Kant in 1755 and Laplace in 1796 each proposed the nebular theory about the origin of the solar system. It is the earliest scientific theory of celestial evolution. The basic arguments of these two nebula theories are similar. They believe that all celestial bodies in the solar system have a history of formation and are gradually evolved from the same primitive nebula according to objective laws - the law of universal gravitation. Kant believed that this primitive nebula was composed of solid particles of different sizes. "Celestial bodies began to form where the attraction was strongest." Gravity brought the particles closer to each other, and the larger particles attracted the smaller particles to form larger clumps. And the clump is getting bigger and bigger, and the central part with the strongest gravity attracts the most material, forming the sun first. When the outer particles fall toward the central body under the attraction of the sun, they collide with other particles and change direction, becoming a circular motion around the sun. These particles moving around the sun gradually form several gravitational centers. These gravitational centers finally condense into the same direction. The direction of rotation of the planet. The process of moon formation is similar to that of planets. Comets are formed on the periphery of the original nebula. The sun's gravitational pull on them is weak, so the inclination angles of comet orbits are varied. The rotation of a planet is caused by the impact of particles falling on the planet. Kant also used the size of the planetary zone to explain the mass distribution of the planets (at that time, people only knew six major planets: Mercury, Venus, Earth, Mars, Jupiter, and Saturn, ten satellites, and about thirty comets).