a comet system is the stars accepted by planetsRadiant energy。Even if the planet orbits in the same plane as the companion star, the energy it receives will also be changed by the eclipse between stars.S. H. Doyle estimated in his 1970 book Planets That Can Be Inhabited by Human Beings that 10% of the energy received by the planets would not affectResidential environment。This is a controversial issue: the change of solar energy output is far less than this figure, but it has caused climate change and ultimately affected the change of life species.
Habitable zones of other interstellar systems
Other types of star systems, if anyWhite dwarf、neutron star, black holes and other celestial systems are not suitable for the production of life.ForOpen clusterperhapsGlobular clusterThere will be more radiation and particles in the nearby space, and too many gravitational changes will affect the orbits of planets.Globular clusterAnother disadvantage is that they are composed of ancient stars, so they lack heavier elements. As we know,Heavy elementIt not only provides a living environment for life, but also an important part of life.
Out of band life
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AntiphiliaExtreme environmental microorganismsThe discovery of the "livable belt" requires us to reconsider the concept of "livable belt" different from that of a few years ago.The habitable zone is often limited to the habitable zone of animalsextreme environmentOnly a few microorganisms are neededchemical energyAnd water can be deep underground.Such environment can include planets, satellites and even planets outside the habitable zoneasteroidNear Earth surface of.
Although according to James Castin's calculation, Mars is outside the habitable zone“Phoenix Mars Probe”The Mars probe shows thatMartian soilThere is ice below, so the surface of Mars isSiberiaPermafrostSimilar, inIce granuleThe water film on the surface and the liquid water under the ice have the conditions for the existence of life, so Mars may soon be classified as a "livable zone".Another example isEuropaIt may have an ocean hundreds of kilometers deep, which greatly increases the chances of generating and breeding life.It can be seen that even the stars outside the habitable zone may still provide a condition for life.
Research findings
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Livable zone
National Science FoundationOn September 29, 2010, 30% of astronomers reported that they had found a planet "near" the Earth.This planet is about the same size as the Earth and may be habitable.
American researchers published a report on September 29, saying that they usedHawaii"Keck" ofAstronomical telescopeFound this one calledGliese 581gThe planet ofStellar systemThe "livable zone" may be suitable for life.The habitable zone refers to the region where the planets are reasonably close to the stars. In this region, the heat transferred from the stars to the planets is moderate, and the planets are neither too hot nor too cold.This planet revolves around aRed dwarf(a type of star) running, which takes no more than 37 days per week.This red dwarf star is calledGliese 581, about 20 light years (189.21 trillion kilometers) away from the Earth.This distance is not far by astronomical standards.
Researchers found that this planet is in the center of the "habitable zone" near Gliese 581.This area is suitable for the existence of liquid water and life.The researchers said thatsolar systemInMercurySimilarly, this exoplanet is also "locked" by its star, that is, one side of the planet almost always faces the star, and the other side almost always faces the star, so its surface temperature difference is large.
However, according to the researchers, this planet's light dark junction is the most livable.The gravity of this planet is close to that of the earth, which is enough to prevent the atmosphere from "escaping" and enable humans to walk upright.Astronomers think there are millions moreterrestrial planetWaiting for us to explore.
