Excited the minds of scientists for many millennia. There were and are many versions - from purely theological to modern, formed on the basis of data from deep space research.

But since no one happened to be present during the formation of our planet, it remains to rely only on indirect "evidence". Also, the most powerful telescopes are of great help in removing the veil from this mystery.

solar system

The history of the Earth is inextricably linked with the appearance and around which it revolves. And so you have to start from afar. According to scientists, after the Big Bang, it took one or two billion years for galaxies to become approximately what they are now. The solar system, on the other hand, arose, presumably, eight billion years later.

Most scientists agree that it, like all similar space objects, arose from a cloud of dust and gas, since matter in the Universe is distributed unevenly: somewhere there was more of it, and in another place - less. In the first case, this leads to the formation of nebulae from dust and gas. At some stage, perhaps due to external influence, such a cloud contracted and began to rotate. The reason for what happened, probably lies in a supernova explosion somewhere in the vicinity of our future cradle. However, if all are formed in approximately the same way, then this hypothesis looks doubtful. Most likely, having reached a certain mass, the cloud began to attract more particles to itself and contract, and acquired a rotational moment due to the uneven distribution of matter in space. Over time, this swirling clot became more and more dense in the middle. Thus, under the influence of enormous pressure and rising temperatures, our Sun arose.

Hypotheses of different years

As mentioned above, people have always wondered how the planet Earth was formed. The first scientific justification appeared only in the seventeenth century AD. At that time, many discoveries were made, including physical laws. According to one of these hypotheses, the Earth was formed as a result of the collision of a comet with the Sun as a residual substance from the explosion. According to another, our system arose from a cold cloud of cosmic dust.

The particles of the latter collided with each other and connected until the Sun and planets were formed. But French scientists suggested that the specified cloud was red-hot. As it cooled, it rotated and contracted, forming rings. From the latter, the planets were formed. And the sun appeared in the center. The Englishman James Jeans suggested that another star once flew past our star. She pulled out with her attraction the substance from the Sun, from which the planets subsequently formed.

How the Earth Was Formed

According to modern scientists, the solar system arose from cold particles of dust and gas. The substance was compressed and disintegrated into several parts. From the largest piece, the Sun was formed. This piece rotated and warmed up. It became like a disc. From dense particles on the periphery of this gas-dust cloud, planets were formed, including our Earth. Meanwhile, in the center of the nascent star, under the influence of high temperatures and enormous pressure,

There is a hypothesis that arose during the search for exoplanets (similar to Earth) that the more heavy elements a star has, the less likely it is that life will arise near it. This is due to the fact that their large content leads to the appearance of gas giants around the star - objects like Jupiter. And such giants inevitably move towards the star and push small planets out of their orbits.

Date of Birth

The Earth was formed about four and a half billion years ago. The pieces rotating around the red-hot disk became heavier and heavier. It is assumed that initially their particles were attracted due to electric forces. And at some stage, when the mass of this “coma” reached a certain level, it began to attract everything in the area with the help of gravity.

As in the case of the Sun, the clot began to shrink and heat up. The substance is completely melted. Over time, a heavier center formed, consisting mainly of metals. When the Earth was formed, it began to slowly cool, and the crust formed from lighter substances.

clash

And then the Moon appeared, but not the way the Earth was formed, again, according to the assumption of scientists and according to the minerals found on our satellite. The Earth, having already cooled down, collided with a slightly smaller other planet. As a result, both objects completely melted and turned into one. And the substance thrown out by the explosion began to rotate around the Earth. It was from this that the moon was born. It is claimed that the minerals found on the satellite differ from those of the earth in their structure: as if the substance was melted and solidified again. But the same thing happened to our planet. And why didn't this terrible collision lead to the complete destruction of two objects with the formation of small fragments? There are many mysteries.

path to life

Then the Earth began to cool again. Again, a metal core formed, and then a thin surface layer. And between them - a relatively mobile substance - the mantle. Thanks to strong volcanic activity, the atmosphere of the planet was formed.

Initially, of course, it was absolutely unsuitable for human breathing. And life would be impossible without the appearance of liquid water. It is assumed that the latter was brought to our planet by billions of meteorites from the outskirts of the solar system. Apparently, some time after the formation of the Earth, there was a powerful bombardment, the cause of which could be the gravitational influence of Jupiter. Water was trapped inside minerals, and volcanoes turned it into steam, and it fell out to form oceans. Then came oxygen. According to many scientists, this happened due to the vital activity of ancient organisms that could appear in those harsh conditions. But that's a completely different story. And humanity every year is getting closer and closer to getting an answer to the question of how the planet Earth was formed.

