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planetesimal theory of planet formation
Star sub-images can be small objects in millimeter range. (mm)
To hundreds of kilometers in size（km)
They are collided by dust particles to form larger objects, such as planets.
Planetary theory was first proposed by a Soviet astronomer in the name of Victor Savronov.
His theory is widely accepted by astronomers all over the world, but there are other theories.
We\'ll look at the theoretical state and how planets in the solar system form.
Generally speaking, planetary theory holds that during the formation of our planetary system, a protoplanetary disk is formed from the material of the solar nebula.
The Solar Nebula is considered to be a disk. -
Clouds formed by gas and dust left behind by the sun.
Due to accretion, smaller dust particles begin to gather together during collisions, forming larger stars.
When stars reach about a kilometer in size, they begin to attract smaller objects because of their gravity.
Eventually, the stars become large enough to attract larger objects, which in turn form protoplanets and eventually planets.
Many asteroids collide with each other, causing the bodies to split into smaller and smaller ones. However, some eventually converge instead of splitting.
According to this theory, asteroids and Kuiper Belt objects（KBOs)
It is considered a star.
Many planetary satellites are also considered stars.
We will now study this theory in detail because it is related to the formation of terrestrial planets. (
Or inner planets)
And objects in the Alter Cloud and the Kuiper Belt.
The formation of the Earth\'s planets, which occurred billions of years ago, is a complex process.
Planets on Earth are thought to be formed by the accumulation and collision of larger objects, namely stars.
Solar nebulae accumulate a certain proportion of mass in the form of microns. -
Dust particles of the same size are formed in the turbulent atmosphere of exploding stars.
The density of space within the solar nebula allows small clusters to gather in collisions.
Brownian motion and turbulence promote further collision of dust particles.
Based on the turbulent vortices in the nebula, particles begin to settle in the middle plane.
Close to the middle plane, the spatial density further propagates the agglomeration of dust particles.
This process eventually led to the formation of stars about 10 kilometers in size.
Due to the disturbance of gravity, the orbit eccentricity of the planetary embryos increases, resulting in the intersection of orbits.
This means more collisions. Over time, stars roughly the size of planets begin to collide with each other at speeds of more than 10 kilometers per second.
The result is a melted protoplanet.
Nearly 100 million years（Myr)
After the initial accumulation of planets began, Earth and Venus formed several smaller planets.
It should be noted that the process of star collision and accumulation is random or random, and various models must be considered.
Another interesting thing is that the orbit eccentricity of the inner planet will not be affected by further collisions, leading to near-circular orbits.
Mass planets are considered to be a great disturbance of gravity, affecting the formation of terrestrial planets.
At present, the understanding of the formation of exoplanets is relatively vague.
There are two theories commonly considered to be the formation of exoplanets.
One is the core accretion theory and the other is the disk instability.
In theory, both can form exoplanets, but both have difficulties.
For simplicity, we only study the core accretion theory.
Stars, in the form of ice dust particles, begin to accumulate as a result of collisions.
This process is similar to the formation of planets on Earth.
Stars eventually form planetary embryos, but planetary embryos are of greater mass.
The accretion of stars results in the accretion of a 10-Earth core. -mass cores.
The size of the core increases when the nebular gas is obtained. With the increase of the core size, the nebular gas continues to become thicker.
The accumulation rate of gases at a certain point increases, which is greater than that of solids.
This led to the collapse of nebular gas onto the original planet, which eventually depleted the surrounding gas disk.
The formation of an atmosphere rich in hydrogen and helium encloses a solid core, the classic structure of a gaseous giant planet.
The planet has a nearly circular orbit.
The biggest problem in this star initial accretion model is the time scale involved.
Observations of young stars show that gas disks around the stars disappear within a few miles.
The process of nuclear proliferation is longer than that.
Another option is the disk instability model, which also has its problems.
Turbulence is a factor in star formation. Traditional star formation theories include dust particles sticking to a disk.
This may be offset or disturbed by turbulence in the disk.
In order to form clumps, they need to be in the low vorticity region.
This is to avoid the rotational breakage of the mass.
In addition, clumps need to be self-sufficient-
Gravity to avoid bursting due to punch pressure generated by ambient gas.
The formation rate of stars depends on the rotation rate of the largest vortices in the disk.
Compared with the disk, if the largest vortices rotate slowly, the rate of star formation will increase.
Formation rate increases in order-of-
When a person moves outward across the snow line, the range may even be greater.
Snow lines are the distance from the original sun in the solar nebula, where the temperature is cold enough for water, methane and other hydrocarbons to condense into solid ice particles.
This means that the surface density of solids has increased by two times.
The modal size of the star increases the square root of the distance from the star.
Orbital migration of planets-
Astronomical simulations show that planets with masses less than m（
Soil Core Quality)
Orbital migration is usually type I in the solar disk with the smallest mass.
Type I migration usually occurs when a planet loses angular momentum and moves inward in a disk.
Type II migration is common for planets with mass greater than m.
Type II migration of planets means that planets have cleared the distance of type I migration, so they can form outside the disk.
These results are based on migration simulations between a three-dimensional Kepler disk and a giant planet.
Binary clusters are clusters that surround each other and influence the development of planetary systems.
Their role in development is not very clear.
In theory, however, binary clusters exist around distant stars.
Some stars in our solar system are considered remnants of this binary system.
Asteroids and Kuiper Belt objects are quasars, such as Vesta.
It is believed to be the result of collisions and splits between two larger stars.
Many such asteroids are thought to be formed by stars.
Kuiper Belt objects are also believed to be based on the theory of stars.
Astronomers generally believe that about three years.
Eight billion years ago, most of the stars from the solar system were ejected to distant galaxy margins, where we found the Oort Cloud and the Kuiper Belt.
What can we learn from stars? The stars that still exist in our solar system are like time capsules, which provide us with a glimpse of the past.
Many things about the formation of the solar system and the initial solar nebula can be explained from the study.
For this reason, meteorites are of particular interest to astronomers.
Although their chemical properties can be changed by solar radiation, the matter inside stars has remained intact since the formation of the solar system. Credits1. Safronov, V. S.
Evolution of Protoplanetary Nebulae and Formation of Earth and Planets. Moscow, 1969: Nauka 2. Marcy, G. W. and Butler, R. P. , Ann. Rev. Astr. Astrophys. 36, 37, 1983.