In planetary science andAstrobiologyMiddle, Earth Difference Hypothesis（English: Rare Earth hypothesis)earthuppermulticellular organismThe formation ofAstrophysicsandgeologyCombination of events and environments.The word "earth earth" comes from a book written byPeter Wald(Peter Ward) andDonald E. Brownie（Donald E. Brownlee）[1]"Earth: Why Complex Life Is Uncommon in the Universe"Astrophysics、AstrobiologyPlanetary Science Explores the Mystery of the Birth of Life).
The Earth Difference Hypothesis isCarl Sagan andFrank DrakeProposedMediocrity principleThe opposite concept.The mediocrity principle holds that the earth is just located in the ordinaryBarred spiral galaxyA commonPlanetary systemAn ordinary one inRocky planet, so the wholeuniverseIs full of complex life.Wald and others pointed out that planets, planetary systems and galactic regions suitable for complex life, such as the Earth, the solar system and our region in the Milky Way, are very rare.
Chinese name
Earth Difference Hypothesis
Foreign name
Rare Earth hypothesis
Main ideas
The earth is different from other celestial bodies
The Earth Difference Hypothesis points out that the formation of complex life requires the combination of multiple accidental conditions.These conditions include galaxiesHabitable zonePlanetary system with similar conditions, planet size, and a huge natural satellite (such asMoon）The planet has the magnetic sphere and the corresponding plate movement, the lithosphere, the atmosphere, and the ocean, giant glaciers and asteroid impacts.
The conclusion of the Earth Difference Hypothesis is that complex life is unusual, which may beFermi paradox"If extraterrestrial life is common, why don't they appear?" provides an answer.[2]
Habitable zone of galaxy
Main item: habitable zone
NGC 7331
A planet in a metal deficient region or a high radiation region near the silver core cannot support the existence of life.NGC 7331 is often called the sister galaxy of the Milky Way.The Earth Difference Hypothesis points out that the universe, including most regions of our Milky Way Galaxy, cannot support complex life, which is called Dead Zones by Wald et al.The habitable zone is also closely related to the distance to the Silver Heart.The farther away from the silver core:
1. The amount of metal in stars decreases, but the metal is formedterrestrial planetRequired.
2. Silveryblack holeReleasedX-rayAnd gamma ray intensity.This kind of radiation is harmful to complex life.
3. As the density between stars decreases, the possibility of planets and micro planets being interfered by the gravity of nearby stars and other influences decreases.Therefore, the farther a planet is from the silver center, the less likely it will be impacted by an asteroid.
The above (1) excludes the periphery of the galaxy, (2) and (3) exclude the inner region of the galaxy, globular clusters and spiral arms of spiral galaxies.These spiral arms are not objective objects, but regions characterized by high star formation rate, which move slowly in the galaxy in waves.As it moves from the center of the galaxy to its farthest extreme, its ability to support life increases and then decreases.
Even if somePlanetary systemIn the region suitable for complex life, it must be maintained for a long time so that complex life can evolve.If the star's orbit is elliptical, it will pass through some parts of the galaxySpiral arm。But if the star's orbit is circular, its revolution speed is equal to the rotation speed of the spiral arm, so the star will only slowly enter the spiral arm area.Therefore, the proponents of the Earth Difference Hypothesis point out that a star capable of breeding life must have a nearly circular orbit around the silver center.
Lineweaver et al. calculated that the diameter of the habitable zone of the Milky Way is 7 to 9 thousand seconds, which only contains no more than 10% of the stars in the Milky Way.According to a conservative estimate of the number of all stars in the Milky Way, this number is about 20 to 40 billion stars.However, Gonzalez et al. estimated that the number was only half of the above;That is, up to 5% of stars in the Milky Way are located in habitable zones.
The sun's orbit around the silver center is almost circular.The solar revolution period is 226 million years, which is very similar to the rotation period of the Milky Way.Karen Masters calculated that every 100 million years, the revolution of the sun will make it pass through one of the main spiral arms of the Milky Way.On the contrary, the Earth Difference Hypothesis speculates that since the formation of the sun, it has not passed through any spiral arm of the Milky Way [5].But some studies have shown that some mass extinction events are consistent with the spiral arms of the sun passing through the Milky Way in the past.[3]
Main star
Diagram of habitable zone
This map of habitable zone shows where Earth like planets can exist (but microbes may exist outside the habitable zone in the map).The formation of complex life requires liquid water.The habitable zone of planets is an annular region centered on the main star.If the planet is too far or too close to the main star, its surface temperature will not have the conditions to produce liquid water (although in various distances, it will produceEuropaGeneral subsurface moisture is possible).Kasting et al. estimated that the habitable zone of the solar system is between 0.95 and 1.15 astronomical units.