When a star is young, it is always surrounded by a primordial rotating disk of gas and dust from which space objects are formed. Astronomers are always on the hunt for such structures, because they can capture the moment of not only the formation of a star, but also fix the process of planet formation.

However, finding such disks around brown dwarfs or very low-mass stars is extremely difficult. But this time, scientists discovered four (!) new low-mass objects surrounded by disks at once.

Three of them are extremely small - only 13 or 18 times the mass of Jupiter. The fourth is about 120 times more than the mass of Jupiter (For comparison: the Sun is 1000 times larger than Jupiter).

Even more interesting is that the age of the two stars is approximately 42 and 45 million years. It turns out that these are the youngest objects ever found surrounded by active planet-forming disks.

Finding a cloud of gas and dust belonging to an extremely low mass brown dwarf is even more interesting, because its further development will reveal a lot about the evolution of stars and planets.

How does the formation and development of celestial structures take place?

In the gas and dust disk, grains of dust collide, combine, forming larger fragments that increase to stones, then the stage of planetesimals, planetary embryos begins, and finally, the stage of transformation into rocky terrestrial planets (some of which become the core of gas giants) begins.

Astronomers typically identify gas and dust clouds as follows: the star heats the surrounding dust, which acquires properties that make it visible through telescopes with infrared cameras.

How to understand - whether the formation of the planets is completed?

However, some discs show that the formation of celestial bodies is not ongoing, but has already been completed. These disks are formed from fragments left after the process of planet formation and as a result of subsequent collisions of already created celestial objects. Ultimately, these dust residues dissipate into interplanetary space.

Some discs even represent a transitional stage between the phases of planet formation and its end.

It is important for scientists to distinguish between these types of disks, because as a result they will be able to better chart the birth and change over time of planetary systems, including the solar system.

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Since the New Horizons probe flew past Pluto last year, there have been repeated suggestions that the dwarf planet could have an ocean of liquid under its icy shell. The latest study analyzed the internal and surface features of Pluto and allows us to state with a high probability that the ocean still exists, but its volume and[…]

Approximately 4.54 billion years ago, our planet appeared. Scientists cannot accurately describe all the features of its formation, but the generally accepted theory of the birth of the Earth has numerous scientific confirmations.

At first, there was a huge molecular cloud in place of the solar system. It split and a protosolar nebula was formed from one of its parts, which began to shrink under the influence of gravity. In the core of the nebula, thermonuclear reactions began, from which our Sun was formed.

The young star was surrounded by a dense protoplanetary cloud consisting of gases and dust. In this gas and dust formation, local centers of attraction began to form, protoplanets (planetesimals) were born.

Protoplanets collided, attracted the remains of gas and dust matter. As a result, the Earth, Mars, Neptune, Venus, etc. were formed.

How the planet appeared: video

How the Earth Formed: An Educational Video for Kids

Another eight hundred million years passed and life was born on the cooled Earth.

On the scale of space, the planets are just grains of sand, playing an insignificant role in the grandiose picture of the development of natural processes. However, these are the most diverse and complex objects in the universe. None of the other types of celestial bodies has a similar interaction of astronomical, geological, chemical and biological processes. No other place in space can give rise to life as we know it. In the last decade alone, astronomers have discovered more than 200 planets.

The formation of planets, long considered a calm and stationary process, in reality turned out to be quite chaotic.

The astonishing variety of masses, sizes, compositions and orbits has led many to wonder about their origins. In the 1970s the formation of the planets was considered an ordered, deterministic process - a pipeline in which amorphous gas and dust disks turn into copies of the solar system. But now we know that this is a chaotic process, with a different outcome for each system. The planets that were born survived the chaos of competing mechanisms of formation and destruction. Many objects died, burned up in the fire of their star, or were thrown into interstellar space. Our Earth could have had long-lost twins now wandering in dark and cold space.

The science of planet formation lies at the intersection of astrophysics, planetary science, statistical mechanics, and nonlinear dynamics. In general, planetary scientists are developing two main directions. According to the progressive accretion theory, tiny dust particles stick together to form large clumps. If such a block attracts a lot of gas to itself, it turns into a gas giant, like Jupiter, and if not, into a rocky planet like Earth. The main disadvantages of this theory are the slowness of the process and the possibility of gas dissipation before the formation of the planet.

In another scenario (the theory of gravitational instability), it is stated that gas giants are formed by a sudden collapse, leading to the destruction of the primary gas-dust cloud. This process mimics the formation of stars in miniature. But this hypothesis is highly controversial, since it assumes the presence of strong instability, which may not occur. In addition, astronomers have found that the most massive planets and the least massive stars are separated by a “void” (bodies of intermediate mass simply do not exist). Such a "failure" indicates that the planets are not just low-mass stars, but objects of a completely different origin.

