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The formation and evolution of the solar system

Formation and evolution of the solar system

solar system The formation and evolution of Molecular cloud Small Gravitational collapse 。 Most of the collapsed mass is concentrated in the center to form the sun, and the rest is flattened to form a Protoplanetary disk , and then formed planet , satellites, meteorites and other small solar systems sphere systems 。

This is called Nebula hypothesis The widely accepted model of Emmanuel Swedenburg 、 Immanuel Kant and Pierre Simon Laplace Propose. Its subsequent development is related to astronomy, physics, geology and Planetology And other scientific fields are intertwined. Since the advent of the space age in the 1950s and the 1990s Extrasolar planets This model is challenged and further refined in the process of interpreting new discoveries.

Since its formation, the solar system has undergone considerable changes. Many moons are formed in a disk of gas and dust surrounding their parent star. Other moons are believed to have been captured or come from Huge collision (This is the case for the Earth's satellite Moon). Collisions between celestial bodies continue to occur and are the center of the evolution of the solar system. The positions of planets often migrate, and some planets have shifted to each other. such Planetary migration It is believed to have played a major role in the early evolution of the solar system.

Like the birth of the sun and planets, they will eventually die. In about 5 billion years, the sun will cool and expand outward many times its diameter (becoming a Red Giant ）, throw away its outer layer to become Planetary nebula , and leave it known as White dwarf The remains of a star. In the distant future, the sun's circling planets will be gradually swept away by the gravity of the passing stars. Some of them will be destroyed, and others will be thrown into interstellar space. Finally, after trillions of years, the sun will be alone and there will be no other celestial bodies in the orbit of the solar system^[1] 。

Chinese name: The formation and evolution of the solar system
Principle: huge Molecular cloud Small Gravitational collapse Most of the collapsed mass is concentrated in the center

From: 4.6 billion years ago
Model: Nebula hypothesis
Proposed time: eighteenth century

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seven years
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historical background

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One of the initiators of Pierre Simon Laplace nebula hypothesis

The thought about the origin and destiny of the world can be traced back to the earliest known written records; However, in most of those times, no one tried to link such a theory with the existence of the "solar system" for the simple reason that people generally did not believe that the solar system as we know it existed at that time. The first step towards the theory of the evolution and formation of the solar system is the widespread recognition of the heliocentric theory, which places the sun at the center of the system and places the earth in orbit around it. This theory was conceived for thousands of years, but it was not widely accepted until the end of the 17th century. The first recorded use of the term "solar system" was in 1704.

The standard theory of the formation of the current solar system: Nebula hypothesis , since it was Emmanuel Swedenburg 、 Immanuel Kant , and Pierre Simon Laplace It has been adopted and rejected repeatedly since it was proposed. A major criticism of this hypothesis is that it clearly fails to explain the lack of the sun relative to its planets angular momentum^[6] 。 However, since the research on new stars in the early 1980s showed that, as predicted by the nebula hypothesis, they were surrounded by disks of cold gas and dust, which led to the re acceptance of this hypothesis.

To understand how the sun will continue to evolve requires some understanding of its source of energy. Arthur Eddington yes Einstein Of relativity His confirmation led him to realize that the sun's energy came from its core nuclear fusion 。 In 1935, Eddington further proposed that other elements may also be formed in stars. Fred Hoyle To further elaborate this hypothesis, it is believed that the stars that have evolved into red giant stars will produce many elements heavier than hydrogen and helium in their cores. When the red giant finally throws off its outer layer, these elements will be recycled to form other stars^[2] 。

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Presolar nebula

Main term: Solar nebula

The nebula hypothesis claims that the solar system consists of Light year Spanned Molecular cloud The debris is formed in the process of gravitational collapse. A few decades ago, the traditional view still believed that the sun was formed in a relatively isolated environment, but the study of ancient meteorites found that short-lived isotopes (such as Iron - 60 ）The element can only form in the explosion and stars with short life span. This shows that several times during the formation of the sun Supernova Burst. The shock wave of one of the supernovae may have created an ultradense region in the molecular cloud, leading to the collapse of this region, thus triggering the formation of the sun. Because only massive, short-lived stars can produce supernova explosions, the sun must be in a large star birth region (probably similar to Orion Nebula ）Formation.