The habitable zone will change with the type and age of the host star.Habitable zone and carbon dioxidegreenhouse effectClosely related, on earthcarbon dioxideIs enough to raise the average temperature of the earth's surface (from its original temperature) by 40 ° C.[4]
Suppose the star has a rocky planet in its habitable zone, likeSiriusorVegaIsothermal stars have a wide habitable zone, but there are two problems:
1. It is generally believed that rocky planets are formed in the region close to the parent star, but they are likely to be located outside the habitable region because they are too close to the parent star.This does not rule out the possibility of life on natural satellites of gas planets.
2. The life of hot stars is short. They will change into red giant stars in only one billion years. Such a short time is not enough for the evolution of advanced life.
These ideas exclude the possibility that giant, high-energy stars such as F6 or O type can breed multicellular life.
Globular clusters are unlikely to support life
On the other hand, smallRed dwarfA habitable zone with a small radius.This causes one side of the planet to always face the main star, while the other side is always dark. This situation is calledTidal locking 。The result of tide locking is that one side of the planet is extremely hot and the other side is extremely cold.In addition, planets located in habitable zones with small radius will be affected by the main star flare, which will ionize their atmosphere, thus endangering the survival of complex life.Proponents of the Earth Difference Hypothesis ruled out the possibility that such a planetary system would have life.However, some astrobiologists believe that these planetary systems may be habitable in a few cases.This is the main controversy in theory, because these K to M stars account for 82% of all thermonuclear stars.
Proponents of the Earth Difference Hypothesis believe that the right type of main star is between F7 and K1, and such stars are uncommon.G type stars like the sun (between the hotter F type and the colder K type) account for only 9% of the total number of thermonuclear stars in the Milky Way.
Older stars, such as red giant stars andWhite dwarfAre unlikely to support life.stayGlobular clusterandElliptical galaxyRed giant stars are very common.White dwarfs are dying stars that have passed through the red giant stage.After a star becomes a red giant, its diameter will increase significantly.If a planet is in the habitable zone when its parent star is still in the young and middle-aged stage, it will be vaporized by its parent star when it becomes a red giant.
The energy release of a star in its life cycle should only change very slowly.Variable stars, such asCepheid variableThe possibility of supporting life is very small.If the energy release of the main star suddenly decreases, the water on the planet will freeze.On the contrary, if the energy release of the main star increases significantly, the oceans on the planet will evaporate, causing the greenhouse effect.
Life cannot be created without complex chemical compositions, which must contain metals, namely, elements other than hydrogen, helium and lithium.This means that a planetary system rich in metals is a necessary condition for the existence of life.The only known mechanism that can make metals and disperse them into the universe is the explosion of supernovae.The existence of metals in stars can be determined by the absorption spectrum. Studies have found that many or even most stars lack metals.The early universe, globular clusters, stars formed in the early universespiral galaxyThe stars in other galaxies and all stars located in the outer regions of the galaxy are characterized by low metal content.Therefore, it is believed that the main stars with sufficient metal to support the survival of complex life are generally located inLarge spiral galaxyThe quiet edge area is far away from the high radiation of the silver core, which is another reason why it supports life.
Planetary system
Nebulas that can breed stars can also breed stars like Jupiter andSaturnLow metal gas planet.butJupiter like planetThere is no solid surface necessary for the survival of complex life (although their satellites may have solid surfaces).Wald and Browne believed that the planetary system supporting the survival of complex life should be more or less similar to our solar system, including the inner planets with solid surfaces and the outer planets with gaseous surfaces.But recent research has questioned this view.
Uncertainties of Jupiter
Jupiter
Jupiter is the fifth and largest planet in the solar system.When Ward and Brownie wrote the book, their view of Jupiter like planets was that they could keep asteroids away from the planets that gave birth to life, and prevent them from being hit by asteroids.But recent computer simulations show that the reality is more complex.Compared with the number of asteroid impacts prevented by Jupiter, the number of asteroid impacts caused by Jupiter is more than three times.If Jupiter is replaced by a Saturn sized object, it will cause about twice the number of asteroid impacts.