Despite the fact that scientists continue to argue, most consider the successive accretion scenario to be more likely. In this article, I will rely on it.

1. The interstellar cloud is shrinking

Time: 0 (starting point of the planet formation process)

Our solar system is located in a galaxy where there are about 100 billion stars and clouds of dust and gas, mostly the remnants of stars of previous generations. In this case, dust is just microscopic particles of water ice, iron, and other solids that have condensed in the outer, cool layers of a star and are ejected into outer space. If the clouds are cold and dense enough, they begin to collapse under the force of gravity, forming clusters of stars. Such a process can last from 100 thousand to several million years.

Surrounding each star is a disk of remaining matter, enough to form planets. Young disks contain mostly hydrogen and helium. In their hot inner regions, dust particles evaporate, while in the cold and rarefied outer layers, dust particles remain and grow as steam condenses on them.

Astronomers have found many young stars surrounded by such disks. Stars between 1 and 3 million years old have gaseous disks, while those older than 10 million years have faint, gas-poor disks, as the gas is “blowed out” either by the newborn star itself or by nearby bright stars. This time range is exactly the epoch of planetary formation. The mass of heavy elements in such disks is comparable to the mass of these elements in the planets of the solar system: a pretty strong argument in defense of the fact that planets are formed from such disks.

Result: the newborn star is surrounded by gas and tiny (micrometer-sized) dust particles.

Cosmic dust balls

Even gigantic planets began as humble bodies—micron-sized dust particles (the ashes of long-dead stars) floating in a spinning disk of gas. With distance from the newborn star, the temperature of the gas drops, passing through the "line of ice", beyond which the water freezes. In our solar system, this boundary separates the inner rocky planets from the outer gas giants.

1. Particles collide, stick together and grow.
2. Small particles are carried away by the gas, but those larger than a millimeter are decelerated and spiral towards the star.
3. At the line of ice, the conditions are such that the frictional force changes direction. Particles tend to stick together and easily unite into larger bodies - planetesimals.

2. The disk acquires structure

Time: about 1 million years

Dust particles in a protoplanetary disk, moving chaotically along with gas flows, collide with each other and sometimes stick together, sometimes collapse. The dust grains absorb light from the star and re-emit it in the far infrared, transferring heat to the darkest inner regions of the disk. The temperature, density, and pressure of the gas generally decrease with distance from the star. Due to the balance of pressure, gravity and centrifugal force, the speed of rotation of gas around the star is less than that of a free body at the same distance.

As a result, dust particles larger than a few millimeters are ahead of the gas, so the headwind slows them down and forces them to spiral down towards the star. The larger these particles become, the faster they move down. Meter-sized blocks can halve their distance from a star in just 1,000 years.

As the particles approach the star, they heat up, and gradually water and other low-boiling substances called volatiles evaporate. The distance at which this happens - the so-called "line of ice" - is 2-4 astronomical units (AU). In the solar system, this is just something in between the orbits of Mars and Jupiter (the radius of the Earth's orbit is 1 AU). The line of ice divides the planetary system into an inner region, devoid of volatile substances and containing solid bodies, and an outer region, rich in volatile substances and containing icy bodies.

Water molecules evaporated from dust particles accumulate on the ice line itself, which serves as a trigger for a whole cascade of phenomena. In this region, a gap occurs in the gas parameters, and a pressure jump occurs. The balance of forces causes the gas to accelerate its movement around the central star. As a result, the particles that enter here are influenced not by a head wind, but by a tail wind, which drives them forward and stops their migration into the disk. And since particles continue to flow from its outer layers, the line of ice turns into a band of its accumulation.

Accumulating, the particles collide and grow. Some of them break through the ice line and continue their migration inward; when heated, they become covered with liquid mud and complex molecules, which makes them more sticky. Some areas are so filled with dust that the mutual gravitational attraction of particles accelerates their growth.

Gradually, dust grains collect into kilometer-sized bodies called planetesimals, which, in the last stage of planet formation, scoop up almost all of the primary dust. It is difficult to see the planetesimals themselves in the forming planetary systems, but astronomers can guess about their existence from the fragments of their collisions (see: Ardila D. Invisible planetary systems // VMN, No. 7, 2004).

Result: many kilometer-long "building blocks" called planetesimals.

