ORIGIN OF THE EARTH

The earth is believed to have been formed from a small part of the sun. Most of the theories concerned with the origin of the earth emphasise that the planet originated as a hot gaseous mass which on cooling, turned, first into a liquid and than into a solid. One of the earliest theories was put forth by Kant, which is popularly known as the gaseous hypothesis.

A more popular theory was advanced by Laplace which is called the nebular hypothesis. This theory considers earth as having been formed through the solidification of a ring thrown away by a cooling and rotating nebula (sun). This ring was one of the nine such rings which formed various planets. A more plausible theory put forth by Jeans and Jeffreys assumed the presence of two nebulae instead of one as assumed ,by Laplace. The theory advanced by Jeans and Jeffreys is called the tidal hypothesis and it belongs to the group of the binary star theories (the theories explaining the origin of the earth and the other planets on the basis of the presence of two nebulae). According to this theory a large nebula wandering in the space came very close to another smaller nebula (sun) and its gravitational pull caused a huge tidal upsurge of matter on the surface of the smaller nebula.

As the larger nebula moved away from the smaller one, the matter rising as a tidal wave from the surface of the smaller nebula was pulled towards it and was drawn to a distance from which it could not come back to the parent body. However, it could not follow the larger nebula also and as the larger nebula moved away, the rising tongue of matter was detached from the smaller nebula. On cooling, this matter condensed to form the planets, including the earth, and they started revolving around the sun. This hypothesis is considered to be highly probable and close to reality. The cigar-shaped arrangement of the planets going away from the sun, with the smallest planets located closest and farthest from the sun and the larger ones occupying intermediate positions, strengthens, this view.

MODERN THEORIES :-

However, scientists in later period took up the problems of origin of universe rather than that of just the earth or the planets. The most popular argument regarding the origin of the universe is the Big Bang Theory. It is also called expanding universe hypothesis

The Big Bang Theory :-

The Big Bang Theory consider the following stages in the development of the
Universe:

All matter forming the universe existed in one place in the form of a tiny ball (singular atom). At the Big Bang, the tiny ball exploded violently. This led to a huge expansion, which continues even today. Within the first three minutes from the Big Bang event, the first atom began to form. Within 300,000 years matter. The Universe became transparent.

With greater evidence becoming available about the expanding Universe, the scientific community at present favours the expanding Universe hypothesis.