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# Chapter 1: Origin of the Earth The Earth is a member of the planetary system of the sun. Theories explaining the origin of the Earth can be divided into two groups: - **External force theories:** Material is pulled out from the sun by an external force such as a gravitational pull that results f...

# Chapter 1: Origin of the Earth The Earth is a member of the planetary system of the sun. Theories explaining the origin of the Earth can be divided into two groups: - **External force theories:** Material is pulled out from the sun by an external force such as a gravitational pull that results from the dynamic encounter or near-collision of the sun with another star. These theories are also known as catastrophic theories. - **Natural or evolutionary theories:** The planets became isolated masses of matter as the material of the solar system condensed into the sun. ## The Nebular Hypothesis The earliest theory for the origin of the Earth was put forward by Kant and Laplace (in 1796). It is known as the Nebular Hypothesis. The solar system evolved from a single, large, flat, rotating nebula that extended beyond the position of the most distant planet. As the nebula 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 inward gravitational attraction. At this position, a ring of matter was left while the contraction of the remaining matter continued. In this 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.