Rise of the oligarchs

The billions of kilometer-long planetesimals formed in stage 2 then assemble into bodies the size of the Moon or Earth, called embryos. A small number of them dominate their orbital zones. These "oligarchs" among the embryos are fighting for the remaining substance

3. The embryos of the planets are formed

Time: 1 to 10 Ma

The surfaces of Mercury, the Moon and asteroids covered with craters leave no doubt that during the formation period, planetary systems look like a shooting range. Mutual collisions of planetesimals can stimulate both their growth and destruction. The balance between coagulation and fragmentation leads to a size distribution in which small bodies are mainly responsible for the surface area of ​​the system, while large ones determine its mass. The orbits of bodies around a star may initially be elliptical, but over time, deceleration in the gas and mutual collisions turn the orbits into circular ones.

Initially, the growth of the body occurs due to random collisions. But the larger the planetesimal becomes, the stronger its gravity, the more intense it absorbs its low-mass neighbors. When the masses of planetesimals become comparable to the mass of the Moon, their gravity increases so much that they shake the surrounding bodies and deflect them to the sides even before the collision. This limits their growth. This is how "oligarchs" arise - the embryos of planets with comparable masses, competing with each other for the remaining planetesimals.

The feeding zone of each embryo is a narrow strip along its orbit. Growth stops when the embryo absorbs most of the planetesimals from its zone. Elementary geometry shows that the size of the zone and the duration of extinction increase with distance from the star. At a distance of 1 AU the embryos reach a mass of 0.1 Earth mass within 100 thousand years. At a distance of 5 AU they reach four Earth masses in a few million years. The embryos can become even larger near the ice line or at the edges of disk ruptures where planetesimals are concentrated.

The growth of the "oligarchs" fills the system with a surplus of bodies aspiring to become planets, but only a few succeed. In our solar system, the planets, although distributed over a large area, are as close to each other as possible. If one more planet with the mass of the Earth is placed between the terrestrial planets, then it will unbalance the entire system. The same can be said about other known planetary systems. If you see a cup of coffee filled to the brim, you can almost be sure that someone overfilled it and spilled some liquid; it is unlikely that you can fill the container to the brim without spilling a drop. It is just as likely that planetary systems have more matter at the beginning of their lives than at the end. Some objects are ejected from the system before it reaches equilibrium. Astronomers have already observed free-floating planets in young star clusters.

Result:"oligarchs" are the embryos of planets with masses in the range from the mass of the Moon to the mass of the Earth.

Giant Leap for the Planetary System

The formation of such a gas giant as Jupiter is the most important moment in the history of the planetary system. If such a planet is formed, it begins to control the entire system. But for this to happen, the nucleus must collect gas faster than it spirals towards the center.

The formation of a giant planet is hindered by the waves it excites in the surrounding gas. The action of these waves is not balanced, it slows down the planet and causes it to migrate towards the star.

The planet attracts gas, but it cannot settle until it cools. And during this time, it can spiral quite close to the star. A giant planet may not form in all systems

4. A gas giant is born

Time: 1 to 10 Ma

Probably, Jupiter began with an embryo comparable in size to the Earth, and then accumulated about 300 more Earth masses of gas. Such impressive growth is due to various competing mechanisms. The gravity of the nucleus pulls the gas out of the disk, but the gas compressing towards the nucleus releases energy, and in order to settle, it must be cooled. Therefore, the growth rate is limited by the possibility of cooling. If it happens too slowly, the star can blow gas back into the disk before the nucleus forms a dense atmosphere around it. The bottleneck in heat removal is the transfer of radiation through the outer layers of the growing atmosphere. The heat flux there is determined by the opacity of the gas (mainly depends on its composition) and the temperature gradient (depends on the initial mass of the nucleus).

Early models showed that a planet's embryo would need to have a mass of at least 10 Earth masses to cool quickly enough. Such a large specimen can grow only near the ice line, where a lot of matter had previously accumulated. Perhaps that is why Jupiter is located just behind this line. Large nuclei can also form anywhere else if the disk contains more matter than planetary scientists usually assume. Astronomers have already observed many stars, the disks around which are several times denser than previously thought. For a large sample, heat transfer does not appear to be a serious problem.

Another factor hindering the birth of gas giants is the movement of the embryo in a spiral towards the star. In a process called Type I migration, the embryo excites waves in the gaseous disk, which in turn gravitationally affect its orbital motion. The waves follow the planet, just as its trail follows a boat. The gas on the outer side of the orbit rotates more slowly than the embryo and drags it back, slowing down its movement. And the gas inside the orbit rotates faster and pulls forward, speeding it up. The outer region is larger, so it wins the battle and causes the germ to lose energy and sink to the center of the orbit by a few astronomical units per million years. This migration usually stops at the ice line. Here, the oncoming gas wind turns into a tailwind and begins to push the embryo forward, compensating for its deceleration. Perhaps that is also why Jupiter is exactly where it is.

