The solar system is unique, and its origin is today a mystery that has not been fully discovered, although scientists have been trying for many centuries to reproduce the picture of its creation. We can only accept or reject modern views on the origin solar system, but for humanity it will still be a mystery for many centuries to come. However, there are several scientific assumptions about its occurrence, which we will consider in this article.
The German philosopher Kant suggested in the 18th century that the solar system was formed from a cloud of numerous cold particles in continuous and chaotic motion. Another scientist, the Frenchman Laplace, in 1796 suggested that the origin of the solar system is associated with a constantly rotating nebula, consisting entirely of gas.
Interesting hypotheses The origin of the solar system has been expressed by various scientists at all times. In particular, the English astronomer Hoyle claims that the Sun at the time of birth was a clot of gas and dust nebula, in which there was a magnetic field. At first he rotated high speed, and later due to the influence magnetic field its rotation began to decline.
Another one was put forward by O. Yu. Schmidt. As the scientist suggests, the medium that serves to form planets is a fragment of an interstellar cloud, consisting of a gas and dust mixture. Numerous clusters are formed in it as a result of chaotic collisions of particles. Large formations gradually increase in size and become denser. This is how, from his point of view, the "embryos" of future planets are formed. The impacts that occur during their collisions contribute to the fact that their orbits become like circles, and over time their movement around the Sun becomes stable.
The solar system and its origins are studied in many well-known institutes of the world. The annual international congresses include in the program a mandatory discussion of this issue, and the discussions have already repeatedly taken part in the leading Russian specialists from the Geophysical Institute at the Academy of Sciences.
In-depth research on the topic "The solar system and its origin" is given important place and funds for their implementation are allocated from the state budget. The moment will come, and thanks to the tireless work of scientists, the veil of secrecy will be lifted to be able to learn even more about the origin of our amazing planet.
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 separated by "emptiness" (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 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, condensed in the outer, cool layers of the star and 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 Myr have gaseous disks, while those older than 10 Myr have faint, gas-poor disks, as the gas is blown out of them either by the newborn star itself or by neighboring stars. bright stars. This time range is exactly the epoch of planetary formation. Weight heavy elements in such disks is comparable to the mass of these elements in the planets of the solar system: quite strong argument in defense of the fact that planets form 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.
- Particles collide, stick together and grow.
- Small particles are carried away by the gas, but those larger than a millimeter are decelerated and spiral towards the star.
- At the line of ice, the conditions are such that the frictional force changes direction. Particles tend to stick together and easily combine into more large 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 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 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 planetesimals from their 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 between planets earth type place another planet with the mass of the Earth, 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 a gas giant like Jupiter crucial point in history 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 around it. dense atmosphere. 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 form in any other place if the disk contains more substance 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, since 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 ends up in 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 their movement 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 to the star, dragging the planet with it, which eventually turns out to be 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 formation process without outside help would have gone very slowly: the disk would have dissipated 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 favourable wind on dust grains 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. Wherein great importance has a pace: 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 disk of gas 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 precise 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 influence of mutual perturbations over several million years, the orbits of the nuclei are drawn 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 an embryo the size of Mars crashed into 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 swipe 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 inner region In 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.
Gravitational interaction of nuclei with each other and with 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 region gravitational influence 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. This is probably related to the so-called late epoch of heavy bombardment - a period of very intense collisions with the Moon (and, apparently, with the Earth), which began 800 million years after the formation of the Sun. In some systems, grandiose collisions of formed planets can occur on late stage 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 forever remained 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 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. Even though it allowed us to understand microphysics critical processes, we had no idea about the ways of development of other systems. The amazing variety of planets discovered beyond last decade significantly 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.
The origin of the solar system is directly due to the forces of gravity. It is thanks to them that the Universe, galaxies, stars and planets exist. People who lived many centuries ago assumed that there must be some mysterious forces that gradually control the world. But the first to create mathematical model universal gravitation, was English physicist, mathematician and astronomer Isaac Newton(1642-1727). He laid the foundations of celestial mechanics.
