ORIGIN OF THE EARTH

In general the theories of origin of the solar system can he divided into two groups :

1.  Evolutionary theories
2.  Catastrophic  theories   

    

Evolutionary Theories

The theories which suggest that planets are formed during the evolution of the sun, are called "evolutionary theories". Nebular hypothesis is an example of these theories. 

Catastrophic Theories

 "Catastrophic theories" are those which imagine that planets are formed by some special accident or catastrophe, such as the close approach of two stars or by collision of two stars.

 However, as the stars are so far apart in the galaxy, the possibility of such a catastrophe is extremely rare. The examples of catastrophic theories arc the planetesimal and gaseous tidal hypothesies.

The best known hypothesis es  for the origin of the earth and other planets of the solar system are as follows.

1.  Nebular hypothesis
2.  Planetesimal hypothesis
3.  Gaseous tidal hypothesis
4.  Binary star hypothesis
5.  Gas dust cloud hypothesis

Nebular Hypothesis

 put forward by Kant, the German The nebular hypothesis ‘‘ as philosopher in 1755 and Laplace, the French mathematician in 17%. This hypothesis suggests that the sun and planets, including the earth have formed from a disc-shaped rotating nebula. A vast cloud of hot gas is called nebula". The nebular hypothesis may be summarized as follow.

 

1. Originally there was a large, hot, gaseous nebula which rotated along its axis.
2.  As the gas lost energy by radiation, it became cooler. As a result the nebula contracted inward and its speed of rotation about its, axis increased to conserve angular momentum. Due to this the centrifugal force in the equatorial tone also increased thereby causing the nebula to bulge out in the equatorial zone.
3.  The cooling and contraction of the nebula continued and ultimately a stage came when the centrifugal force became greater than the gravitational attraction acting inward. As a result a gaseous ring was separated out.
4.  The above process was repeated and successive rings of gaseous material were thrown off from the central mass.
5.  In the final stages the rings condensed into planets. Planetoids were formed when one such ring broke into many small fragments.
6.  The central mass of the nebula continued to shrink and finally formed the sun. 

The nebular hypothesis was not favored because it had the following defects. 

• This hypothesis could not explain the energy distribution within the solar system. The sun which possesses most of the mass (about 99.9%) of the solar system, should have gathered maximum angular momentum. However, 98% of the angular momentum is concentrated in the planets and the remaining 2% is present in the sun. In other words, the sun does not rotate fast enough. It should have much higher speed of rotation.
• There was not enough mass in the rings to provide the gravitational attraction for condensation into individual planets.

Planetesimal Hypothesis

  The planetesimal hypothesis was proposed by 

Chamberlin and Moulton in 1904. The main points of this hypothesis are as follows. 

1. The sun existed before the formation of planets. A large passing star approached very close to the sun.
2.  Due to the disruptive forces of the sun and the strong gravitational pull of the passing star, giant masses of gas were torn from the surface of the preexisting sun.
3. The giant masses of gas broke into a large number of small chunks which on cooling gave rise to solid particles, called "planetesimal".
4.  The planetesimal started flying as cold bodies into orbits around the sun in the plane of the passing star. By collision and gravitational attraction, the larger planetesimal swept up the smaller pieces. and thus planets were formed.

 The main flaws in the planetesimal hypothesis are as follows. 

•  Most of the material which was ejected by the explosive action of the sun would come from the interior. It would be so hot that the gases would disperse in the space rather than condense into planets. 
• Although the angular momentum imparted to the planets by a passing star would be greater than that produced the rotation of a nebula, the amount is still less than that observed.
• The space is vast and therefore the probability of a close approach\ of two stars is extremely unlikely.

  Gaseous tidal hypothesis

 This hypothesis was proposed by Jeans and Jeffreys in 1925. The gaseous tidal hypothesis may be summarized as follows. 

1.  A very large star progressively approached close to the sun. Due to the gravitational pull of the star, a gaseous tide was raised on the surface of the sun. As  the star came nearer tide increased in size.  move away, the gaseous tide was detached in size. -. 
2. When the star began to move away, the gaseous tide was detached from the sun. Its shape was like a spindle being thickest in the middle
3. This spindle-shaped gaseous mass soon broke into ten pieces, nine of which condensed into planets and the remaining one which further broke into small pieces, formed the group of planetoids.

 The main objections to the gaseous tidal hypothesis are as follows.

• The passing star is unable to impart the proper angular rnomentwn to the detached gaseous masses.
• The hot gaseous mass pulled away from the sun would not form solid planets but would dissipate into the space.

 

Binary Star Hypothesis

This hypothesis was proposed by Lyttleton in 1938. Before the forma-tion of planets, the sun had a companion star. Another star approached close to these double stars and dragged the companion star away. A gaseous filament was torn from the companion star and it remained close to the sun. The planets were originated from this gaseous filament in the same way as described in the gaseous tidal hypothesis. 

 Recent Theories

Since 1943 there has been a tendency to swing back to theories of Laplacian type. These theories seem to explain well the observed variations in the chemical compositions and densities of the planets. The objection to Laplacian theory, that the sun's angular momentum is too small, has now been removed. The main points of the recent theories are as follows.

1. There was a disc-shaped cloud of gas and dust around the sun.
2.  The planets were formed by gradual aggregation of the dispersed matter in the cloud.

•  Close to the sun where temperatures were highest, only those materials condensed which had high melting points such as metals and rock-forming compounds. Hence the denser planets grew in the hot region lying closer to the sun.
• Volatile materials such as water, methane and ammonia were blown away. They condensed in the cold outer zone of the solar system thereby forming low density planets.

       3. The primitive sun had a considerable magnetic field. It acted as a rotating magnet and                           accelerated the hydrogen ions present in the dust cloud. Due to this acceleration the gases                     moved outwards away from the sun carrying the fine dust with them and leaving only larger               solid masses in the region of inner planets. Thus there was the transfer of momentum from the             sun to the gas ions which slowed down the sun's rotation. This process also explains how the               dust cloud was divided into two regions, an inner gas-free region of solid particles and an                     outer region rich in gases.