The growth of the nucleus, its migration, and the loss of gas from the disk occur almost at the same rate. Which process wins depends on luck. It is possible that several generations of embryos will go through the process of migration without being able to complete their growth. Behind them, new batches of planetesimals move from the outer regions of the disk to its center, and this repeats until a gas giant is eventually formed, or until all the gas has been absorbed, and the gas giant can no longer form. Astronomers have discovered planets like Jupiter around 10% of the sun-like stars they have studied. The cores of such planets may be rare embryos that have survived from many generations - the last of the Mohicans.

The result of all these processes depends on the initial composition of the substance. Approximately one third of the stars rich in heavy elements have planets like Jupiter. It is possible that such stars had dense disks, which allowed the formation of massive seeds that had no problems with heat removal. And, on the contrary, planets rarely form around stars that are poor in heavy elements.

At some point, the mass of the planet begins to grow monstrously fast: in 1000 years, a planet like Jupiter acquires half of its final mass. At the same time, it emits so much heat that it shines almost like the Sun. The process stabilizes when the planet becomes so massive that it turns Type I migration on its head. Instead of the disk changing the orbit of the planet, the planet itself begins to change the movement of gas in the disk. The gas inside the planet's orbit rotates faster than it, so its attraction slows down the gas, forcing it to fall towards the star, i.e. away from the planet. Gas outside the orbit of the planet rotates more slowly, so the planet accelerates it, forcing it to move outward, again away from the planet. Thus, the planet creates a gap in the disk and destroys the supply of building material. Gas tries to fill it up, but computer models show that the planet wins the battle if, at a distance of 5 AU. its mass exceeds the mass of Jupiter.

This critical mass depends on the era. The earlier the planet forms, the greater its growth will be, since there is still a lot of gas in the disk. Saturn has less mass than Jupiter simply because it formed a few million years later. Astronomers have discovered a shortage of planets with masses ranging from 20 Earth masses (that's the mass of Neptune) to 100 Earth masses (the mass of Saturn). This may be the key to reconstructing the picture of evolution.

Result: Jupiter-sized planet (or lack of it).

5. The gas giant is getting restless

Time: 1 to 3 Ma

Oddly enough, many of the extrasolar planets discovered in the last ten years orbit their star at very close distances, much closer than Mercury orbits the Sun. These so-called "hot Jupiters" did not form where they are now, because the orbital feeding zone would be too small to supply the necessary material. Perhaps their existence requires a three-stage sequence of events, which for some reason did not materialize in our solar system.

First, a gas giant must form in the inner part of the planetary system, near the line of ice, while there is still enough gas in the disk. But for this, there must be a lot of solid matter in the disk.

Secondly, the giant planet must move to its current location. Type I migration cannot provide this, since it acts on the embryos even before they accumulate much gas. But type II migration is also possible. The emerging giant creates a gap in the disk and holds back the flow of gas through its orbit. In this case, it must fight against the tendency of turbulent gas to propagate into adjacent areas of the disk. The gas will never stop seeping into the gap, and its diffusion towards the central star will cause the planet to lose orbital energy. This process is quite slow: it takes several million years for the planet to move a few astronomical units. Therefore, the planet must begin to form in the inner part of the system if it is eventually to enter orbit near the star. As this and other planets move inward, they push the remaining planetesimals and germs in front of them, possibly creating "hot Earths" in even closer orbits to the star.

Third, something must stop the movement before the planet hits the star. This may be the magnetic field of the star, clearing the space near the star from gas, and without gas, movement stops. Perhaps the planet excites the tides on the star, and they in turn slow down the fall of the planet. But these limiters may not work in all systems, so many planets can continue to move towards the star.

Result: giant planet in close orbit ("hot Jupiter").

How to hug a star

In many systems, a giant planet forms and begins to spiral toward the star. This happens because the gas in the disk loses energy due to internal friction and settles towards the star, dragging the planet with it, which eventually becomes so close to the star that it stabilizes its orbit.

6. Other giant planets appear

Time: 2 to 10 Ma

If one gas giant managed to form, then it contributes to the birth of the following giants. Many, and perhaps most of the known giant planets have twins of comparable mass. In the solar system, Jupiter helped Saturn form faster than it would have done without it. In addition, he "stretched a helping hand" to Uranus and Neptune, without which they would not have reached their current mass. At their distance from the Sun, the process of formation without outside help would go very slowly: the disk would dissolve even before the planets had time to gain mass.

