The most accepted theory so far, in relation to the formation of our solar system is that it was originated by the contraction of a cloud of interstellar gas due to its own gravity.
To understand it better:
According to this theory, a cloud of gas and interstellar dust began to shrink due to its own gravity. This led to the increase in the temperature of the system, which began to rotate forming a large gas sphere in the center (the Sun) with a flat disk around it.
This is how the Sun formed from the sphere in the center and the planets formed from the disk orbiting in the same plane.
