ORIGINS OF THE SOLAR SYSTEM

The question of the origin of the Solar System attracted the interest of many past investigators, among which Cartesius, Kant and Laplace.
The most recognized theory was, in past centuries, that of the "primordial nebula", that is a huge gas and dust rotating cloud, from which the Sun and the planets would have formed. They would have conserved the same rotation motion of the cloud.
This picture is today considered the most likely one. Other theories have the planets forming after some catastrophic event, such as the collision between the Sun and another star or comet. This would have stripped part of the gas from the Sun, and this stripped material would have been the seed of the planets. However, the chance of such an impact is extremely low, one each 10 billion years. The distances between nearby stars are in fact enormous.

The picture of the formation of the Solar System (and of other planetary systems) is then the following. A large cold cloud of interstellar gas, composed by hydrogen, helium, and a small fraction of heavier elements present as dust, contracts under its own gravitational force. The collapse can be induced by a shock wave (for example, caused by the explosion of a supernova) passing through the cloud, or it can be spontaneous. The contraction lasts several million years, and the cloud starts rotating faster and faster. Its shape becomes that of a disk, whose diameter is about 10 billion kilometers and whose thickness is about 100 million kilometers.
A large amount of gas is stored in the cloud's center, and the gravitational contraction heats it up, from a temperature of about -270^oC  up to about 2000 ^oC. A protostar is born. An accretion disc is formed by the gas rotating around the star, and the gas slowly drops on it, until the time (a few thousand years later) when the stellar wind develops. This is a gas flow from the protostar to the outer regions, which transfers part of its angular momentum to the disk's gas.
In the meantime, the heat and radiation released by the protostar, and the gas flow, vaporize the dust grains of the cloud. The protostar begins its evolution to a star, accreting the gas.

The disk starts cooling through energy radiation. Depending on the amount and distribution of gas, it can be gravitationally stable, or else be unstable and form one or more new protostars. In such a way, a binary or multiple stellar system is formed.

Far from the star, the gas is cold enough so that part of the gas condenses into dust and ice. The dust grains merge due to collisions, until they form small pieces of rock called planetesimals. Planetesimals merge and form protoplanets. The maximum size of protoplanets depend on their distance from the star, and on the chemical composition of the primordial nebula. It is much smaller in the inner regions than in the outer regions, since the protostar tends to disrupt and vaporize dust.

Jovian planets

Terrestrial planets

The difference in size between terrestrial planets and jovian planets supports this scenario. The protoplanet formation requires from about one hundred thousand to some twenty million years.
At this time, the star starts emitting a strong wind which sweeps away the residual gas of the disk. If a protoplanet is massive enough, it can retain part of the gas, so a gaseous planet will form. Otherwise, the gas will be stripped and a terrestrial planet will form. It is then logical that planets closer to the star, being also smaller, belong to the second class.
 

Mercury's surface, showing many craters (Calvin J. Hamilton)

The later evolution of the planetary system is governed by the collisions between its constituent bodies. Impacts of meteorites and planetesimals upon protoplanets and satellites, generate craters on their surfaces, and many of them are still visible. When impacts are are specially violent, they can even move the bodies out of their original orbit. The Solar System went through this phase from 4 to 4.5 billion years ago.

After a few tens of million years, last planetesimals are disrupted by collisions, and the star plus its planets becomes a dynamically stable system. A planetary system is then born.
About 100 million years pass from the initial contraction of the cloud to this event.

Some chemical and geological evidence allow to date the formation of our Solar System to about 4.7 billion years ago.
 

ANIMATIONS

The formation of the Solar System, AVI, 3 Mb (NASA/JPL)

Protoplanetary disc in the Orion nebula, MPEG, 650 Kb (STScI)