Introduction
The Earth is a member of the planetary system of the sun. The principal theories which have been advanced to explain the origin of the earth, can be divided into two groups. All these theories have in common the idea that the planets evolved from the son. They differ as to the manner in which it occurred.
1. In one group, material pulled out of the sun by an external force such as gravitational pull resulting from the dynamic encounter or near-collision of the sun with another star. These are also known as catastrophic theories.
2 The second group holds that the planets became isolated masses of matter as the material of the solar-system condensed into the sun. These are also known as natural or evolutionary theories.
The earlist theory for the origin of the earth was put forward by Kant and Laplace in 1796. It is known as Nebular Hypothesis.
(a) Nebular hypothesis
According to this hypothesis, the solar system evolved from a single, large, flat, rotating nebula that extendedd beyond the position of the most distant planet. As this contracted the mass became increasingly concentrated towards the centre and in doing so it would have rotated more rapidly to conserve angular momentum, until (at some period during the contraction) the speed of the outermost rim of the disc would have become sufficient for the centrifugal force to be as great as the in ward gravitational attraction. At this position a ring of matter was left while the contraction of the remaining matter continued. manner successive rings of matter were left behind the contracting mass. Subsequently the material within each ring was drawn together and planets and satellites were formed.This theory was rejected when it was learnt that the angular momentum of the solar system is concentrated in the planets and not in the sun. This is also not compatible with the idea that the mass of matter rotated more rapidly as it condensed.
In 1944, a German physicist, C.F. Von Weizsacker proposed a modification of the nebular hypothesis.
(b) Proto planet hypothesis of Kuiper
It is the most popular hypothesis within the past few years. It visualises a slightly flattened and slowly rotating disc-shaped solar nebula bulging out from the equator of the sun. In composition this could is similar to the sun and contains mostly hydrogen and helium with small amounts of heavier elements, but the disc and the sun itself are thought of as being cool. The disc containing about one tenth the mass of the sun finally becomes internally unstable and breaks up into smaller concentrations called protoplanets. Within each protoplanet, the heavier elements tend to settle towards the centre and the lighter particles and gases remain in outer shells.(c) Bi-parental origin of the earth.
This theory belongs to the group of dynamic-encounter theories and put forward by Chamberlin and Moulton, as anotheer star approached the sun, tremendous tides were set up on the surface of the sun and thesee tides or filaments of hot gaseswere pulled out from the sun. As the star passed, these arms of gas were given a rotational motion. After the star was gone, the gaseous matter in these arms condensed into solid material and gradually drew together to form planets.Similar hypothcsis indicating biparental origin of the earth were propounded by Jeans and Jeffry and also by Lytileton. But these theories gradually lost its wide acceptance because of the absence of relevant evidences to support it.
Now-a-days, the Big-Bang theory as is being propounded by the American astronomers, that the universe, solar system etc. are the result of an explosion within the nebula, is getting better acceptance