What Is the Modern Theory of Origin of Earth ?

The origin of the earth not deals with the creation of the matter of which the planets are composed, rather the very appearance of this cosmic matter as a single whole body of conceivable state and form having individual existence. The birth of the planet earth must have been due to some unique phenomenon which occurred in the universe in the remote past in which the earth saw the first glimpse of its existence. Because of their similarities in motions and composition, the earth, along with its co-planets and satellites, forms a part and parcel of the solar system, and as such all of them including the Sun must have been formed simultaneously.

Thus, the origin of the earth is intimately related to that of the solar system and the birth of the sun and its member planets, satellites etc. The Earth in particular must have been born due to some unique celestial phenomenon that occurred in very remote past, about 4,500 million years ago.

Subsequently many hypotheses were put forward from time to time. Few of them appeared easily convincing and were taken as most satisfactory for some years but are now regarded as only historically important. An acceptable theory must explain the fundamental regularities of the solar system. The most imperative fundamental regularities are:

(i) Orbital regularities: It has been observed that orbits of all the planets are almost circular, lie in one plane and the revolution is in one and in same direction.

(ii) Distance regularities: The various members of the solar system are distributed at varying distances from the sun is a proved fact.

(iii) Differentiation of planets: Interior of all the planets can be divided into inner and outer with characteristic difference in density

iv) Distribution of angular momentum: Angular momentum (M) is defined as the mass of the body (m) multiplied by its velocity (v) and by the radius of the orbit (R). A body rotating around a central object must have angular momentum. In case of solar system the distribution of angular momentum is such that the sun accounts for less than 2 percent of momentum in spite of the fact that it possesses more than 99 percent of the mass of the solar

Many hypotheses have been put forth relating to the origin of the earth.

There seems to be two basic premises on which the hypotheses are based. (i) A hot ball of fire origin attesting the planet’s birth in a hot state (Hot ball of fire stage).

(ii) A cold origin speaking of the birth of our planet from a cold and dust cloud state.

In another way, the hypotheses advanced so far, may be put into two major groups such as condensation hypotheses and fragmentation hypotheses. The two groups postulate diametrically opposite mode of origin of our planet and hence of the solar system. Both the groups of hypotheses have merits and demerits.

It is beyond the scope to present here an account of all the hypotheses. Only some important hypotheses which enjoy scientific acceptance and popularity during some period or the other are discussed below. It may be mentioned here that more than one hypothesis put forth by individual scientists carry the same or nearly same idea in some modified ways and as such the related hypotheses are treated in a combined form in the following description. Some of them belong to “Condensation (evolutionary hypotheses)” school and the others to

 Condensation/ Evolutionary Hypotheses: Hypotheses under this group postulate the birth of the planets through the process of condensation of diffused gas and particles. In other words, condensation hypotheses speak of the formation of the planets and the solar system by gathering together or coagulation of diffused matter scattered in space. The earliest theory for the origin of earth was put forwarded by Kant and Laplace.

(i) Nebular Hypotheses: These were propounded by Kant (1755), Laplace (1796) and Helmond later on. Kant’s original idea was largely modified by Laplace. The solar system originated through condensation of matter from a rotating, hot and gaseous body of diffused nature called the nebula. The nebula cooled down through radiation of heat energy. As it cooled down, it contracted. More and more contraction caused increase of the speed of rotation of the nebula. As the speed had become very high in course of time, bulging of the equatorial zone of the nebula took place under increased centrifugal forces. It then flattened into a disk. As a result of this, a gaseous ring of diffused matter was separated and then thrown off into the space. This thrown-off-ring gradually condensed to form a sphere which revolved round the nebula and also rotated around its own axis. Thus, the planet was formed (Fig. 1.4). In succession, more rings were thrown off

to form other planets. The satellites formed around the planets before their solidification in the same way as the planets evolved. The remainder of the nebula formed the sun. This mode of origin of the earth was very popular during 18th and 19th centuries but thereafter the idea got gradually weakened. Kant and Laplace only differed in the mode and time of separation of the rings. While Kant advocated formation of nine separate rings, from the nebula, Laplace advocated for one primary ring first from which other rings were thrown in rotating secondary rings to form planets etc.

(ii) Meteoric Cloud Hypotheses: A meteoric cloud origin of the earth and the solar system was advocated by Darwin and Lockyer (mid 20th century), who thought of the existence of cloudy matter in form of bodies varying in size. These were called meteoroids which constantly collided with each other to unite together under gravitational pull. This process continued to form lager unions, which became the centres of condensations to form planets and satellites.

According to Otto Schmidt (1943) in his meteoric hypothesis (also known as Interstellar Dust hypothesis) all planets including the earth were derived from some foggy matter occurring in the central part of our galaxy. As the sun-a star moved very close to this centre of the galaxy, it attracted the foggy matter which was detached I thrown off in form of particles called meteoroids, which revolved round the sun. The meteoroids ultimately got together (united) to form bigger bodies which gave rise to planets and satellites. He postulated a cold origin of earth.

(iii) Dust Cloud Hypotheses: Propounded by Weizacker in 1944, Kuiper in 1951 and Urey in mid 20th century the ideas were laid down under the general head of Dust-cloud hypotheses. The solar system coagulated from a vast cloud of dust forming the inter-stellar matter. This cloud consisted of dust and diffused gas from

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