The solar system includes the sun and its nine planets. We shall now describe how the solar system was initially formed. That is, we shall now discuss how the sun and the planets were formed.
1. How Sun was Formed :
The sun was formed about 5000 million years ago from the dense clouds of mainly hydrogen gas, some helium and dust. The over dense and cold cloud of hydrogen gas shrank (collapsed) under its own gravitational force to form a huge, dark ball of hydrogen called protostar. The collisions of hydrogen molecules in the protostar raised its temperature greatly which started the nuclear fusion reactions.
The nuclear fusion reactions in which hydrogen was converted into helium, produced a tremendous amount of energy due to which the protostar began emitting heat and light, and became the sun (which is a star). The sun is now a middle aged star. The sun would become a red-giant star and die out as white dwarf star after about 5000 million years from now.
2. How Planets were Formed :
When the sun was formed about 5000 million years ago, then the left-over gases (mainly hydrogen and some helium) formed a disc shaped cloud around the sun. These left-over gases were very cold and condensed (shrank) to form small chunks of matter called planetesimals. These planetesimals started rearranging themselves by constant collisions among themselves, breaking up and then reassembling to form bigger objects. These bigger objects started attracting smaller objects by their gravitational force to form various planets.
In this way, the small chunks of matter called planetesimals were gradually converted into planets. During the formation of planets, many large sized objects were hurled at random in all directions. Some of these objects also collided with the newly formed planets and satellites. For example, the craters seen on the surface of the earth’s satellite ‘moon’, are due to the impact of the objects which hit its surface during the evolution of planets. The craters present on the surface of moon is a concrete evidence of the initial disorder in the solar system.