It was on the basis of Newton's work that empirical laws Kepler. The theory of the motion of comets and the moon was created. Newton scientifically explained the precession of the earth's axis. All this is still considered huge contribution into science. But the German philosopher Immanuel Kant (1724-1804) was the first to express his ideas about the formation of the Sun and planets.
In 1755, his work "The Universal natural history and the theory of the sky. "In it, the philosopher suggested that all celestial bodies and the luminary itself arose from a nebula, which was originally a huge gas and dust cloud. Kant was the first to talk about cosmogony- the origin of the world.
This requires primary material and gravitational forces. But divine intervention in this issue not required. That is, the world arose as a result physical laws and God had nothing to do with it. At the time, that was a pretty bold statement.
Three stages in the formation of the solar system
Modern views on the origin of the solar system largely coincide with Kant's conclusions. No wonder he, according to Bulgakov, constantly had breakfast with the Devil himself. Therefore, the philosopher knew what he was saying, and today's learned minds largely agree with him.
The main theory suggests that a giant cloud of gases and dust existed at the site of the current solar system 5 billion years ago. It had huge dimensions, and was stretched in space for 6 billion km. Similar dust clouds exist in many corners of the vast universe. Most of them are made up of hydrogen. This is the gas from which stars are originally formed. Then, as a result of a thermonuclear reaction, the inert gas helium begins to be released. The share of other substances accounts for only 2%.
At some point, the dust cloud received an external powerful impulse, which is a huge release of energy. It could have been the shock wave generated by the explosion. supernova. And it is possible that there was no external influence. Just due to the law of attraction, the cloud began to decrease in volume and condense.
This process gave rise to gravitational collapse. That is, there was a rapid compression of the cosmic mass. As a result of this, an incandescent core appeared in the center with a very high density. The rest of the mass dispersed along the edges of the core. And since everything in space rotates around its axis, this mass has acquired the shape of a disk.
The core decreased in size, increasing its temperature and density. As a result, it has been transformed into protostar. This is the name of a star in which there are prerequisites for the start of a thermonuclear reaction. And the gas cloud around the core became more and more dense.
Finally, in the core, the temperature and pressure reached a critical value. This triggered the start of a thermonuclear reaction, and hydrogen began to turn into helium. The protostar ceased to exist, and instead a star arose called the Sun. This whole process lasted about one million years. Not much by space standards.
And then another process followed. Gas and dust clouds revolving around the Sun began to converge into dense rings. Each of them formed a clot with a higher density. Moreover, the heaviest substances were in the center of the clot, and the lungs created outer shell. This is how the cores of the planets surrounded by gases were formed.
To put it quite simply, we can say that the star "blew away" from the nearest nuclei gas envelopes. This is how small planets were formed, orbiting near the Sun. it Mercury, Venus, Earth and Mars. And other planets were at a great distance from the star. Therefore, they retained their "gas coats". They are currently known as gas giant planets: Jupiter, Saturn, Uranus and Neptune. All these transformations took another 4 million years.
Subsequently, satellites appeared around the planets. So the Moon appeared near the Earth. The rest of the planets also acquired satellites. And, in the end, a single space community was formed, which exists to this day.
This is how science explains the origin of the solar system. By the way, this theory inherent in other stellar formations, which in space infinite set. Who knows, maybe somewhere in the black abyss there is a similar star system. There is intelligent life, and, consequently, there is some kind of civilization. It is quite possible that someday people will meet brothers in mind. This will become the most outstanding event our history.
Kant's theory
For many centuries, the question of the origin of the Earth remained the monopoly of philosophers, since factual material almost completely absent in this area. The first scientific hypotheses regarding the origin of the Earth and the solar system, based on astronomical observations, were put forward only in 18th century. Since then, more and more new theories have not ceased to appear, in accordance with the growth of our cosmogonic ideas. The first in this series was the famous theory, formulated in 1755 German philosopher Immanuel Kant. Kant believed that the solar system arose from some primary matter, previously freely dispersed in space. Particles of this matter moved into various directions and, colliding with each other, lost speed. The heaviest and densest of them, under the action of gravity, connected with each other, forming a central bunch - the Sun, which, in turn, attracted more distant, smaller and lighter particles.