The Orion Nebula photographed by the Hubble Space Telescope

Some of these collapsed gas regions, called "pre solar nebulae", will form the solar system. The diameter of this area is 7000 to 20000 Astronomical unit (AU) Its mass just exceeds that of the sun. Its composition is similar to that of the sun. from Primordial nucleosynthesis Generated Elements hydrogen 、 helium ,, and a small amount of lithium It constitutes 98% of the mass of the collapsed nebula. The remaining 2% of the mass is made of metals produced in previous generations of stellar nucleosynthesis Heavy element form. In the later years of these stars, they eject these heavy elements into Interstellar matter 。

because angular momentum Conservation, the nebula rotates faster when it collapses. As the nebula condenses, the atoms in it collide with each other more frequently kinetic energy It is converted into heat energy. The center of its mass concentration is more and more hot than the surrounding disk. After about 100000 years, under the mutual competition of gravity, gas pressure, magnetic force and moment of inertia, the contracted nebula flattened into a protoplanetary disk with a diameter of about 200AU, and formed a hot and compact protostar in the center (a star whose internal hydrogen fusion has not yet started).

The sun has developed to this evolution Point is considered as a Taurus T Star Type of star. The study of Taurus T stars shows that they are often accompanied by disks composed of pre planetary matter with a mass of 0.001 to 0.1 solar. These disks stretch hundreds of AU—— Hubble Space Telescope The protodisk with a diameter of 1000 AU in star forming regions (such as the Orion Nebula) has been observed - and it is quite cold, and the hottest can only reach 1000 AU Kelvin 。

In 50 million years, the temperature and pressure of the solar core become so huge that its hydrogen begins to fuse, generating internal energy to resist gravitational contraction until Static equilibrium 。 This means that the sun has become Main sequence star This is a major stage in its life. The main sequence stars generate energy from the fusion of hydrogen in their cores into helium. The sun is also a main sequence star^[3] 。

Planetary formation

See: Protoplanetary disk

Many planets in the solar system are believed to be formed from“ Solar nebula ”The solar nebula is a disk shaped cloud formed by the remaining gas and dust in the formation of the sun. The accepted planetary formation hypothesis is called accretion , where planets begin to form from dust particles in orbits around protostars. Through direct contraction, these particles form blocks with a diameter of one to ten kilometers, and then they collide with each other to form objects with a larger size of about five kilometers（ Microplanet ）。 Through further collisions, they gradually increased their size, increasing by several centimeters every year for the next few million years.

Solar nebula in artist's imagination

Inner solar system (The area within 4 astronomical units of the center diameter) is too warm for volatile molecules such as water and methane to gather, so the micro planets formed there can only be formed by materials with high melting points, such as iron 、 nickel 、 aluminum And stony silicate 。 These stone sky will become terrestrial planet （ Mercury 、 Venus And Mars). These substances are rare in the universe, accounting for only 0.6% of the mass of the nebula, so Earth like planets will not grow too large. Earth like planet embryos grow to 0.05 Earth mass 100000 years after the formation of the sun, and then stop gathering mass; The subsequent collision and merger of these planet sized objects made these Earth like planets grow to their size (see Earth like planets below).

Jupiter like planet (Jupiter Saturn 、 Uranus and Neptune ）Formed further Freezing line In addition, the material between the orbits of Mars and Jupiter is cold enough to keep volatile ice like compounds solid. Ice ratio on the Jupiter like planet terrestrial planet The metal and silicate on the surface are more abundant, making the mass of the Jupiter like planet grow large enough to capture the lightest and richest elements such as hydrogen and helium. Microplanets outside the freezing line have accumulated four times the mass of the Earth in 3 million years. These four Jupiter like planets account for 99% of the mass of all celestial bodies orbiting the sun. Theorists believe that it is no accident that Jupiter is just outside the freezing line. Because the freezing line gathers a large amount of water evaporated from the inward falling ice like material, it forms a low-pressure area, which accelerates the speed of the dust particles surrounding the orbit and stops them from moving towards the sun. In effect, the freezing line acts as a barrier, causing matter to rapidly gather at a distance of about 5 astronomical units from the sun. These excessive substances gathered into an embryo with about 10 Earth masses, and then began to grow rapidly by devouring the hydrogen in the surrounding star disk. It took only 1000 years to reach 150 times the Earth's mass and finally reached 318 times the Earth's mass. Saturn's mass may be significantly smaller because it was formed millions of years later than Jupiter, when it could use less gas.

