Formation and History of the Solar System


4600 million years ago, a dark cloud of interstellar gas and dust started to contract under the influence of its own gravity. This contraction shall have been preceded by a supernova that occurred in a region close to that cloud, which would have been compressed by that explosion. The supernova would have endowed the early nebula with radioactive elements, which later heated and activated the planets of the solar system. It should have also reinforced the cloud contraction process.


A Single Star

Only one star formed in the centre of the cloud (not a binary or multiple system), contrarily to what happens during most of these processes. In a multiple system, the formation of a planetary system would have been made difficult, given the orbital instabilities that it would provoke in the eventual planets that would be formed (unless the stars of the system would be very close or very far from each other).


The Transference of Angular Momentum

A big portion of the angular momentum (the quantity of rotation of the bodies) was transferred from the centre to the outskirts of the cloud (the region of the planets), due to the friction between the inner and outer zones of the nebula or to the "freezing" of the force lines of a strong magnetic field, which would transfer the rotation movement from the central source (the Sun) to those outer regions.


The Formation of the Disk

The solid (dust) and gaseous components of the nebula, initially mixed in a disordered way, were gradually segregated.

Given the rotation of the nebula, this one ended up assuming a flat shape, because this rotation movement favours a centrifugal force (directed towards the exterior) that is perpendicular to the rotation axis (counterbalancing the gravitational force at the equatorial plane), thus not being noticeable along this axis.

It's possible that this process was initially delayed by the gas turbulence. As the gas became scarce (absorbed by the giant planets or expelled from the solar system by the pressure of the intense primordial solar wind) and the bodies grew up, that friction gradually became less prevailing and the materials were finally laid at the equatorial plane of the protoplanetary nebula.


The dust disk of the star Beta Pictoris, similar to what may have been the proto-planetary disk of the Sun.


The First Planetesimals

It's thought that the gaseous protoplanetary disk lasted for only 100 000 years.

In the internal regions, the temperature was so high that only the metals (like the iron and magnesium) and minerals rich in silicates survived and solidified. In the outer regions, the most volatile materials like the water ended up forming solid grains too. According to the most popular model for the formation of the giant planets, it is for that reason that in the outer regions the solid material availability would be much more expressive and, therefore, the planetary accreting process would be faster and more efficient there.

How would that accretion occur? Solid particles with microscopic dimensions would shock against each other, at low speeds, forming grains under the influence of attraction forces (electrostatic, magnetostatic).

When the dust layer overtook the critical density, the gravitational attraction between those particles started to be intense enough for disrupting the disk, which was split into a sequence of thin rings. These ones were subsequently fragmented into a big quantity of sub-condensations (dust clusters contracting under the pressure of their own gravity).

It was through this way that were born the planetesimals (the first population of solid bodies that later was aggregated in planets) were born. It's estimated that only in the internal region there were trillions of those objects travelling around.


In a primordial stage of the planetary formation, little grains adhere to each other in order to build accreted objects like this (D. Richardson, T. Quinn, G. Lake, J. Stadel)


The First Planets

In a time range between less than one million and several hundred million years, some large planets very far away from each other ended up to be formed.

According to one recently revived model that explains the formation of the giant planets, the collapse of the disk material under its own weight (as it happens in the case of the stars) is the responsible factor for the creation of these bodies, which reached the present dimensions some thousand years later. This model approaches the concept of giant planets to the concept of brown dwarfs.

However, the model that still seems to prevail is the one defending that in the region of the giant planets, the availability of volatile (icy) material led to the accumulation of both rocky and icy materials in big bodies, which hold a composition that is moderately similar to the actual comets'.

When a critical value equivalent to some (10 to 15) Earth masses was overtaken, these bodies started to absorb gaseous material too. They started to grow faster and the process would be completed in 10 million to some hundred million years after the genesis of the Solar System. The formation of Jupiter suspended the formation of planets in its adjacent region, which is nowadays called the zone of the asteroids.

Around the giant planets themselves mini-planetary disks were formed, and they generated the satellite systems that presently surround them.


The Future

In about 5 billion years, when the fuel of the nuclear fusion starts to be exhausted in the Sun's core, our star will begin to expand and to be transformed into a red giant. Mercury and Venus will vanish during this phase, being swallowed and vapourized in the interior of the Sun. Later, the Sun will expand even further until it reaches the orbit of Mars.

Finally, the Sun will expel its outer layers, while the core is transformed into a white dwarf, which will dissipate fossil energy until it reaches the condition of a black dwarf. It's probable that in this distant epoch, what are left from the solar system will be just frozen and inactive corpses of the giant planets.



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