The nebular theory is the most widely accepted model explaining the formation and evolution of the Solar System.
According to nebular theory, stars form in massive and dense clouds of molecular hydrogen-giant molecular clouds. They are gravitationally unstable, and matter coalesces to smaller denser clumps within, which then proceed to collapse and form stars. Star formation is a complex process, which always produces a gaseous protoplanetary disk around the young star. This may give birth to planets in certain circumstances, which are not well known. Thus the formation of planetary systems is thought to be a natural result of star formation. A sun-like star usually takes around 100 million years to form.
There is also a theory called Big Bang. According to it, the whole world was formed after the powerful explosion (so it called Big Bang). The explosion has occurred for roughly 20 billion in the past and create a giant ball of fire then it crushed into pieces, and it turned into sun, moon, planet, stars and more…
A widely accepted theory of planet formation, the so-called planetesimal hypothesis, that planets form out of dust grains that collide and stick to form larger and larger bodies
Condensation Theory proposes that the Moon and the Earth condensed individually from the nebula that formed the solar system, with the Moon formed in orbit around the Earth. However, if the Moon formed in the vicinity of the Earth it should have nearly the same composition. Specifically, it should possess a significant iron core, and it does not. Also, this hypothesis does not have a natural explanation for the extra baking the lunar material has received.
Georges Buffon, a naturalist, advocated the Dynamic Encounter Theory. According to this theory, the world was formed out of the molten materials from the Sun when it collided with a comet.
Category: Earth Science