Taurus T, like the young sun, has much more stable and stronger Stellar wind 。 Uranus and Neptune are believed to have formed after Jupiter and Saturn, after the solar wind blew away most of the matter in the astrolabe. As a result, there is very little hydrogen and helium accumulated on these two planets, each not more than twice the mass of the Earth. Uranus and Neptune are sometimes cited as failed nuclei. The main problem with the formation theory of these planets is their formation time. At their location, their cores need hundreds of millions of years to gather. This means that Uranus and Neptune may have formed closer to the sun - near or even between Jupiter and Saturn - before moving outward. (See planetary migration below). In the era of micro planets, the movement of planets was not all inward toward the sun; from Wilt II Stardust samples taken from Kuiper belt Regional migration.

After three to ten million years, the solar wind of the young sun will clear all the gas and dust in the original disk, and blow them into interstellar space, thus ending the growth of planets^[4] 。

Subsequent evolution

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Planets were originally thought to have formed in or near their orbits as we saw them. However, this view changed dramatically in the late 20th century and the early 21st century. It is believed that the solar system looks very different after its initial formation: there are several objects in the inner solar system at least as big as Venus, Outer solar system It is also more compact, Kuiper belt Much closer to the sun.

terrestrial planet

After the end of the planet formation era, there are 50-100 moon to Mars sized Planetary embryo 。 Further growth may be due to the collision and merger of these celestial bodies, which lasted about 100 million years. These celestial bodies exert gravitational force on each other, dragging each other's orbits until they collide and grow larger, until the last four known today terrestrial planet It has taken shape. One such huge collision is believed to have led to the formation of the moon (see satellite below), and another one stripped the early Mercury Housing.

The problem that this model does not solve is that it cannot explain these factors terrestrial planet How did the initial orbit of the two orbits, which had to be quite eccentric circles to collide with each other, form a fairly stable and nearly circular orbit like today's orbit. One of the hypotheses of this "eccentric removal" is that the Earth like planets formed in the gas disk have not been driven away by the sun. Of these residual gases“ Gravitational drag ”It will eventually reduce the energy of the planets and smooth their orbits. However, if such gas exists, it will prevent the Earth like planets from becoming so rounded in the beginning. Another hypothesis holds that gravitational drag does not occur between planets and gases, but between planets and the remaining small bodies. When a large body passes through a small body group, the small body hand is attracted by the gravity of the large body, forming a high-density area in the path of the large body, a "gravitational awakening", thus reducing the large body to enter a more regular orbit.

Asteroid belt

Asteroid belt It is located at the outer edge of the Earth like planetary region, 2 to 4 AU away from the sun. The asteroid belt began to have more than enough material to form more than two or three Earth like planets, and in fact, there were many Microplanet Formed there. Like Earth like planets, the micro planets in this region later merged to form 20 to 30 moon to Mars sized Planetary embryo ； But because it is near Jupiter, it means that the history of this region has changed greatly 3 million years after the formation of the sun. Jupiter and Saturn Track resonance It is particularly strong for the asteroid belt, and the gravitational interaction with more massive planetary embryos makes more micro planets spread into these resonances, causing them to be torn apart after collision with other celestial bodies, rather than condensed and aggregated. With the inward migration of Jupiter after its formation (see below Planetary migration ）Resonance will sweep across the entire asteroid belt, dynamically excite the number of objects in this area, and increase the relative speed between them. The cumulative effect of resonance and planetary embryos can either make the micro planets leave the asteroid belt, or stimulate their orbital inclination and Eccentricity Change. Some massive planetary embryos were also thrown out by Jupiter, while others may have migrated to the inner solar system and played a role in the final concentration of Earth like planets. In this initial depletion period, the total mass of the asteroid belt remaining mainly composed of micro planets under the influence of the giant planets and planetary embryos is less than 1% of the Earth. This is still 10 to 20 times the mass of the main belt, about 1/2000 of the earth's mass. The second depletion stage is believed to occur when Jupiter and Saturn enter the temporary 2:1 orbital resonance, which makes the mass of the asteroid belt drop to nearly the scale (see below).