Thus, a certain number of rotating bodies arose, the trajectories of which mutually intersected. Some of these bodies, initially moving in opposite directions, were eventually drawn into a single stream and formed rings of gaseous matter located approximately in the same plane and rotating around the Sun in the same direction without interfering with each other. In separate rings, denser nuclei were formed, to which lighter particles were gradually attracted, forming spherical accumulations of matter; this is how the planets were formed, which continued to circle around the Sun in the same plane as the original rings of gaseous matter.
Nebular theory of Laplace
In 1796, the French mathematician and astronomer Pierre-Simon Laplace put forward a theory somewhat different from the previous one. Laplace believed that the Sun originally existed in the form of a huge incandescent gaseous nebula (nebula) with an insignificant density, but colossal dimensions. This nebula, according to Laplace, originally rotated slowly in space. Under the influence of gravitational forces, the nebula gradually contracted, and the speed of its rotation increased. The resulting increasing centrifugal force gave the nebula a flattened and then a lenticular shape. In the equatorial plane of the nebula, the ratio between attraction and centrifugal force changed in favor of the latter, so that in the end the mass of matter accumulated in equatorial zone nebula, separated from the rest of the body and formed a ring. From the nebula that continued to rotate, new rings were successively separated, which, condensing at certain points, gradually turned into planets and other bodies of the solar system. In total, ten rings separated from the original nebula, disintegrating into nine planets and a belt of asteroids - small celestial bodies. The satellites of individual planets were formed from the substance of the secondary rings, torn off from the hot gaseous mass of the planets.
Due to the continued compaction of matter, the temperature of the newly formed bodies was exceptionally high. At that time, our Earth, according to P. Laplace, was a hot gaseous ball that glowed like a star. Gradually, however, this ball cooled down, its matter passed into liquid state, and then, as it cooled further, a hard crust began to form on its surface. This crust was enveloped in heavy atmospheric vapors, from which water condensed as it cooled.
These two theories complemented each other, so in the literature they are often referred to under common name as the Kant-Lallas conjecture. Since science did not have more acceptable explanations at that time, this theory had many followers in the 19th century.
Jeans theory.
A new theory proposed in 1916 by James Jeans, according to which a star passed near the Sun and its attraction caused the emission of solar matter, from which planets subsequently formed, was supposed to explain the paradox of the angular momentum distribution. However, at present, experts do not support this theory. In 1935, Russell proposed that the Sun was a double star. The second star was torn apart by gravitational forces during a close approach to another, third star. Nine years later, Hoyle theorized that the Sun was a double star, with the second star going all the way through evolution and exploding as a supernova, shedding its entire envelope. From the remnants of this shell, the planetary system was formed. In the 1940s, Soviet astronomer Otto Schmidt suggested that the Sun captured a cloud of dust as it orbited the Galaxy. From the substance of this huge cold dust cloud formed cold dense pre-planetary bodies - planetesimals. Elements of many of the theories listed above are used by modern cosmogony.
Schmidt's theory.
In 1944, the Soviet scientist O. Yu. Schmidt proposed his theory of the origin of the solar system. According to O. Yu. Schmidt, our planetary system was formed from matter captured from a gas-dust nebula through which the Sun once passed, which even then had an almost "modern" appearance. However, no difficulty with torque planets do not arise, since the initial moment of the cloud matter can be arbitrarily large. Beginning in 1961, this hypothesis was developed by the English cosmogonist Littleton, who made significant improvements to it. It is easy to see that the block diagram of the "accretion" hypothesis of Schmidt-Littleton coincides with the block diagram of the "capture hypothesis" of Jeans-Wulfson. In both cases, the "almost modern" Sun collides with a more or less "loose" space object, capturing parts of its substance. It should be noted, however, that in order for the Sun to capture a sufficient amount of matter, its velocity relative to the nebula must be very small, of the order of one hundred meters per second. Considering that the speed internal movements elements of the cloud should not be less, then, in essence, we are talking about the Sun "stuck" in the cloud, which, most likely, should have a common origin with the cloud. Thus, the formation of planets is associated with the process of star formation.
