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The origin of the planets
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Douglas Lin "In the World of Science" No. 8, 2008
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About the author Douglas N. C. Lin, like many scientists of his generation, became interested in astronomy under the influence of the launch of the first satellite in 1957.
He was born in New York and grew up in Beijing.
He studied at McGill University in Montreal, defended his dissertation at the University of Cambridge, then worked at Cambridge and Harvard Universities, and later began teaching at the University of California, Santa Cruz.
He is the founder and director of the Institute of Astronomy and Astrophysics of the Kavli Foundation at Peking University.
As a lover of skiing, he knows firsthand about ice particles and ice lines.
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On the scale of space, planets are just grains of sand that play 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 such an interaction of astronomical, geological, chemical and biological processes.
Life as we know it cannot originate in any of the other places in the cosmos.
In the last decade alone, astronomers have discovered more than 200 planets.
The formation of planets, which has long been considered a calm and stationary process, in fact turned out to be very chaotic.
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The amazing variety of masses, sizes, composition and orbits made many people think about their origin.
In the 1970s, the formation of planets was considered an ordered, deterministic process a conveyor belt on which amorphous gas dust disks turn into copies of the Solar system.
But now we know that this is a chaotic process, assuming a different result for each system
The planets that were born survived in 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 long lost twins, now wandering in the dark and cold space.
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More detailed
A young giant planet captures gas from the disk around a newborn star.
Image "In the world of Science"
The science of planet formation lies at the intersection of astrophysics, planetary science, statistical mechanics and nonlinear dynamics
In general, planetary scientists develop two main directions.
According to the theory of sequential accretion, tiny dust particles stick together, forming large blocks.
If such a block attracts a lot of gas, 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 scattering 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 copies the formation of stars in miniature.
But this hypothesis is very controversial, because it assumes the presence of a 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.
Ten years ago, scientists studying the formation of planets based their theories on a single example — our Solar system.
But now dozens of emerging and dozens of already formed planetary systems have been discovered, and no two of them are the same.
The main idea of the leading theories of planet formation is as follows: small dust particles stick together and capture gas.
But these processes are complex and confusing.
The struggle of competing mechanisms can lead to completely different results.
Despite the fact that scientists continue to argue, most consider the scenario of sequential accretion more likely.
In this article, I will rely on it.
1. The interstellar cloud is shrinking Time: 0 (the 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, the dust is just microscopic particles of water ice, iron and other solid substances 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 contract under the influence of gravity, forming clusters of stars.
Such a process can last from 100 thousand to several million years.
Each star is surrounded by a disk of the remaining matter, which is enough for the formation of planets.
Young disks mainly contain
hydrogen and helium.
In their hot inner regions, dust particles evaporate, and in the cold and rarefied outer layers, dust particles persist and grow as steam condenses on them.
Astronomers have discovered many young stars surrounded by such disks.
Stars aged from 1 to 3 million years have gas disks, while those that have existed for more than 10 million years have weak, gas poor disks, since the gas "blows" out of it either the newborn star itself or neighboring bright stars.
This time range is exactly the epoch of the formation of planets.
The mass of heavy elements in such disks is comparable to the mass of these elements in the planets of the Solar system: a fairly strong argument in defense of the fact that planets are formed from such disks.
The result: the newborn star is surrounded by gas and tiny (micron sized) dust particles.
Even giant planets began with modest bodies — micron dust particles (the ashes of long dead stars) floating in a rotating gas disk.
With the distance from the newborn star, the temperature of the gas drops, passing through the "ice line", beyond which the water freezes.
In our Solar system, this boundary separates the inner solid planets from the outer gas giants.
1. Particles collide, stick together and grow.
2. Small particles are carried away by gas, but those that are larger than a millimeter are slowed down and spiral towards the star.
3. At the ice line, the conditions are such that the friction force changes direction.
The particles tend to stick together and easily combine into larger bodies planetesimals.
2. The disk acquires the structure of Time: about 1 million years, dust particles in the protoplanetary disk, moving chaotically along with gas flows, collide with each other and sometimes stick together, sometimes collapse.
The dust particles absorb the star's light and re emit it in the long wave infrared range, transferring heat to the darkest inner regions of the disk.
The temperature, density, and pressure of the gas as a whole decrease with distance from the star.
Due to the balance of pressure, gravity and centrifugal force, the rotation speed of the 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 to the star.
The larger these particles become, the faster they move down.
Meter sized blocks can reduce their distance from the star by half in just 1000 years.
As the particles approach the star, they heat up, and gradually water and other substances with a low boiling point, called volatiles, evaporate.
The distance at which this happens, the so called "ice line", is 2-4 astronomical units
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 ice line divides the planetary system into an inner region devoid of volatile substances and containing solids, and an outer region rich in volatile substances and containing icy bodies.
On the ice line itself, water molecules accumulate, evaporated from dust particles, which serves as a trigger for a whole cascade of phenomena.