The great impact period of the inner solar system may have played a role in obtaining the water composition (~6 × 10 kg) of the asteroid belt from the Earth. Water is too volatile to exist during the formation of the earth. It must have been sent from a cooler place outside the solar system. The water may be brought by the planetary embryos and small micro planets thrown away from the asteroid belt by Jupiter. Some discovered in 2006 main-belt comets It is also considered to be one of the sources of water on the earth. In contrast, from Kuiper belt Comets from far away regions bring only about 6% of the earth's water. The germ theory hypothesis believes that life itself may be sown on the earth in this way, although this idea is not widely accepted^[5] 。

Planetary migration

Main term: Planetary migration

Simulations of Outer Planets and Kuiper Belt

According to the nebula hypothesis, the two outer planets are in the "wrong position". For the wrong position of Uranus and Neptune, see Nice model As for Jupiter and Saturn asteroids, see the Grand Tack model of Jupiter. Uranus and Neptune Of the region Solar nebula Their low density and longer orbital cycle time make their formation seem very unreasonable. These two planets are believed to have formed near the orbits of Jupiter and Saturn, which have more matter, but later moved to their current positions after hundreds of millions of years.

The migration of outer planets is also necessary to explain the existence and characteristics of the outermost region of the solar system.

Neptune Beyond, the solar system extends to Kuiper belt 、 Ecliptic discrete objects and Oort Cloud These three sparse small ice like celestial bodies are believed to be the most observed comet The place of origin. At their distance from the sun Solar nebula The speed of gathering before dispersion is too slow to form a planet, so the original disk lacks enough material density to form a planet. The Kuiper Belt is 30 to 55 AU away from the sun, and the farther ecliptic discrete objects extend to 100 AU, while the distant Oort Cloud starts at about 50000 AU. But at first, the Kuiper Belt was much closer and denser to the sun, and its outer edge was about 30AU away from the sun. Its inner edge is just outside the orbits of Uranus and Neptune. The orbits of Uranus and Neptune were much closer to the sun when they were formed (maybe 15-20AU), and the position is opposite. Uranus is farther from the sun than Neptune.

After the formation of the solar system, the orbits of giant planets continued to change slowly, mainly due to the interaction between them and a large number of remaining micro planets. After 500 million to 600 million years (about 4 billion years ago), Jupiter and Saturn entered 2:1 resonance ； Saturn does not circle the sun until Jupiter circles it twice. This resonance has caused gravitational thrust to the outer planets, thus allowing Neptune to cross the orbit of Uranus and "plough" into the Coca Iber Belt. These planetary groups spread most of the small icy bodies inward, while they themselves moved outward. These micro stars then disperse the next planet they encounter in a similar way, moving the planet's orbit outward and moving themselves inward. This process continued until the micro planets interacted with Jupiter, whose strong gravity made their orbits highly elliptical, and even threw them straight out of the solar system. This causes Jupiter to move slightly inward. These celestial bodies dispersed by Jupiter into highly elliptical orbits formed Oort Cloud ； Those objects that are less dispersed by the migrating Neptune formed the current Kuiper belt And ecliptic discrete objects. This situation can explain the low density of today's Kuiper Belt and ecliptic discrete objects. These scattered objects, including Pluto, began to be bound by Neptune's gravity and pulled into Track resonance 。 Eventually, the friction in the disk makes the orbits of Uranus and Neptune round again.

Compared with the outer planets, the inner planets did not migrate significantly in the history of the solar system, because their orbits remained stable during the big impact period.

Late heavy bombing and subsequent

Main term: Late Heavy Bombardment

Meteor Crater in Arizona

The gravitational interference caused by the migration of the outer planets will send a large number of asteroids to the inner solar system, seriously depleting the original zone until it drops to today's particularly low mass level. This event may have triggered the late heavy bombing about 4 billion years ago and 500 to 600 million years after the formation of the solar system. The heavy bombing of this period lasted for hundreds of millions of years, as evidenced by the geological remnants in the solar system, such as Mercury and the apparent meteorite craters on the moon. The earliest evidence of life on the earth can be as early as 3.8 billion years ago, almost immediately following the end of the late heavy damage.

Astronomers believe that meteorite impacts are a regular part of the evolution of the solar system (if not very frequent now). The evidence that meteorite impacts continue to occur is that comet Shoemaker Levy 9 hit Jupiter in 1994 and meteor crater 。 Therefore, the process of planetary convergence is not over yet, and may also pose a threat to life on Earth.

Outer solar system The evolution of Supernova And via Interstellar cloud influence. The surface of the outer celestial bodies of the solar system may have been brought about by the solar wind, micro meteorites and neutral components of interstellar matter space weathering 。

After heavy bombing later, Asteroid belt The evolution of depends mainly on collision. Massive objects have enough gravity to retain any material splashed out by strong impact, but this is usually not the case in the asteroid belt. As a result, many larger objects will split in the collision, while the remnants of less intense collision will sometimes merge to form new objects. The formation of satellites around some asteroids can only be explained by the fact that matter flew out of the parent body but did not have enough energy to completely escape its gravity and thus gathered.

