Theories about the origin of the Earth

The Earth came into being around 4.54 billion years ago. In general, the theories about the origin of the Earth and Solar System can be divided into two groups: (i) evolutionary theories, and (ii) catastrophic theories. The earlier theories include mainly the evolutionary. The evolutionary theories regarding the origin and evolution of Earth are the Nebular Hypothesis, the Revised Nebular Hypothesis as well as Binary Star Theories of origin of the Earth.

Catastrophic theories include the modern theories of the origin of the Earth. These theories include Big Bang Theory, Star Formation Theory, Planet Formation Theory, etc.

Evolutionary Theories:

The theories which suggest that planets are formed during the evolution of the Sun are called Evolutionary Theories. The Nebular Hypothesis, the revised Nebular Hypothesis, and Binary Stary Theory are examples 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 their collision. However, as the stars are so far apart in the galaxy. The possibility of such a catastrophe is extremely rare. An example of catastrophic theories is the planetesimal and gaseous tidal hypotheses.

The best-known hypotheses for the origin of the Earth and other planets of the solar system are as follows.

Nebular Hypothesis

The Nebular Hypothesis was put forward by Kant, the German philosopher in 1755, and Laplace, the French mathematician in 1796. This hypothesis suggests that the Sun and planets, including the Earth, have formed from a disc-shaped rotating nebula. A nebula is a vast cloud of gases and vapors. The Nebular Hypothesis may be summarized as follows.

1. Originally there was a large, hot, gaseous nebula that 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 zone 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. Above the process was repeated and successive rings of gaseous material were thrown off from the central mass.
5. In the final stage, 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.

Drawbacks of Nebular Hypothesis: The Nebular Hypothesis was not favored by many scientists because of the following defects.

1. 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 angular momentum is concentrated in planets and the remaining 2% is present in the sun. In other words, the sun does not rotate fast enough. It should have a much higher speed of rotation.
2. There was not much 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 the planets. A large passing star approach

   

   hed 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 pre-existing sun.
3. The giant masses of gas broke into a large number of small chunks which on cooling gave rise to solid particles, called “planetesimals”.
4. The planetesimals started flying as cold bodies into orbits around the sun in the plane of the passing star. By collision and gravitational attraction, the larger planetesimals swept up the smaller pieces, and thus planets were formed.

The main flaws in the planetesimal hypothesis are as follows. 

1. 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 gasses would disperse in space rather than condense into planets.
2. Although the angular momentum imparted to the planets by a passing star would be greater than that produced by the rotation of a nebula, the amount is still less than that observed.
3. 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 raised on the surface of the sun. As the star came nearer, the tide increased 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. 

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

Binary Star Hypothesis

This hypothesis was presented by Russel and Lyttleton in 1937.

Before the formation 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 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 the 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 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.
   1. Close to the sun where temperatures were highest, only those materials consensed which had high melting points such as metals and rock-forming compounds. Hence the denser planets grew in the the hot region lying closer to the sun.
   2. 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.

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