The first gas giant turns out to be useful for several reasons. At the outer edge of the gap formed by it, the matter is concentrated, in general, for the same reason as on the line of ice: the pressure difference causes the gas to accelerate and act as a tailwind on dust particles and planetesimals, stopping their migration from the outer regions of the disk. In addition, the gravity of the first gas giant often throws neighboring planetesimals into the outer region of the system, where new planets form from them.

The second generation of planets is formed from the material collected for them by the first gas giant. At the same time, pace is of great importance: even a small delay in time can significantly change the result. In the case of Uranus and Neptune, the accumulation of planetesimals was excessive. The embryo became too large, 10-20 Earth masses, which delayed the onset of gas accretion until the moment when there was almost no gas left in the disk. The formation of these bodies was completed when they collected only two terrestrial masses of gas. But these are no longer gas, but ice giants, which may turn out to be the most common type.

The gravitational fields of the second generation planets increase chaos in the system. If these bodies formed too close together, their interaction with each other and with the gaseous disk could throw them into higher elliptical orbits. In the solar system, the planets have almost circular orbits and are sufficiently distant from each other, which reduces their mutual influence. But in other planetary systems, the orbits are usually elliptical. In some systems, they are resonant, that is, the orbital periods are related as small integers. It is unlikely that this was laid during the formation, but it could have arisen during the migration of the planets, when gradually the mutual gravitational influence tied them to each other. The difference between such systems and the solar system could be determined by different initial gas distributions.

Most stars are born in clusters, and more than half of them are binaries. Planets may not form in the plane of the stars' orbital motion; in this case, the gravity of the neighboring star quickly rearranges and distorts the orbits of the planets, forming not such flat systems as our solar system, but spherical ones, resembling a swarm of bees around a hive.

Result: company of giant planets.

Addition to the family

The first gas giant creates the conditions for the birth of the next. The strip cleared by him acts as a fortress ditch, which cannot overcome the substance moving from outside to the center of the disk. It collects on the outside of the rift, where it forms new planets.

7. Earth-like planets form

Time: 10 to 100 Ma

Planetologists believe that Earth-like planets are more common than giant planets. While the birth of a gas giant requires a fine balance of competing processes, the formation of a rocky planet must be much more difficult.

Before the discovery of extrasolar Earth-like planets, we relied only on data about the solar system. The four terrestrial planets—Mercury, Venus, Earth, and Mars—are mostly made up of substances with high boiling points, such as iron and silicate rocks. This indicates that they were formed inside the ice line and did not migrate noticeably. At such distances from the star, the embryos of the planets can grow in a gaseous disk up to 0.1 Earth mass, i.e., no more than Mercury. For further growth, it is necessary that the orbits of the embryos intersect, then they will collide and merge. The conditions for this arise after the evaporation of gas from the disk: under the action of mutual perturbations, over several million years, the orbits of the nuclei are stretched into ellipses and begin to intersect.

It is much more difficult to explain how the system stabilizes itself again, and how the terrestrial planets ended up in their current nearly circular orbits. A small amount of the remaining gas could provide this, but such a gas should have prevented the initial "blurring" of the orbits of the nuclei. Perhaps, when the planets are almost formed, there is still a decent swarm of planetesimals. Over the next 100 million years, the planets sweep away some of these planetesimals, and the rest are deflected towards the Sun. The planets transfer their erratic motion to the doomed planetesimals and move into circular or near-circular orbits.

According to another idea, the long-term influence of Jupiter's gravity causes the nascent terrestrial planets to migrate, moving them into regions with fresh matter. This influence should be stronger on resonant orbits, which gradually shifted inward as Jupiter descended to its current orbit. Radioisotope measurements indicate that asteroids formed first (4 million years after the formation of the Sun), then Mars (after 10 million years), and later Earth (after 50 million years): as if a wave raised by Jupiter passed through the solar system. If it had not encountered obstacles, it would have moved all the planets of the terrestrial group to the orbit of Mercury. How did they manage to avoid such a sad fate? Perhaps they have already become too massive, and Jupiter could not move them much, or maybe strong impacts threw them out of Jupiter's range.

Note that many planetary scientists do not consider the role of Jupiter to be decisive in the formation of solid planets. Most sun-like stars are devoid of planets like Jupiter, but have dust disks around them. This means that there are planetesimals and embryos of planets from which objects like the Earth can form. The main question that observers must answer in the next decade is how many systems have earths but no Jupiters.

