Answer: A solar nebula collapsed.
Explanation:
The most accepted theory of formation of solar system is collapse of molecular cloud having dust and gas - A solar nebula.
Nebula is the birth place of a star. A solar nebula- where Sun was born formed as the dust and gases started clumping together due to gravity. The gravitational collapse of the nebula led to the beginning of the fusion reaction which marked the birth of the star- the Sun. The remnant mass formed planets, moons, asteroids and other constituents of the solar system.
