According to computer calculations, the initial mass of the gas-dust cloud that gave rise to the Solar System was equal to more than 104 Sun's masses. The initial size of the cloud was much greater than the current size of the Solar System. Its composition was similar to the one that is now found in dense cold interstellar nebulas, i.e., 99% of cosmic gas and 1% of cosmic dust.
The theory of four-stage formation of the planetary system is the most commonly recognized theory.
The planetary system is formed from the same protostar dust matter as the star is made of, and planets form at the same time as the star does. The central part of the cloud compresses independently, and becomes the protostar. The remaining part of the cloud, with a mass that is approximately ten times smaller than the central part of the cloud, continues to slowly rotate around the central bulge. Each peripheral fragment compresses independently. The gas condenses into solid matter bypassing the liquid phase. The particles are formed on the basis of the larger solid dust particles. The larger are the grains that are being formed, the quicker they move towards the central part of the dust cloud.
A fraction of the substance in question, which possesses an excess rotation momentum, forms a thin gas-dust layer called the gas-dust disk. A protoplanet cloud - called dust subdisk - is formed around the protostar. Gradually, the protoplanet cloud grows flatter and flatter, and becomes subject to strong compaction. Gravitational instability results in formation of separate small cold bundles of matter, which collide with each other and form progressively more massive bodies called the planetesimals. Parts of planetesimals disintegrate as a result of collisions, while other parts amalgamate. Eventually, a swarm of pre-planet bodies with around 1 km in diameter is formed. Billions of such pre-planet bodies came into being during the formation of the Solar System.
During the next stage, preplanetary bodies amalgamate into planets. The formation of planets continues for millions of years. Meanwhile, the protosun' temperature increases. The radiation heats up the protosun cloud's inner region up to 400 K. Solar winds and light pressure force the lightweight chemical elements (hydrogen and helium) out from the outskirts of the young star. This results in the varying chemical composition of the future planets.
As soon as the protoplanet's mass reaches 1-2 masses of the Earth, it becomes capable of entrapping an atmosphere of its own. Protojupiter increases its mass by dozens of times at the expense of gases that it traps for hundred of years.
Giant planets are able to entrap gas "blankets" that were formed at the peripherals of our Solar System. The cores of giant planets were formed first. They were followed by an acquisition of hydrogen or helium "blankets" around the cores of these planets.
Associated gas or dust mini disks emerged in the vicinity of giant planets. Planetary rings and numerous satellites were later formed from that matter.
A border at which water vapors condensed was located in the vicinity of future Jupiter. According to recent calculations, the volatiles were in gaseous state at closer distances, within future Jupiter's asteroid belt. As a result, the formation of preplanetary bodies was accelerated in the area of future Jupiter, while it was relatively smaller in the asteroid belt area. This explains why the growth speed of massive Jupiter was higher than that of the protoplanets that were located closer to the Sun. However, soon after the "birth" of Jupiter, it became an obstacle in the way of an asteroid belt. Affected by the gravitation of the giant planets, a significant quantity of matter was pushed away to the outskirts of the Solar System, and turned into comets.
The planets that formed closer to the center of the Solar System were far less massive. The solar wind blew away small particles and gas from the location of their formation. The heavier particles remained. The Earth's growth lasted for hundreds of millions of years. Its interior warmed up to 1000-2000 K because of gravity compression. An increase in temperature was promoted by falls of large bodies, resulting in formation of crates with increased temperature sources below them. Decay of radioactive elements, mainly uranium, thorium and potassium, is the other - and principal - source of the Earth's heat.
The age of the Earth, the Moon and the Solar System was determined with the advent of radiological methods. This age is around 4.6 billion years.
The Sun, the central body of Solar System, is a typical star of main sequence, whose balance is determined by the equality of gas pressure and gravity forces. The Sun is 5 billion years old, and it will continue to radiate a virtually constant flow of energy for approximately 5 billion years more by virtue of nuclear reactions within its interior.
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