The origin of the solar system

The most accredited theory about the formation of the solar system is the Theory of Planetesimal Accretion by Safronov. In this theory, the difference between the rocky inner planets and the gaseous external jovian planets is explained, as well as the formation of the main belt asteroids. The existence of comets and their main role in the origin of life on Earth is another very interesting topic of the early phases of the solar system's formation.

The first steps: from the nebula to planets

Almost 5 billion years ago, the actual solar system was only a cloud made of gas (mostly hydrogen and helium) and very diffuse dust grains (carbon and silicate). This cloud, called primordial nebula, had a very low density and temperature, and on the all, it had a mass that is today estimated being 1.1 masses of the actual Sun.
In these conditions, a gravitational collapse occurred, making the temperature and density of the central part of the nebula augment. In other words, the nucleus of the cloud started to collapse gravitationally under its own attraction, until it reached a high enough temperature for nuclear reaction to start. At this point hydrogen started burning, and the protostar was born. At the end of this phase it is believed that 90 % of the global mass of the nebula had already been captured by the protostar.

But what about the other 10% of material (gas and dust)? Around the protostar the material that has not been incorporated in the star rotates around it, forming a disk. During this phase, that is called disk phase and that can last up to 100 millions years, the grains of dust grow in size very rapidly (this phenomenon being called accretion). After a relatively short period (100000 years) the grains will have grown into objects of some Km of diameters called planetesimals.
These planetesimal have a composition that depends on the region where they have formed: if in the inner parts the temperatures are higher and gas
sublimate, leaving rocky planetesimals, in the outer parts of the disk we can instead find ices.
At this point of the evolution, the star is embedded in a disk shape structure made of several rings of planetesimals of different composition. The accretion of planets is now possible. This accretion is due to the impacts between planetesimals that can glue together, forming growing objects with a composition that depends on the formation's place and that is still respected by the actual structure of the solar system (where, in the inner parts, wet find rocky planets, while in the outer parts, planets are gaseous).
Asteroids and comets are leftover planetesimals that have not been incorporated into a planet during this period.

The planets of the solar system and their different obliquity

The period of heavy bombardment

At this point of the evolution, a period of heavy bombardment is thought to have taken place (about from 4 to 3.5 millions years ago). At this phase, planets were already formed and planetesimals had time to grow into very big objets (the size of the Moon) that can have very violent impacts with the planets. Most of the craters today seen on the Moon and on satellites without atmosphere are due to that period.
Under this heavy bombardment, life wasn't of course possible: on the early Earth, oceans vaporized and the fragile carbon-based molecules, upon which life is based, could not survive. In other words, the influx of interplanetary debris was so strong that the proto-Earth was far too hot for life to have formed.

The dark, buttered side of the Moon.

The objects of the main belt
The asteroids today inhabiting the main belt, can be seen as the remnants of a never-born planet. In the zone of the actual main belt, as well as in the totality of the disk, there was a big number of growing planetesimals. To make two bodies stick during a collision, their relative speed must necessarily be smaller than 100 m/s otherwise the two bodies will undergo a collisional fragmentation giving birth to smaller bodies. Now, in the zone of the actual Main Belt, at the time of the heavy bombardment period, Mars was accreting at about 1.5 A.U., while at 5.2 A.U we could find Jupiter, with a mass much bigger than Mars (and of the planetesimals). The presence of Jupiter deviated a big number of planetesimals sending them in the inner zones of the solar system, making the concentration in the zone change, and the speeds of the bodies in the actual main belt zone become greater. For this reason the collisions became destructive and only the bigger embryos continued accreting (becoming the bigger asteroids).

The solar system today

Life on Earth
Life on Earth began at the end of the period of heavy bombardment, 3.8 billion years ago. The earliest known fossils on Earth date from 3.5 billion years ago and there is evidence that biological activity took place even earlier, just at the end of the period of late heavy bombardment. So the window when life could have begun is very short: just as soon as life could have formed on our planet, it did.
A very interesting question about life formation is about the presence of water on Earth: if at the time of life formation there was little water and carbon-based molecules, how were these building blocks of life delivered to the Earth's surface so quickly? The answer may involve the collision of comets with the Earth, since comets contain abundant supplies of both water and carbon-based molecules.

What about comets?
Comets are the leftover building blocks of the outer solar system formation process, and offer clues to the chemical mixture from which the giant planets formed some 4.6 billion years ago.
The Kuiper Belt also  holds significance for the study of the planetary system on at least two levels. First, it is likely that the Kuiper Belt objects are extremely primitive remnants from the early accretional phases of the solar system. In fact, according to this theory, the inner, dense parts of the pre-planetary disk condensed into the major planets much faster than the outer parts could have done(probably within a few millions to tens of millions of years). In the outer parts of the disk - and so, in the Kuiper Belt- accretion progressed much more slowly and only very small objects were formed.
Curiously, it seems that the Oort Cloud objects were formed closer to the Sun than the Kuiper Belt objects. In fact, the small objects formed near the giant planets, would have been ejected from the solar system by gravitational encounters. Those that didn't escape entirely, formed the distant Oort Cloud. Small objects formed farther out, had no such interactions and remained as the Kuiper Belt objects.

Images of this page: courtesy of NASA