In this area, there is a gap 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 falling here are under the influence of a tailwind, not a counter wind, which pushes them forward and stops their migration into the disk.
And as particles continue to flow from its outer layers, the ice line turns into a band of its accumulation.
Accumulating, the particles collide and grow.
Some of them break through the ice line and continue to migrate inside; when heated, they are 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 the particles accelerates their growth.
Gradually, the dust particles gather into kilometer sized bodies called planetesimals, which at the last stage of planet formation rake almost all the primary dust.
It is difficult to see the planetesimals themselves in the forming planetary systems, but astronomers can guess their existence from the fragments of their collisions (see: Ardila D. Invisible planetary systems / / VMN, No. 7, 2004).
The result: a lot of kilometer long "building blocks" called planetesimals.
The growth of oligarchs Billions of kilometer long planetesimals formed at stage 2 are then collected into bodies the size of the Moon or Earth, called embryos.
A small number of them dominate in their orbital zones.
These "oligarchs" among the embryos are fighting for the remaining substance
3.
The embryos of planets are formed Time: from 1 to 10 million years, the cratered surfaces of Mercury, the Moon and asteroids leave no doubt that during the formation of planetary systems are similar to 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, and large ones determine its mass.
The orbits of bodies around a star may initially be elliptical, but over time, braking in 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 intensely it absorbs its low mass neighbors.
When the masses of the 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 is how they limit 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 absorption increase with distance from the star.
At a distance of 1 AU, the embryos reach a mass of 0.1 of the Earth's 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 the disk breaks, where planetesimals are concentrated.
The growth of" oligarchs " fills the system with an excess of bodies that strive to become planets, but only a few succeed.
In our Solar system, although the planets are distributed over a large space, they are as close to each other as possible.
If another planet with the mass of the Earth is placed between the Earth type planets, 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 be almost sure that someone has filled it and spilled some liquid; ma 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 life than at the end.
Some objects are thrown out of the system before it reaches equilibrium.
Astronomers have already observed free flying planets in young star clusters.
The result: "oligarchs" are the embryos of planets with masses ranging from the mass of the Moon to the mass of the Earth.
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 has formed, it begins to control the entire system.
But for this to happen, the embryo must collect gas faster than it moves in a spiral to the center.
A giant leap for a planetary system.
Image "In the world of Science" The formation of a giant planet is hindered by the waves that it excites in the surrounding gas.
The action of these waves is not balanced, it slows down the planet and causes its migration towards the star.
The planet attracts gas, but it canot settle until it cools down.
And during this time, it can come quite close to the star in a spiral.
A giant planet may not form in all systems
4.
A gas giant is born Time: from 1 to 10 million years, Jupiter probably began with an embryo comparable in size to the Earth, and then accumulated about 300 more Earth masses of gas.
Such an impressive growth is due to various competing mechanisms.
The gravity of the embryo attracts gas from the disk, but the gas compressing to the embryo releases energy, and in order to settle, it must cool down.
Therefore, the growth rate is limited by the possibility of cooling.
If it happens too slowly, the star can blow the gas back into the disk before the embryo forms a dense atmosphere around itself.
The bottleneck in heat removal is the transfer of radiation through the outer layers of the growing atmosphere.
The heat flow 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 embryo).
Early models showed that the embryo of a planet for sufficiently rapid cooling should have a mass of at least 10 Earth masses.
Such a large specimen can grow only near the ice line, where a lot of matter has previously collected.
Perhaps that is why Jupiter is located just behind this
a line.
Large embryos can be formed in any other place, if the disk contains more matter than planetologists usually assume.
Astronomers have already observed many stars, the disks around which are several times denser than previously assumed.
For a large sample, heat transfer does not seem to be a serious problem.
Another factor complicating the birth of gas giants is the movement of the embryo in a spiral to the star.
In a process called type I migration, the embryo excites waves in the gas disk, which in turn gravitationally affect its movement along the orbit.
The waves follow the planet, as its trail stretches behind the boat.
The gas on the outer side of the orbit rotates slower than the embryo and pulls it back, slowing down the movement.
And the gas inside the orbit rotates faster and pulls forward, accelerating it.
The outer region is larger, so it wins the battle and causes the embryo to lose energy and descend to the center of the orbit by several astronomical units in a 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 braking.
Perhaps that is also why Jupiter is exactly where it is.
The growth of the embryo, its migration and the loss of gas from the disk occur at almost the same pace.
Which process will win depends on luck.
It is possible that several generations of embryos will go through the migration process without being able to complete their growth.
Behind them, new batches of planetesimals are moving from the outer regions of the disk to its center, and this is repeated until a gas giant is eventually formed, or until all the gas is dissolved, and the gas giant can no longer form.
Astronomers have discovered Jupiter like planets in about 10% of the studied sun like stars.
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.