The most important epoch for our planet was the period between 30 and 100 million years after the formation of the Sun, when a Mars-sized embryo crashed into the proto-Earth and gave rise to a huge amount of debris from which the Moon was formed. Such a powerful blow, of course, scattered a huge amount of matter throughout the solar system; therefore, Earth-like planets in other systems may also have satellites. This strong blow was supposed to disrupt the Earth's primary atmosphere. Its present-day atmosphere mainly originated from gas trapped in planetesimals. The Earth formed from them, and later this gas came out during volcanic eruptions.

Result: terrestrial planets.

Explanation of non-circular motion

In the inner region of the solar system, planetary embryos cannot grow by capturing gas, so they must merge with each other. To do this, their orbits must intersect, which means that something must disrupt their original circular motion.

When nuclei are formed, their circular or almost circular orbits do not intersect.

The gravitational interaction of the nuclei with each other and with the giant planet perturbs the orbits.

The germs combine into an earth-type planet. It returns to a circular orbit, mixing the remaining gas and scattering the remaining planetesimals.

8. Cleanup operations begin

Time: 50 million to 1 billion years

At this point, the planetary system is almost formed. Several secondary processes continue: the collapse of the surrounding star cluster, capable of destabilizing the orbits of the planets with its gravity; internal instability that occurs after the star finally destroys its gaseous disk; and finally the continued dispersal of the remaining planetesimals by the giant planet. In the solar system, Uranus and Neptune are throwing planetesimals out into the Kuiper Belt, or towards the Sun. And Jupiter, with its powerful gravity, sends them to the Oort cloud, to the very edge of the gravitational influence of the Sun. The Oort cloud can contain about 100 Earth masses of matter. From time to time, planetesimals from the Kuiper Belt or the Oort Cloud approach the Sun, forming comets.

Scattering planetesimals, the planets themselves migrate a little, and this can explain the synchronization of the orbits of Pluto and Neptune. Perhaps the orbit of Saturn was once located closer to Jupiter, but then moved away from it. Probably, the so-called late epoch of strong bombardment is connected with this - a period of very intense collisions with the Moon (and, apparently, with the Earth), which occurred 800 million years after the formation of the Sun. In some systems, grandiose collisions of formed planets may occur late in development.

Result: The end of the formation of planets and comets.

Messengers from the past

Meteorites are not just space rocks, but space fossils. According to planetary scientists, these are the only tangible witnesses to the birth of the solar system. It is believed that these are pieces of asteroids, which are fragments of planetesimals that never participated in the formation of planets and remained forever in a frozen state. The composition of meteorites reflects everything that happened to their parent bodies. It is striking that traces of the long-standing gravitational influence of Jupiter are visible on them.

Iron and stone meteorites apparently formed in planetesimals that experienced melting, as a result of which the iron separated from the silicates. Heavy iron sank to the core, while light silicates accumulated in the outer layers. Scientists believe that the heating was caused by the decay of the radioactive isotope aluminum-26, which has a half-life of 700 thousand years. A supernova explosion or a nearby star could "infect" the protosolar cloud with this isotope, as a result of which it fell into the first generation of planetesimals in the solar system in large quantities.

However, iron and stone meteorites are rare. Most contain chondrules - small millimeter-sized grains. These meteorites - chondrites - arose before the planetesimals and never experienced melting. It appears that most of the asteroids are not associated with the first generation of planetesimals, which were most likely ejected from the system under the influence of Jupiter. Planetary scientists have calculated that the region of the current asteroid belt used to contain a thousand times more matter than it does now. Particles that escaped Jupiter's claws or later fell into the asteroid belt coalesced into new planetesimals, but by that time there was little aluminum-26 left in them, so they never melted. The isotopic composition of chondrites shows that they formed about 2 million years after the beginning of the formation of the solar system.

The glassy structure of some chondrules indicates that before entering the planetesimals, they were sharply heated, melted, and then quickly cooled. The waves driving Jupiter's early orbital migration must have turned into shock waves and could have caused this sudden heating.

There is no single plan

Before the era of the discovery of extrasolar planets, we could only study the solar system. Despite the fact that this allowed us to understand the microphysics of the most important processes, we had no idea about the development of other systems. The astonishing diversity of planets discovered over the past decade has greatly expanded the horizon of our knowledge. We are beginning to understand that extrasolar planets are the last surviving generation of protoplanets that have experienced formation, migration, destruction, and continuous dynamic evolution. The relative order in our solar system cannot be a reflection of some general plan.

From trying to figure out how our solar system formed in the distant past, theorists have turned to research to make predictions about the properties of yet undiscovered systems that may be discovered in the near future. So far, observers have noticed only planets with masses on the order of Jupiter's near sun-like stars. Armed with a new generation of instruments, they will be able to search for terrestrial-type objects, which, according to the theory of successive accretion, should be widely distributed. Planetary scientists are just beginning to realize how diverse worlds are in the universe.