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Satellites exist on most planets and other celestial bodies Around. these ones here Natural satellite There are three possible source mechanisms:

It is generated simultaneously from the planetary disk (only in the case of large gas planets);
From the wreckage of the impact (if there is a sufficiently large impact at a shallow angle), and
Capture passing objects.

Jupiter and Saturn have several large moons, such as Io 、 Europa 、 Ganymede and titan They come from the star disk surrounding the two planets in the same way as the two planets formed from the star disk surrounding the sun. The huge size of these satellites and their proximity to the planet reveal their origin. The capture mode cannot have these characteristics, and their gaseous characteristics mean that they cannot be formed from the impact debris. The outer satellites of large gas planets generally have small eccentric orbits with arbitrary inclination angles, which are the expected characteristics of captured objects. Most of these satellites orbit in the opposite direction of the rotation of their host star. The largest irregular satellite is Neptune's satellite Triton , which is believed to be captured Kuiper Belt Objects 。

Satellites of solid objects in the solar system come from collision and capture. Two small moons of Mars Phobos and Phobos Is believed to be a captured asteroid. The Earth's moon is thought to have formed in a single giant oblique collision. It is estimated that the impact object may have a mass close to that of Mars, and the collision may occur at the end of the big impact. The collision bumped some of the mantle layers of the impacting celestial body into the orbit and formed the moon. This impact may be the last in a series of mergers that formed the Earth. It has been further speculated in the past that objects about the size of Mars were formed from the Earth Sun Lagrange point The middle stable part (L four Or L five ）And then drifted away from its position. Pluto's moon Charon may also have been formed by a big impact; Pluto Charon and Earth Moon systems are two examples of only "satellites accounting for at least 1% of the mass of larger bodies" in the solar system.

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Astronomers predict that all the hydrogen in the core of the solar system as we know it today will converge into helium, that is, in the evolution of stars Herotu From Main sequence star There will be no drastic changes before the transition to a red giant. Even so, the solar system will continue to evolve by then.

Long term stability

With the long-term uncertainty factor of planetary orbits, the solar system is chaotic. A remarkable example of this kind of chaos is the Neptune Pluto system, which is at 3:2 Track resonance 。 Although orbital resonance is stable, Pluto is predicted to be in the next 10 to 20 million years（ Lyapunov time ）But we can't get any accuracy. Another example is the Earth's axial tilt , affected by friction caused by tides between the mantle and the moon（ see below ）In the next 1.5 to 4.5 billion years, it will show a chaotic state.

The orbits of planets will be in a chaotic state after a long time, for example, the Lyapunov time range of the entire solar system is 2 million to 230 million years.

In all cases, this means that the position of a planet in its orbit will eventually become impossible to predict with any certainty (therefore, for example, the time of winter and summer will become uncertain), but in some cases the orbit itself may change dramatically. Such chaos is most obvious in the change of orbital eccentricity, and the orbits of some planets become significantly more or less elliptical.

Eventually, the solar system will stabilize in the next few billion years, and the planets will not collide with each other or be thrown out of the solar system. After that, about 5 billion years later, the eccentricity of Mars will reach 0.2, so that it will be in a rendezvous orbit with the Earth, which will lead to potential collisions. In the same time period, the eccentricity of Mercury will increase, and the close encounter with Venus may theoretically throw it out of the solar system or send it on the road of collision with Venus or the Earth.

Sanitation environment system

Satellite evolution is caused by Tidal force Driven by. Due to the gravitational difference along the diameter of the main planet, the orbiting satellite will cause Tidal bulge 。 If the satellite orbits in the same direction as the rotation of the planet, and the rotation of the planet is faster than the rotation period of the satellite, the protrusion will be pulled regularly to lead the satellite. under these circumstances, angular momentum It is transmitted from the rotation of the main planet to the revolution of the satellite. The satellite obtains energy and gradually moves out in a spiral shape. The transfer of the main planet is slower over time.

Neptune and its moon Triton

The Earth and the Moon are an example of this. Today, the moon Tidal locking On Earth; Its revolution around the earth is equal to its rotation around its axis, which means that it always faces the earth on the same side. The moon will stay away from the earth, and the rotation of the earth will continue to slow down. About 50 billion years, if these two worlds can The expansion of the sun They will tide lock each other if they survive; Each side will only see the other side in one hemisphere. Another example is the Galilean moon of Jupiter (and many small moons of Jupiter) and Saturn Many of the larger satellites of.

