Most of the modern theories of the origin of the Solar System are based on the nebular hypothesis proposed by Kant (1755) and then by Laplace (1796). About 4600 million years ago (4600 mya) a slowly rotating interstellar cloud of gas (mostly hydrogen) enriched with the remnants of earlier stars that had died collapsed inwards, its materials being drawn by gravity towards the centre. As it contracted it spun faster in order to preserve its angular momentum (vr).
The cloud finally collapsed along its spin axis and became a flat disc with a bulge at its centre. At this bulge the pressure and heat was eventually sufficient to initiate hydrogen nuclear fusion and the Sun was born.
Accretion is the action of small particles colliding and sticking together to form larger masses, for example, falling snowflakes collide and stick together to form clusters of snowflakes. Accretion was probably the process by which most of the planets and their satellites were formed.
Grains of dust collided and accreted to form pebble-sized bodies that eventually fell into the plane of the nebula. The pebbles then gravitated together to form bodies of up to several hundreds of kilometres in diameter. These planetesimals accreted to make planet-sized masses called protoplanets. It has been calculated that it took roughly 100 million years for the protoplanets to form from the smaller masses.
The Solar System has three broad categories of material: gaseous (hydrogen, helium, neon, argon, etc.), icy (water, methane, ammonia, etc.) and rocky (iron, iron sulphide, silicates, etc.). Each group is distinguished by its solidification temperature: gaseous (0-50K), icy (100-200K) and rocky (500-2000K).
The bodies of the Solar System are made of various combinations of the three groups. The Sun contains mainly gaseous materials with icy and rocky materials as gases. The terrestrial planets are mainly rocky and metallic; Jupiter and Saturn mostly gaseous; Uranus, Neptune, Pluto and the comets are mostly icy.
With the temperature dropping outwards from the Sun, different temperatures allowed different chemical compounds to condense into the grains that eventually made up the proto-planets. If the temperature was too high for a material to condense then it could not end up in a protoplanet.
With the temperature at the nebula’s centre above 2000K, the known composition of the planets gives a condensation sequence related to a certain temperature gradient outwards from the centre: 1400K for Mercury, 900K for Venus, 600K for Earth, 400K for Mars and 200K for Jupiter.
The four planets closest to the Sun were warmed by its solar radiation. Nearly all their lighter gases were carried away by solar winds (streams of charged particles, mostly protons and electrons, escaping from the Sun), leaving these four planets consisting primarily of iron, nickel and silicates (a mixture of silicon and oxygen). Remote from the Sun’s heat, the outer planets formed primarily from frozen lighter gases, hydrogen, helium, ammonia and methane.
Not all the planetesimals ended up in the large bodies. Some of the rocky/metallic ones became asteroids and icy ones the nuclei of comets. Most of them were thrown into the outer system by gravitational influences of large masses, such as Jupiter. The Oort Cloud (0.8 to 3.2 light-years distant) now contains these primordial icy bodies.
Ancient rocky fragments left over from the process still fall to Earth from time to time as meteorites. Radioactive dating of these puts the formation of Earth and the other planetary bodies as essentially complete about 4600 mya.
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