Fesenkov's theory.
Probably, the age of the Moon and the Earth is close to the age of the Sun, Academician V. Fesenkov believed in 50-60 years. And the substance of which they are composed, arose from the near-solar gas-dust nebula, and not from interstellar clusters. According to Fesenkov, the Moon and the Earth are “children of the young Sun”, which, rotating and gradually thickening, gave rise to vortex condensations around itself - future planets and their satellites. With regard to the Moon, the scientist turned out to be right; its origin is indeed connected with the explosion of the young Sun.
The solar system consists of a central celestial body - the star of the Sun, 9 large planets revolving around it, their satellites, many small planets - asteroids, numerous comets and the interplanetary medium. The major planets are arranged in order of distance from the Sun as follows: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto. The last three planets can only be observed from Earth through telescopes. The rest are visible as more or less bright circles and have been known to people since ancient times.
One of important issues associated with the study of our planetary system - the problem of its origin. The solution to this problem has a natural-scientific, ideological and philosophical meaning. For centuries and even millennia, scientists have tried to figure out the past, present and future of the universe, including the solar system. However, the possibilities of planetary cosmology to this day remain very limited - so far only meteorites and samples of lunar rocks are available for experiment in the laboratory. Limited and opportunities comparative method research: the structure and patterns of other planetary systems are still not well understood.
By now, many hypotheses about the origin of the solar system are known, including those proposed independently by the German philosopher I. Kant (1724–1804) and the French mathematician and physicist P. Laplace (1749–1827). The point of view of I. Kant was the evolutionary development of a cold dusty nebula, the entrance of which first arose a central massive body - the Sun, and then the planets were born. P. Laplace considered the original nebula to be gaseous and very hot, in a state of rapid rotation. Compressing under the influence of the force of universal gravitation, the nebula rotated faster and faster due to the law of conservation of angular momentum. Under the influence of large centrifugal forces, arising during rapid rotation in the equatorial belt, rings were successively separated from it, turning into planets as a result of cooling and condensation. Thus, according to the theory of P. Laplace, the planets were formed before the sun. Despite this difference between the two hypotheses under consideration, they both come from the same idea - the solar system arose as a result of regular development nebulae. This is why such an idea is sometimes called the Kant-Laplace hypothesis.
According to modern ideas, the planets of the solar system formed from cold gas and dust cloud that surrounded the Sun billions of years ago. This point of view is most consistently reflected in the hypothesis of the Russian scientist, Academician O.Yu. Schmidt (1891–1956), who showed that the problems of cosmology can be solved by the concerted efforts of astronomy and the Earth sciences, primarily geography, geology, and geochemistry. At the heart of the hypothesis O.Yu. Schmidt is the idea of the formation of planets by combining solids and dust particles. The gas and dust cloud that emerged near the Sun initially consisted of 98% hydrogen and helium. The remaining elements condensed into dust particles. The chaotic movement of gas in the cloud quickly ceased: it was replaced by the calm movement of the cloud around the Sun.
Dust particles are concentrated in the central plane, forming a layer increased density. When the density of the layer reached a certain critical value, its own gravitation began to "compete" with the gravitation of the Sun. The dust layer turned out to be unstable and disintegrated into separate dust clots. Colliding with each other, they formed many continuous dense bodies. The largest of them acquired almost circular orbits and in their growth began to overtake other bodies, becoming potential embryos of future planets. Like more massive bodies, neoplasms attached to themselves the remaining matter of the gas and dust cloud. In the end, nine large planets formed, the movement of which in orbits remains stable for billions of years.
