Formation and evolution of the Solar System

The planets of the solar system is believed to have formed out of a collapsed and spinning cloud of gas and dust. The outer parts of this cloud became a disk surrounding the young Sun as it condensed and ignited. From this cloud and its gas and dust, the various planets formed. The inner solar system got swept clear of the gas early on, and so what formed there were rocky planets. Farther out, the gravitational effects of Jupiter made it impossible for the masses to come together, leaving behind the asteroid belt. In the outer solar system, Jupiter and Saturn developed as large gas giants, while farther out Uranus and Neptune are known are ice giants because their cores are believed to be made of ice.

The planets were originally believed to have form in place and as we see them now. However, this view has undergone radical change in the late 20th century and the beginning of the 21st century. Currently, it is believed that the solar system looked very different after its initial formation, with five objects at least as massive as Mercury being present in the inner solar system (instead of the current four), the outer solar system being much more compact than it is now, and the Kuiper belt starting much farther in than it does now.

The inner solar system was "completed" by a collision between the young Earth and a Mars-sized object (being the "fifth" inner solar system object alluded to above) which resulted in the fomation of the Moon.

The outer solar system is now believed to have been shaped by planetary migrations. As the gravity of the planets perturbed the orbits of the Kuiper belt objects, many were scattered inwards by Saturn, Uranus, and Neptune, while Jupiter often kicked those objects out of the solar system altogether. As a result, Jupiter migrated inwards while Saturn, Uranus, and Neptune migrated outwards. A major breakthrough in the understanding of how this lead to the current solar system structure occurred in 2004. If turns out this if Jupiter started taking fewer than two orbits around the Sun for every time that Saturn orbited the Sun once, this migration pattern would put Jupiter and Saturn into a 2:1 resonance when the orbital period of Jupiter became exactly twice that of Saturn's. This resonance would have put Uranus and Neptume into highly elliptical orbits, with there being a 50% chance that they would have exchanged places. As their orbits got more elliptical, they would have plowed into the your Kuiper belt. The interaction between the planets and the Kuiper belt after Jupiter and Saturn passed through the 2:1 resonance explains the orbital charactertistics and axial tilts of the giant outer planets. The resultant scattering of Kuiper belt objects also would explain the late bombardment phase of the history of the solar system.