If the satellite rotates faster than the main planet or its revolution direction is different from the rotation direction of the main planet, different situations will occur. In both cases, the tidal bulge lags behind the satellite in orbit. In the former case, the transmission of angular momentum is reversed, the rotation of the main planet is accelerated, and the orbit of the satellite is reduced. In the latter case, the signs of the angular momentum of rotation and revolution are opposite, so transmission leads to reduction of each other's strength. In both cases, Tidal deceleration Cause the satellite spiral to approach the main planet until it is torn by tidal pressure and may generate Planetary ring The system either crashes into the surface of a planet or into its atmosphere. Such fate is waiting for Mars' moon Phobos (Between 30 and 50 million years), Neptune's Triton (over 3.6 billion years) Triton 、 Thalassa , Jupiter's Ganymede 15 and Ganymede 16 , and Uranus at least 16 small moons. Uranian Tianwei Ten It may even collide with its adjacent satellite. Neptune's Triton could also enter nearby Despina Track.

The third possibility is that the main planet and satellite Tidal locking 。 In this case, the tidal bulge will stay under the satellite, without the transmission of angular momentum, and the orbital period will not change. Pluto And Charon is an example of this situation.

In 2004 Cassini Huygens Before the arrival of the spacecraft, Saturn Ring It was once thought to be much younger than the solar system and would not exist for another 300 million years. The gravitational action with the moon is expected to gradually sweep the outer circumference of the ring towards the planet, and the friction of the meteor and the gravity of Saturn will remove the rest of the ingredients, leaving behind the body of Saturn without rings. But the data from the Cassini trip led scientists to revise this early view. Observations showed that the ice like material with a width of 10 km continued to break and regenerate, maintaining the renewal of the ring. Saturn's rings are much larger than those of other gas giants. Such a huge mass is believed to have maintained Saturn's ring since its formation 4.5 billion years ago, and will continue to maintain it in the next few billion years.

Solar and planetary environments

In the long run, the biggest change in the solar system will come from the changes brought about by the aging of the sun itself. As the sun burns off its hydrogen supply, it will become hotter and burn off the remaining fuel faster. As a result, the sun is 10% brighter every 1.1 billion years. In a billion years, as the solar radiation output increases, its Habitable zone It will move outward, and the earth's surface will be so hot that liquid water cannot continue to exist on the earth's surface. At this time, all life on the ground will disappear. Water vapor from sea level, a strong greenhouse gases It can accelerate the temperature rise and potentially end all life on the earth earlier. At this time, the surface temperature of Mars may gradually rise, and the water and carbon dioxide It will be released into the atmosphere, producing a greenhouse effect to warm the planet until it reaches the same conditions as today's Earth, providing a living place for future life. After 3.5 billion years, the surface environment of the earth will become similar to today's Venus.

After about 5.4 billion years, all the hydrogen in the solar core will be fused into helium. The core will no longer support gravitational collapse, and will begin to shrink, heating a shell around the core until the hydrogen inside begins to fuse. This will make its outer layer expand rapidly, and the star will enter the red giant stage of its life. In 7.6 billion years, the sun will expand to a radius of 1.2AU - 256 times its size. In its Red giant branch Because of the huge increase in surface area, the surface of the sun will be much cooler (about 2600K) luminosity It will increase a lot, reaching 2700 times the sunlight. When the sun becomes a red giant, it will produce strong Stellar wind , which will take away 33% of its own mass.

Ring nebula, a planetary nebula similar to what the sun will become

When the sun expands, Mercury Like Venus, it will almost certainly be swallowed up. The fate of the earth is not very clear. Although the sun will devour the Earth's orbit, the loss of mass (and therefore the weaker gravity) of this star will cause the planet's orbit to move outward. If it were just like this, the earth might escape from the fire, but the research in 2008 believed that the earth would still be swallowed up due to the outer tidal action that is not closely attached to the sun. At this time, Kuiper belt Of Pluto And Karen, it is possible to reach the surface temperature that can sustain life.