Taking into account the physical characteristics, all the planets are divided into two groups. One of them consists of relatively small terrestrial planets– Mercury, Venus, Earth and Mapca. Their substance is distinguished by a relatively high density: on average, about 5.5 g / cm 3, which is 5.5 times higher than the density of water. Another group is made up giant planets: Jupiter, Saturn, Uranus and Neptune. These planets have huge masses. Thus, the mass of Uranus is equal to 15 Earth masses, and Jupiter - 318. The giant planets consist mainly of hydrogen and helium, and the average density of their matter is close to the density of water. Apparently, these planets do not have a solid surface similar to the surface of the terrestrial planets. Special place occupied by the ninth planet - Pluto, discovered in March 1930. It is closer in size to the terrestrial planets. Recently it was discovered that Pluto is a double planet: it consists of a central body and a very large satellite. Both celestial bodies revolve around a common center of mass.
In the process of planet formation, their division into two groups is due to the fact that in parts of the cloud far from the Sun, the temperature was low and all substances, except hydrogen and helium, formed particulate matter. Among them, methane, ammonia and water prevailed, which determined the composition of Uranus and Neptune. The composition of the most massive planets - Jupiter and Saturn, in addition, turned out to be a significant amount of gases. In the region of the terrestrial planets, the temperature was much higher, and all volatile substances (including methane and ammonia) remained in a gaseous state, and, therefore, were not included in the composition of the planets. The planets of this group were formed mainly from silicates and metals.
The process of formation of the solar system cannot be considered thoroughly studied, and the proposed hypotheses cannot be considered perfect. For example, the considered hypothesis did not take into account the influence electromagnetic interaction during the formation of the planets. Clarification of this and other questions is a matter for the future.
Sun
The central body of our planetary system is Sun- the closest star to the Earth, which is a hot plasma ball. This is a gigantic source of energy: its radiation power is very high - about 3.86 10 23 kW. Every second the Sun radiates such an amount of heat that would be quite enough to melt the layer of ice surrounding Earth, a thousand kilometers thick. The sun plays an exceptional role in the origin and development of life on Earth. Only a small part reaches the earth solar energy, which supports gaseous state earth's atmosphere, the surfaces of land and water bodies are constantly heated, the vital activity of animals and plants is ensured. Part of the solar energy is stored in the bowels of the Earth in the form hard coal, oil, natural gas.
At present, it is generally accepted that thermonuclear reactions occur in the interior of the Sun at enormous temperatures - about 15 million degrees - and monstrous pressures, which are accompanied by the release of huge amount energy. One of these reactions may be the synthesis of hydrogen nuclei, in which the nuclei of the helium atom are formed. It is calculated that every second in the interior of the Sun 564 million tons of hydrogen are converted into 560 million tons of helium, and the remaining 4 million tons of hydrogen are converted into radiation. thermonuclear reaction will continue until the supply of hydrogen runs out. They currently make up about 60% of the Sun's mass. Such a reserve should be sufficient for at least several billion years.
Almost all of the Sun's energy is generated in its central region, from where it is transferred by radiation, and then in the outer layer - is transferred by convection. Effective temperature the surface of the Sun - the photosphere - about 6000 K.
Our Sun is not only a source of light and heat: its surface emits streams of invisible ultraviolet and X-rays, as well as elementary particles. Although the amount of heat and light sent to the Earth by the Sun remains constant for many hundreds of billions of years, the intensity of its invisible radiations varies significantly: it depends on the level solar activity.
There are cycles during which solar Activity reaches its maximum value. Their periodicity is 11 years. During the years of greatest activity, the number of sunspots and flares increases by solar surface, on Earth arise magnetic storms, the ionization of the upper layers of the atmosphere increases, etc.
The sun exerts a noticeable influence not only on such natural processes, how is the weather, terrestrial magnetism, but also on biosphere- animal and vegetable world Land, including per person.
It is assumed that the age of the Sun is at least 5 billion years. This assumption is based on the fact that, according to geological data, our planet has existed for at least 5 billion years, and the Sun was formed even earlier.
Moon
Just as our earth revolves around the sun, Moon is a natural satellite of our planet. The moon is smaller than the earth, its diameter is about one quarter of the earth's diameter, and its mass is 81 times less mass Earth. Therefore, the force of gravity on the Moon is 6 times less than on our planet. The weak force of attraction did not allow the Moon to retain the atmosphere, for the same reason there can be no water on its surface. Open bodies of water would quickly evaporate, and the water vapor would escape into space.
