The solar system formed from a cloud of gas and dust that contracted gravitationally, flattened into a disc shape, and gave birth to the Sun. Planets formed from tiny solid particles that collided and stuck together, with gas giants forming further out and rocky planets forming closer to the Sun. The solar nebula theory explains the key features of the solar system and is supported by evidence from other parts of the Milky Way.
Our solar system is thought to have formed about 4.6 billion years ago from a large cloud of gas and dust, several light-years in diameter, known as a nebula. This cloud consisted mostly of hydrogen gas, with minor amounts of the elements that make up the solar system today. According to the solar nebula theory, part of this cloud began to contract gravitationally, possibly due to the disturbance of a nearby supernova or the passage of another star, and in doing so the slow initial rotation of the cloud began to increase as it contracted. causing it to flatten into a disc shape. As more material accumulated in the center of the disk, the density and temperature increased, reaching the point where hydrogen atoms began fusing, forming helium and releasing enormous amounts of energy, resulting in the birth of the Sun. , asteroids and comets formed from the leftover material.
After a while, further collapse was arrested by the Sun reaching hydrostatic equilibrium. The solar wind from the young Sun dispersed much of the material in the solar nebula, reducing its density, and the nebula began to cool. Apart from the three lightest elements — hydrogen, helium and lithium — the elements of which the solar nebula was composed were formed either by the nuclear fusion of now disappeared stars or, in the case of elements heavier than iron, created by supernovae. Simple covalent molecules, including water, methane and ammonia, and ionic molecules, such as metal oxides and silicates, would also have been present. Initially, due to the high temperatures in the disk, these compounds would have been gaseous, but as cooling occurred most of the elements and compounds condensed into tiny particles; metals and ionic compounds condensed first due to their higher boiling and melting points.
Near the disk’s center, metals, metallic compounds, and silicates predominated, but further out, where temperatures were cooler, large amounts of ice condensed out of the nebula. In this outer region, hydrogen gas and helium were also abundant; these gases were largely dispersed by the solar wind closest to the Sun. Tiny solid particles collided and stuck together, forming larger and larger objects that began to attract more material through gravitation, eventually leading to the formation of planets . In the inner solar system, the lack of ice, hydrogen and helium has led to the formation of the relatively small planets Mercury, Venus, Earth and Mars, composed largely of rock. Further out, the ice and mineral particles aggregated, forming larger bodies that were able to hold the light gases hydrogen and helium through their relatively strong gravitational fields, giving rise to the “gas giant” planets, Jupiter, Saturn, Uranus and Neptune.
The solar nebula theory explains a number of key features of our solar system. The fact that the planets – with the exception of Pluto, which is no longer considered a planet – all lie more or less in the same plane, and the fact that they all orbit the Sun in the same direction, suggests that they originated in a disk surrounds the Sun. The presence of relatively small rocky planets in the inner solar system and gas giants in the outer region also fits this model well.
Beyond Neptune, the outermost planet, is the Kuiper belt, a region of relatively small objects composed of rock and ice. It is thought that Pluto may have originated here and that comets are Kuiper belt objects that have been thrust into orbits that take them into the inner solar system. The Kuiper belt is also well explained by the solar nebula theory as resulting from residual ice and rocky material dispersed too thinly to have formed planets.
Further evidence to support this theory comes from other parts of the Milky Way. Astronomers can study parts of our galaxy where stars are currently forming, such as the Orion Nebula, a large volume of gas located in the constellation Orion. Most of the new stars in this nebula are surrounded by disks of gas and dust from which planets are thought to eventually form.
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