The sun as the main sequence star and the sun that will become a red giant star in the future

Gradually, the hydrogen burning in the shell around the solar core will increase the mass of the core until it reaches 45% of the current solar mass. At this time, the density and temperature are so high that helium begins to coalesce into carbon, resulting in Helium flashover ； The radius of the sun will shrink from about 250 times to 11 times the radius of the main sequence star. Therefore, its luminosity will drop from 3000 times to 54 times today's level, and its surface temperature will rise to about 4770K. The sun will become a Horizontal branch The star, which burns helium in its core smoothly, probably like it burns hydrogen. The helium fusion phase will last only 100 million years. In the end, it still has to rely on its outer hydrogen and helium reserves, and expand for a second time to become Asymptotic giant bifurcation Stars. The sun's luminosity will rise again, reaching 2090 times the luminosity, and it will cool to about 3500K. This stage will last for 30 million years, and then, in the course of another 100000 years, the remaining outer layer of the sun will be lost, and a huge flood of material will be thrown out to form a halo (misleading) called Planetary nebula 。 The ejected material will contain helium and carbon generated by the sun's nuclear reaction, and continue to enrich the interstellar material with heavy elements for future generations of stars.

This is a relatively peaceful ending Supernova There is nothing similar. Our sun is too small to make such an evolution. If it is possible, any observer who witnessed this event will see that the wind speed of the solar wind increases significantly, but it is not enough to completely destroy a planet. However, the loss of the planet's material can send the surviving planets into orbit into chaos: some will collide, some will be thrown out of the solar system, and the rest will be torn apart by tidal action. After that, all the sun has left is one White dwarf A very dense celestial body, 54% of its original mass, but only the size of the Earth. At first, the white dwarf was about 100 times as bright as the sun. It will be completely Degeneracy Carbon and oxygen Composition, but will never reach the temperature where these elements can be fused. Therefore, the white dwarf sun will gradually cool and become darker.

With the death of the sun, its gravity acting on such celestial bodies as planets, comets and asteroids will weaken with the loss of its mass. If the Earth and Mars were still alive at this time, their orbits would be about 1.85 and 2.8AU. They and the rest of the planets will become dark, cold shells, completely without any form of life. They will continue to circle their star, and their speed will be slowed down due to the increase of distance from the sun and the decrease of solar gravity. Two billion years later, when the sun cools to 6000-8000K, the carbon and oxygen in the solar core will cool, and the remaining 90% of its mass will form a crystal structure. Finally, in billions of years, the sun will stop shining completely and become a black dwarf star.

Galactic interaction

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Galactic interaction

The solar system follows a distance Galaxy Silver core About 30000 light years of circular orbit alone. Its speed is about 220 kilometers per second. The solar system completes a revolution around the silver center, that is Galactic Year About 220~250 million years. Since the formation of the solar system, it has been rotating like this for at least 20 weeks.

Some scientists speculate that the path of the solar system in the Milky Way is on the Earth Fossil record Periodicity observed in Mass extinction A factor of. One hypothesis suggests that the vertical oscillation caused by the sun's revolution around the galactic center makes it regularly pass through the galactic plane. When the sun's orbit takes it out of the galactic plane, Galactic tide When it enters the galactic disk every 20 to 25 million years, it will be affected by the far stronger "disk tide". According to the mathematical model, the flow of Comet Oort will increase four times, leading to a greatly increased possibility of destructive impact.

However, there are other theories that the sun is close to the galactic plane, but the last mass extinction occurred 15 million years ago. Therefore, the vertical position of the sun cannot alone explain such periodic extinction, which occurs when the sun passes through the Milky Way Spiral arm When. Spiral arms are not only numerous, but their gravity can interfere Oort Cloud It is also the location of the highly dense region of bright blue giant stars, which exist for a short time and explode violently into supernovae.

Galaxy Collision and Planetary Interference

Although most galaxies in the universe are far away from the Milky Way, we Local galaxy group The largest galaxy in the Andromeda galaxy But it is crashing into the Milky Way at the speed of 120 kilometers per second. In 2 billion years, Andromeda Galaxy and the Milky Way Galaxy will collide, Tidal force Twist their peripheral arms into huge Tidal tail Both of them are deformed. When such initial interference occurs, astronomers calculate that the solar system has a 12% chance of being pulled from the Milky Way to the periphery, and a 3% chance that it will be captured by the gravity of the Andromeda Galaxy as part of it. After a further series of sideswipes, the solar system has a 30% chance of being thrown out Superheavy black hole Will be merged. Finally, in about 7 billion years, the Milky Way and Andromeda Galaxy will merge to form a huge Elliptical galaxy 。 In the merger, if there is enough gas, the increased gravity will attract the gas to the center of the forming elliptical galaxy. This will lead to a short period of intense star formation called starbursts. In addition, the gas falling inward will fill the newly formed black hole , turn it into a Active galactic nucleus 。 These interactions will probably push the solar system into the outer halo of the new galaxy and protect it from the radiation of these collisions.