The surface of the moon is very uneven: it is covered with mountain ranges, ring mountains - craters and dark ridges of flat areas called seas, on which small craters are observed. It is assumed that the craters are of meteorite origin, that is, they were formed at the sites where giant meteorites fell.
Starting from 1959, when the Soviet automatic station Luna-2 first reached the surface of the Moon, and up to the present time, spacecraft have brought a lot of information about our natural satellite. In particular, the age of lunar rocks delivered to Earth by spacecraft was determined. The age of the youngest rocks is about 2.6 billion years, while the age of older rocks does not exceed 4 billion years.
A loose layer formed on the surface of the Moon, covering the main rock - ragolith, consisting of fragments igneous rocks, slag-like particles and solidified drops of molten magma. It is assumed that about 95% of the rocks covering the lunar surface are in a magmatic state.
Temperature lunar surface is 100–400 K. The moon is at an average distance from the Earth of 384,400 km. Having overcome such a distance, on July 21, 1969, the American astronaut N. Armstrong first set foot on the surface of the Moon - an old fairy-tale dream of a man's flight to the Moon came true.
terrestrial planets
United in one group of planets: Mercury, Venus, Earth, Mars - although they are close in some characteristics, but still each of them has its own unique features. Some characteristic parameters of the terrestrial planets are presented in Table. 5.1.
Table 5.1
The average distance in the table. 5.1 is given in astronomical units (AU); 1 a.u. equal to the average distance of the Earth from the Sun (1 AU = 1.5 10 8 km.). The most massive of these planets is the Earth: its mass is 5.89 10 24 kg.
Significantly different planets and the composition of the atmosphere, as can be seen from Table. 5.2, where chemical composition atmospheres of Earth, Venus and Mars.
Table 5.2
Mercury- the most minor planet in earth group. This planet could not keep the atmosphere in the composition that is characteristic of the Earth, Venus, Mars. Its atmosphere is extremely rarefied and contains Ar, Ne, He. From Table. 5.2 it can be seen that the Earth's atmosphere is characterized by a relatively high content of oxygen and water vapor, due to which the existence of the biosphere is ensured. On the Venus and mars the atmosphere contains a large amount carbon dioxide with a very low content of oxygen and water vapor - all these are characteristic signs of the absence of life on these planets. No life and Mercury: lack of oxygen, water and high daytime temperature (620 K) hinder the development of living systems. The question of the existence of some forms of life on Mars in the distant past remains open.
The planets Mercury and Venus do not have satellites. Natural satellites of Mars Phobos and Deimos.
giant planets
Jupiter, Saturn, Uranus and Neptune are the giant planets. Jupiter- the fifth in distance from the Sun and the most big planet Solar system - located at an average distance from the Sun of 5.2 AU. Jupiter is a powerful source of thermal radio emission, has a radiation belt and an extensive magnetosphere. This planet has 16 satellites and is surrounded by a ring about 6 thousand km wide.
Saturn is the second largest planet in the solar system. Saturn is surrounded by rings (see Fig. 5.4), which are clearly visible through a telescope. They were first observed in 1610 by Galileo using the telescope he created. The rings are a flat system of many small satellites of the planet. Saturn has 17 moons and has a radiation belt.
Uranus- the seventh planet in the solar system in order of distance from the Sun. There are 15 satellites revolving around Uranus: 5 of them were discovered from Earth, and 10 were observed using the Voyager 2 spacecraft. Uranus also has a ring system.
Neptune- one of the planets most distant from the Sun - is located at a distance of about 30 AU from it. Its orbital period is 164.8 years. Neptune has six moons. Remoteness from the Earth limits the possibilities of its research.
Planet Pluto does not belong to the terrestrial group, nor to the giant planets. This is a relatively small planet: its diameter is about 3000 km. Pluto is considered to be a double planet. Its satellite, approximately 3 times smaller in diameter, moves at a distance of only about 20,000 km from the center of the planet, making one revolution in 4.6 days.
A special place in the solar system is occupied by the Earth - the only living planet.
5.7. Earth is a planet in the solar system