Obstruction of revolution
The gas planet cannot be too close to a celestial body that is pregnant with life. Unless the celestial body is its satellite, the gas planet will hinder the revolution of that celestial body.
NewtonMotion can cause chaos in the rotation of planets, especially in a highEccentricity of trackThe gas giant planets in the planetary system.
Considering the factor of stable revolution, a giant gas planet (called“Hot Jupiter”）Our planetary system is ruled out as habitable.It is believed that when hot Jupiter formed, they were far away from their parent star than they are now, but then moved to the current orbital orbit.In this process, they may have disastrous hindered the orbit of all planets in the habitable zone.
Planetary volume
A planet too small can not maintain enough atmosphere and have a large area of oceans.It tends to have rugged terrain, such as high mountains and deep valleys.Their cores will cool rapidly.Their plate movement can not last as long as the large planets or there is no plate movement at all.
according toUniversity of ArizonaAccording to the astronomer Michael Meyer, rocky planets like Earth may be common in the universe:
Our observation shows that in 20% to 60% of sun like stars, there is evidence that the formation process of rocky planets is similar to that of the Earth, which is very exciting.
– Michael Mayer
Michael's research team found cosmic dust near newly formed sun like stars, which they regarded as a by-product of the formation of rocky planets.
Giant natural satellite
In the solar system, natural satellites such as the moon are unusual, because some of the rocky planets outside the Earth do not have their own satellites (such as Mercury and Venus), while some only have very small natural satellites (such as Mars).
The big collision theory speculated that the formation of the moon was the result of the collision between a body with the size of Mars and the early Earth, which gave the tilt angle and rotation speed of the Earth's rotation axis.Rapid rotation reduces the daily temperature change rate of the earth and maintains photosynthesis.The earth difference hypothesis further points out that the inclination of the rotation axis (relative to the orbital plane) cannot be too large or too small.A planet with an excessively inclined axis will encounter extreme seasonal changes in climate.On the contrary, a planet with too small tilt angle of its axis lacks seasonal changes to promote life evolution.In this regard, the earth is "just right".The gravity of the giant natural satellite can also stabilize the inclination of the planet's rotation axis.Without this effect, the change of rotation axis inclination will be very chaotic, which may cause that complex life cannot be born on land.
If the earth does not have such a natural satellite as the moon, the tide change of the ocean will be very small only by the gravity of the sun.
The giant natural satellite of the planet can increase the possibility of plate movement on the planet through the effect of tidal force on the planet crust.In addition, in the absence of inhomogeneous crust, it may not produce the large-scale plate movementmantleConvection.But there areMarsThe evidence shows that, even without the above mechanism, there has been plate movement on Mars before.
If the impact of a giant celestial body is the only way for a planet to obtain a giant natural satellite, then the planets in the habitable zone around the star need to form aDouble planetThe system is large enough to impact the celestial body to create a huge natural satellite.Such an impact celestial body may not be able to form.
Plate movement
Unless the chemical composition of the planet makes plate movement possible, no plate activity will occur on the planet.The persistent heat energy required for plate movement is known to come from the deep radiation inside the planet.The continents on the planet must also be made of granite floating on the denser basalt.Taylor(Taylor) stressed that the subduction zone (an essential part of plate movement) needs lubrication from rich water sources.On Earth, subduction zones exist only at the bottom of the ocean.
Inertial alternating event
Many evidences indicate that the Cambrian explosion periodContinental driftVery often.In fact, the continent can move from the Arctic to the equator or from the equator to the Arctic in less than 15 million years.Kirschvink et al. put forward the following controversial opinion: the uneven distribution of the continental mass relative to the axis of rotation leads to the 90 ° change of the Earth's axis of rotation.This results in drastic changes in climate and ocean currents in the short term and affects the whole earth.They call this "Inertial Interchange Event", which has not been proved by science and is extremely unusual if it happens.If such an event is more thanPoriferaandCoral reefFor the development of more complex creatures, we have another reason why complex creatures are rare in the universe.
Earth difference formula
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The following discussion refers to Cramer.Wald and Brownie derived the formula of earth difference from Drake formula.According to this equation, the number of Earth like planets with complex life in the Milky Way is:
Earth difference formula
Including:
● N * is the number of stars in the Milky Way.This figure is not easy to estimate, because it is very difficult to estimate the mass of the Milky Way, not to mention that there is almost no information about tiny stars.N * is at least 100 billion, and may be as high as 500 billion (if there are many stars with low visibility in the Milky Way).
