SueÃ±o (venda)

The solar nebulous is the cloud of gas (or Disque of accretion) from which our Solar system was formed.

This assumption of nebula was proposed for the first time in 1755 by Emmanuel Kant, by supposing that the Nébuleuse rotates slowly, condensing and being flattened gradually under the effect of the Gravité, to form later star S and Planet S. a similar model was proposed in 1796 by Pierre-Simon Laplace.

Formation and evolution

According to the theory, nebula would have had a initial Diamètre of 100 UA and a mass from 2 to 3 times the current mass of the Sun. With time, gravity because condensation of the gas cloud and, whereas the Pressure and the Densité increased, the emergence of a Protostar in the center of nebula. This primitive system was heated by the Friction molecules, the fusion appearing only well later. Because of the principle of conservation of the Angular momentum (which implies the acceleration of rotation during the fall), nebula entirely did not crumble on itself, but led to the formation of a disc. A disc protoplanétaire was formed, orbits about it around protostar.

Inside the system, the lightest elements such as the Hydrogen and the Hélium were involved out of the central area by the Solar wind and the Pression of radiation, leaving behind the heavier elements and the particles of dust, which will form the Protoplanète S. In the external part of solar nebula, the ice and the volatile gases pûrent to remain, which will result in the formation from Planet S " minérales" in the interior part, and gas planets in the external part.

After 100 million years, the temperature in protostar reached a level such as the thermonuclear reactions started - the protostar became the Sun then. About at the same time, the interior planets were formed, that is to say there is approximately 4,6 billion years.

The the Moon S of the gas giant seem to be formed in the same way, agglomerating starting from accretion discs formed around these giant planets themselves. A contrario , the the Moon would have been rather formed following the collision of the Ground with an object of big size (comparable with that of Mars).

The theories Kant-Laplace and quasi-Collision

At the end of the 19^ème century, the theory Kant-Laplace was criticized by James Clerk Maxwell, which showed that if the matter of known planets had one day distributed around the Sun in the form of disc, the forces of the differential Rotation would have prevented the creation of planets. Another objection mentioned that the Sun had a Angular momentum lower than than the Kant-Laplace theory indicated. During decades, the majority of the astronomers preferred the theory of the quasi-collision ( near-collision théory in English), according to which the planets would have been formed by the approach of another star not far from the Sun. Thus, a great quantity of matter would have been attracted apart from the Sun and other star by the effect of their forces of mutual tide, matter which would have then condensed out of planets.

But of the objections also appeared against the theory of the quasi-collision, and during the years 1940, the Kant-Laplace theory was modified and became accepted overall. In the modified version, the mass of the Protoplanète S is regarded as being larger, and the anomaly of the angular momentum allotted to the magnetic forces.

The modern models of formation of the planets have had for refined summer.

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