The common misconception is that such collisions would interfere with the planetary orbits of the solar system. Although the passing stars may separate the planets from the solar system and send them into galactic space, the distance between stars is so huge that the Milky Way and Andromeda galaxy The interference to a single star system caused by the collision is negligible. Although the solar system as a whole may be affected by these events, the sun and planets themselves are not expected to be disturbed.

However, as time goes by Cumulative probability Increase, the interference to the planet is inevitable. Apocalyptic Large extrusion or Large tear No, it is calculated that the passing star will completely strip all the planets of the dead sun within 10 billion years. This marks the end of the solar system. Although the sun and planets may exist, the solar system, in any sense, will no longer exist.

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The time frame for the formation of the solar system is Radioisotope determination Method. Scientists estimate that the solar system is about 4.6 billion years old. The oldest known mineral particle on the earth is about 4.4 billion years old. Because the surface of the earth is often Erosion 、 volcano Activities and Plate movement After transformation, such old rocks are rare. Scientists use Solar nebula Meteorites formed during early condensation were used to estimate the age of the solar system. Almost all meteorites (see Magic Valley Meteorite ）Both were found to be 4.6 billion years old, indicating that the solar system is at least as old.

The study of the disks of other stars has also contributed a lot to the establishment of the timetable for the formation of the solar system. Stars between 1 million and 3 million years old are mostly gas rich, while star disks over 10 million years old contain little to no gas, indicating that the giant gas planets in their interior have stopped generating.

Chronology of solar system evolution

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Note: All times and years in this chronology should only be regarded as Order of magnitude Indicators.

Chronology of solar system evolution
stage	Time from sun formation	event
Pre solar system	Billions of years before the formation of the solar system	Previous generations of stars lived and died Heavy element Throw as Interstellar matter , from which the solar system was formed.
Pre solar system	Before the formation of the solar system~5 × 10 seven year	If the solar system is in a Orion nebula The same star forming region is formed, and the massive stars experience formation, live their lives, die, and explode into supernovae. One of the supernovae triggered the formation of the solar system.
The formation of the solar system	0–1×10 five year	The Presolar Nebula forms and begins to collapse. The sun began to form.
	1×10 five –5×10 seven year	The sun is a Taurus T Star Of type Protostar 。
	1×10 5-7 year	Outer planets form. ten seven Year, Original astrolabe The gas in is blown away, and the formation of outer planets may be completed.
	1×10 7-8 year	Earth like planets and moons formed. At the beginning of the great collision, water was sent to the earth.
Main sequence star	5×10 seven year	The sun becomes a Main sequence star 。
	2×10 eight year	The oldest rock formation on earth.
	5–6×10 eight year	Jupiter The orbital resonance with Saturn moved Neptune to the Kuiper Belt. Late Heavy Bombardment The inner solar system begins.
	8×10 eight year	The earliest known life appeared on the earth.
	4.6×10 nine year	The sun is still a main sequence star, every 10 nine The annual warming is about 10%.
	6×10 nine year	sunlight Habitable zone Move out of the Earth's orbit, and possibly move to the orbit of Mars.
	7×10 nine year	Galaxy and Andromeda galaxy start Fairy Galaxy Collision 。 There is a small possibility that the solar system will be captured by Andromeda before the two galaxies merge completely.
Post main sequence star	10–12×10 nine year	The sun depletes the hydrogen in its core, ending life in the main sequence stage. The sun starts at Herotu Up to Red Giant Branch , suddenly become brighter (to 2700 times), larger (to 250 times the radius), colder (to 2600K): the sun becomes Red Giant 。 Mercury, Venus and Earth may be swallowed.
Post main sequence star	~12×10 nine year	The sun burns through helium Horizontal branch and Asymptotic giant bifurcation 30% of the mass is lost in the later main sequence stage. Asymptotic giant star branching with jet Planetary nebula The end, leaving the core of the sun to become a White dwarf 。
Solar debris	>12×10 nine year	The white dwarf sun no longer generates energy, starts to cool and dim continuously, and finally moves towards Black dwarf Phase.
Solar debris	ten fifteen year	The sun cools to 5K. The gravity of the passing stars stripped the planets from their orbits, and the solar system no longer existed.

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