About a third of the stars rich in heavy elements have Jupiter type planets.
It is possible that such stars had dense disks, which allowed the formation of massive embryos that did not have problems with heat dissipation.
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 the Type I migration "upside down".
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.
The gas outside the planet's orbit rotates slower, so the planet accelerates it, forcing it to move outward, again from the planet.
Thus, the planet creates a gap in the disk and destroys the supply of building material.
Gas is trying to fill it, but computer models show that a planet wins the battle if, at a distance of 5 AU, its mass exceeds the mass of Jupiter.
This critical mass depends on the epoch.
The earlier the planet is formed, the greater its growth will be, since there is still a lot of gas in the disk.
Saturn has a smaller mass than Jupiter, simply because it formed several million years later.
Astronomers have found a shortage of planets with masses from 20 Earth masses (this is the mass of Neptune) to 100 Earth masses (the mass of Saturn).
This may be the key to restoring the picture of evolution.
The result: A planet the size of Jupiter (or its absence).
The history of the birth of worlds Based on radioisotope dating of meteorites and observations of near star disks, scientists have recreated the history of the formation of planets
1.
from 0 to 100 thousand years — a star is formed in the center of the disk, and nuclear fusion begins in it.
2. from 100 thousand to 2 million years dust particles stick together in planetary embryos with masses from the moon to the earth.
3. 2 million years — the first gas giant is formed and sweeps out asteroids of the first generation.
4. 10 million years — a gas giant stimulates the formation of other giants and Earth like planets.
By this time, there was almost no gas left.
5. 800 million years — the rearrangement of the planets continues about a billion years after its beginning.
5. The gas giant becomes restless Time: from 1 to 3 million years Oddly enough, many extrasolar planets discovered over the past ten years orbit their star at a very close distance, much closer than Mercury is around the Sun.
These so called "hot Jupiters" were not formed where they are now, because the orbital feeding zone would be too small to supply the necessary substance.
Perhaps, for their existence, a three stage sequence of events is needed, which for some reason has not been realized in our Solar System.
First, a gas giant should form in the inner part of the planetary system, near the ice line, 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 should move to its current location.
Type I migration cannot provide this, because it acts on the embryos even before they gain a lot of gas.
But type II migration is also possible.
The forming giant creates a gap in the disk and holds back the flow of gas through its orbit.
In this case, it must fight the tendency of the turbulent gas to propagate into adjacent areas of the disk.
The gas will never stop oozing into the gap, and its diffusion to the central star will cause the planet to lose orbital energy.
This process is quite slow: it takes several million years to move the planet by several astronomical units.
Therefore, the planet should begin to form in the inner part of the system, if it eventually has to enter an orbit near the star.
As this and other planets move inward, they push the remaining planetesimals and embryos in front of them, possibly creating "hot Earths" in orbits even closer to the star.
Third, something must stop the movement before the planet falls on the star.
This may be the magnetic field of the star, clearing the space near the star from gas, and without gas, the 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.
The result: a giant planet in a close orbit ("hot Jupiter").
How to embrace a star In many systems, a giant planet forms and begins to spiral closer to 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 turns out to be so close to the star that it stabilizes its orbit
6.
Other giant planets also appear Time: from 2 to 10 million years If one gas giant managed to form, then it contributes to the birth of a trace the life of 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 happened without its help.
In addition, he "extended 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 would be very slow without outside help: 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 substance concentrates, in general, for the same reason as on the ice line: the pressure drop 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 are formed from them.
The second generation of planets is formed from the matter collected for them by the first gas giant.
At the same time, the 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 beginning of gas accretion until there was almost no gas left in the disk.
The formation of these bodies was completed when they gained only two Earth masses of gas.
But these are not gas giants, but ice giants, which may be the most common type.
The gravitational fields of the second generation planets increase chaos in the system.
If these bodies have formed too close, their interaction with each other and with the gas disk can 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, i.e. the orbital periods are correlated as small integers.
It is unlikely that this was laid down during the formation, but it could arise 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 a different initial distribution of gas.
Most stars are born in clusters, and more than half of them are binary.
Planets may not form in the plane of the orbital motion of stars; in this case, the gravity of a 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: the company of giant planets.
Addition to the family
The first gas giant creates the conditions for the birth of the next ones.
The strip cleared by him acts as a fortress moat, which cannot be overcome by the substance moving from the outside to the center of the disk.
It gathers on the outer side of the gap, where new planets are formed from it.
7. Earth type planets are formed Time: from 10 to 100 million years, planetary scientists believe that Earth like planets are more common than giant planets.
Although the birth of a gas giant requires an exact balance of competing processes, the formation of a solid planet should be much more difficult.
Before the discovery of extrasolar Earth like planets, we relied only on data about the Solar system.
The four planets of the Earth group Mercury, Venus, Earth and Mars are mainly composed of substances with a high boiling point, such as iron and silicate pores
