Origin of earth

These are some of theories given explaining the origin of Earth.

Nebular Hypothesis: It was the first theory which tried to explain the origin of Earth and Solar System, after the works of Newton. It was first proposed in 1734 by Emanuel Swedenborg, and then further developed by the German philosopher, Immanuel Kant. He argued that gaseous clouds—nebulae, which slowly rotate, gradually collapse and flatten due to gravity and eventually form stars and planets. He further suggested the possibility that other nebulae might also be similarly large and distant disks of stars. For the first time the scope of astronomy was extended beyond the solar system to galactic and extragalactic realms. A similar model was proposed in 1796 by Pierre-Simon Laplace. The Laplacian nebular model dominated in the 19th century, it encountered a number of difficulties. The main problem was angular momentum distribution between the Sun and planets. The planets have 99% of the momentum, and this fact could not be explained by the nebular model. As a result this theory of planet formation was largely abandoned at the beginning of the 20th century.

Planetesimal Theory: It was proposed by Thomas Chamberlin and Forest Moulton in 1901. As per this theory, the Sun and the planets are thought to have formed from a rotating dust cloud formed from the remnants of a supernova explosion. On condensation, this cloud formed a central star and a rotating disc, the material of which separated into rings of dust grains that began to stick together. Larger and larger clumps formed in each ring and eventually collected into bodies the size of present-day asteroids, or planetesimals. After numerous collisions, these bodies eventually formed the nuclei of the various planets of our Solar System. It ultimately was discarded in the 1940s by the realization it was incompatible with the angular momentum of Jupiter.

Tidal Model: It was proposed by James Jeans in 1917. This explained the origin of the solar system as a result of a close encounter between the Sun and a second star. Jeans concluded in 1916 that the tidal interaction between the Sun and a passing star would raise tides on the Sun resulting in the loss of a single cigar-shaped filament of hot gas, rather than separate streams of gas as in the Chamberlin and Moulton scenario. This hot gas would then condense directly into the planets instead of going through a planetesimal stage. The central section of the "cigar" would give rise to the largest planets – Jupiter and Saturn – while the tapering ends would provide the substance for the smaller worlds. It was not able to explain the angular momentum of the Sun.

Binary or three star theories: It was given by Russel Henry. This was slight modification of the passing star and tidal model. As per this theory, the Sun was orginally a part of a binary system and the second star of this system then underwent a very close encounter with a third star. This encounter ejected a gaseous filament in which the planets formed.

Big Bang: It is widely accepted theory right now. It is based on the principle of ever expanding universe. As per the theory, in the beginning the universe existed in form of a 'cosmic egg' of very very small volume, infinite mass and infinite density. Then the explosion of egg happened and the universe started expanding. It predicts the age of universe as 13.7 billion years, the expansion is still going on. The expansion here refers to expansion of space between the galaxies.