Story of our planet EARTH
About 4.5 million years ago in the history of Earth, a very old star exploded in a supernova. This created heavy elements that were spread around the blast zone, as well as cohesive little groups of atoms called molecules. In the beginning the debris only existed as a nebula, a huge cloud of gas and dust. Over time this cloud started clumping together and a new solar system formed, due to gravity and the random movement of particles. In the middle of this cloud a new star started coming together. Elsewhere in the nebula other clumps of matter came together to form planets in orbit around the new star. So began our own solar system.
The four gas giants, Jupiter, Saturn, Uranus, and Neptune formed when lighter elements in the outer nebula came together. While this happened, there were heavier elements on which the sun's gravitational pull has a stronger effect, which made these elements remain in the inner solar system. These came together to form the four concrete planets; Mercury, Venus, Earth, and Mars.
Concrete planets are formed through collision under normal circumstances. If two objects meet in a head on collision, they would destruct, but in most cases collisions in space are not head on. Often the two objects would just glance each other, while in other cases one would overtake the other moving in the same direction. When this happens the objects often come together to form a larger object more capable of not surviving in a future collision.