Although the Sun and the planets of the solar system differ greatly from each other, they are believed to have all been formed at the same time. For a time it was thought that planets originated from other stars and were caught by the gravitational pull of the Sun.
It seems that the Sun was formed out of a compressed primordial_nebula as previously described. This occurred about 4.5 billion years ago. Other masses of gas and dust existed further out from the center of the dense cloud. These contracted into solid bodies. At first only small lumps were formed and these often collided, sometimes fusing together and other times breaking up into smaller pieces. They all orbited the central point, the Sun. This process continued for many millions of years until finally solid bodies moved along specific orbits. The initially glowing hot objects were eventually longer hit by other objects. They reached their current size and cooled down.
Mercury was formed close to the Sun. It was and still is so hot there that iron vapor and dust could easily be compressed to great density, which could explain the thick iron core of this planet. Silicates on the other hand, do not easily form at such temperatures, which could explain the relatively thin silicate crust. All the following inner planets Venus, Earth and Mars are spheres with a metal core and a crust of rock.
The outer planets, which are larger, probably continued to exist in the form of clouds for a longer time. Their mass was probably not sufficient to cause the ignition of core fusion as it occurred in the Sun. They also contracted into spheres and began to orbit the Sun. The nine planets of the solar system had now come into being.