Translation: V. G. Surdin

Additional literature:
1) Towards a Deterministic Model of Planetary Formation . S. Ida and D.N.C. Lin in Astrophysical Journal, Vol. 604, no. 1, pages 388-413; March 2004.
2) Planet Formation: Theory, Observation, and Experiments. Edited by Hubert Klahr and Wolfgang Brandner. Cambridge University Press, 2006.
3) Alven H., Arrhenius G. The evolution of the solar system. M.: Mir, 1979.
4) Vityazev A.V., Pechernikova G.V., Safronov V.S. Terrestrial planets: Origin and early evolution. Moscow: Nauka, 1990.

In the first hundreds of years of its existence, the Earth experienced numerous cataclysms that left deep scars on its surface. Over the billions of years that have passed since then, wind and water erosion, global climate change have almost erased the traces of the primitive era. But they can still be found. Examples of planets forming around other stars today, as well as sophisticated computer models, help to understand the history of our planet.

The solar system formed from the same initial cloud of gas and dust as the Sun itself. Such clouds, called nebulae, are often invisible unless illuminated by stars. They are mostly made up of the lightest element, hydrogen, but also contain small amounts of helium and heavier elements formed in previous generations of stars and released after their death.

No one knows for sure what set a certain nebula on the path that led to the emergence of the solar system. It could have been a blast from a nearby supernova, the force of gravity from a passing star, or simply a passage through a cloud of denser material as the nebula orbited. Whatever the trigger, 4.5 million years ago, something sent the nebula to collapse.

CONCENTRATED SUBSTANCE

As the center of the cloud became denser, it began to exert a greater influence on its surroundings, pulling it inward, until, after a light year, the original cloud condensed and became a few light hours wide. The concentration of matter led to the fact that the solar nebula began to rotate faster.

As a result, the nebula flattened out and took on the shape of a disk with a bulge in the center. The bulge, containing about 90% of the mass of the nebula, became our star, the Sun, but continued to be surrounded by gas and dust - the raw material for the formation of the planetary system.

In the immediate vicinity of the Sun, the cloud was dominated by dust from heavy elements that form complex chemical compounds. Dust particles stuck together when they collided, lighter ones tended to evaporate under conditions of harsh solar radiation. They were then blown away from the inner solar system and recondensed in cooler areas where they helped form.

As the clumps of dust got bigger, the risk of them colliding with each other increased, and eventually a few of them got bigger enough to have an effective attraction force.

GROWING PLANETS

The resulting planetosimals began to rapidly collect material from their surroundings. The exponential growth continued until a few dozen objects, varying in size between the Moon and Mars, dominated the inner solar system. The constant bombardment of objects' surfaces heated them to their melting point.

At this stage, the planetosimals stopped their growth. However, most of them ended up in elongated, intersecting orbits, causing them to collide and increase in size by merging with each other. Each of these interplanetary collisions released a huge amount of energy to help keep the planetozimals hot.

LAND OF THE AGE OF HADEA

Last but not least, a colossal collision with a Mars-sized world called Theia resulted in the . On Earth, the most significant events were the eruptions of a significant part of the planetary mantle and the absorption of the core of Theia by the core of the Blue Planet. After the echoes of the shocks subsided, the Earth finally found its current form. The early era of the Earth's history is often called the Hadean period ("Hades" the ancient Greeks called hell). Gases from the molten interior formed a dense atmosphere, but the impact that formed the Moon ripped away most of the atmosphere.

According to traditional views, at that time the surface of the Earth shook from violent volcanic activity, because of which it was constantly updated. Probably, by that time a thin surface crust had formed - these could be minerals with a high content of heavy elements with a high melting point, such as iron and magnesium. However, this dense material must have sunk into the molten rock below.

The gas released from all this activity has created a high-pressure atmosphere, possibly high in carbon dioxide. In turn, this led to a suffocating greenhouse effect, similar to what is observed today on Venus. Despite temperatures above 200 °C, the water vapor released during gas evolution condensed into a liquid, thus forming oceans with hot water. However, recent studies of samples of some of the oldest rocks on Earth challenge the traditional view.

INTENSE ROTATION

Whatever the conditions on the surface, something else made the young Earth unrecognizable to the modern visitor. Theia's influence has caused our planet to rotate very rapidly, with a five-hour cycle of day and night. The rapid rotation caused the Earth to be 1800 km wider at the equator than from pole to pole. Since then, however, the ebb and flow of the moon has slowed its movement, so the current equatorial diameter is only 43 km larger than the polar one.

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