● ne is the average number of planets in the habitable zone of stars.In the time required for complex life evolution, the average temperature of the planet surface should be moderate (neither too high nor too low), so that the water on the planet can always be maintained in a liquid state.Restricted by this condition, the habitable area is quite narrow.So ne=1 is probably the upper limit.
We assume that.According to the earth difference hypothesis, the product of the other nine earth difference parameters (all fractions) will not be larger than 10 − 10, and it is likely that the actual value is only 10 − 12.In the latter case, N may be as small as 0 or 1.Walter and Brownie did not really calculate the value of N, because most of the following earth's different parameters can only be estimated.
● fg is the proportion of stars located in the habitable zone of the galaxy (Vald and Browne et al estimated this parameter to be 0.1).
● fp is the proportion of stars with planets in the Milky Way.
● fpm is the specific gravity of a rocky planet (non gas planet).
● fi is the proportion of habitable planets with microorganisms.
● fc is the proportion of planets with complex life.
● fl is the proportion of complex life in the life cycle of the planet.
● fm is the proportion of habitable planets with huge satellites.
● fj is the proportion of planetary systems with giant gas planets.
● fme is the proportion of planets that only cause a low number of extinction events.
The difference between the earth difference formula and Drake's formula is that it does not take into account the factor that complex creatures evolve into intelligent creatures with technology (it is worth mentioning that neither Wald nor BrownieEvolutionary biologyHome).
Hypothesis support
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The following books (arranged from easy to difficult) support the idea of the Earth Difference Hypothesis:
● Stuart Ross Taylor, an expert who studies the solar system, firmly believes in the hypothesis of the Earth's difference in his book, but that book mainly introduces the solar system and its formation, rather than discussing the authenticity of the hypothesis.Taylor's conclusion is that the solar system may be extremely unusual, because it is the result of many accidental factors and events.
● Physicist Stephen Webb mainly elaborates and refutes some candidate solutions to Fermi's paradox in his book.At the end of the book, the Earth Difference Hypothesis becomes one of the few reservation schemes.
● Paleontologist Simon Conway Morris believes that the evolutionary process is convergent.The fifth chapter of the book is devoted to the earth difference hypothesis.Although Morris agrees that the Earth is probably the only planet supporting complex life in the Milky Way, he believes that it is quite possible for complex life to evolve into intelligent life.
● Physical cosmologists John D. Barrow and Frank J. Tipler (19863.2, 8.7, 9) forcefully defended the hypothesis that "humans are likely to be the only intelligent life in the Milky Way Galaxy and even the whole universe", but their books only comprehensively studied how the principle of human selection and physical laws skillfully promote complexity in nature, not focusing on the above hypothesis.
● In the book Singularity Approaching, computer pioneer Raymond Kurzweil believes that from the perspective of technological singularity, the earth must be the first planet to evolve a life with emotion and technology.Although there may be other planets similar to the Earth, the Earth must be the most evolved.Otherwise, we can find evidence that other civilizations have experienced technological singularity.
criticism
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Criticism of the Earth Difference Hypothesis comes from the following different aspects.
By the beginning of 2009, there were more than 300 known extrasolar planets, and the number of newly discovered planets was increasing.Dr. Alan Boss of Carnegie University of Science and Technology estimates that there may be as many as 100 billion rocky planets in the Milky Way alone.Dr. Booth believes that many of these planets have simple life forms and there may be thousands of civilizations in the Milky Way.Dr. Booth guessed that the sun like stars have an Earth like planet on average.
University of EdinburghResearchers from the University of California are trying to find out how many intelligent civilizations exist in the universe.Research indicates that there may be thousands of intelligent civilizations.
Biological evolution
The core of the earth difference hypothesis is biological evolution: microorganisms may be common in the universe, but advanced life may not be.Simon Conway Morris is the only evolutionary biologist who has ever talked about the hypothesis of Earth's difference.The Earth Difference Hypothesis holds that complex life can only evolve on Earth like planets or suitable satellites of planets.Some biologists, including Jack Cohen, believe that this hypothesis is limited and unimaginable. They regard the hypothesis as a circular argument.According to astronomer David Darling, the Earth Difference Hypothesis is neither a hypothesis nor a conjecture, it only describes how life on the earth develops.
