MAJOR HYPOTHESES ON THE ORIGIN OF THE EARTH
The origin of the earth and the solar system has aroused curiosity since ancient times. A number of hypotheses have been put forth from time to time to explain this. Many of these hypotheses are based on imagination and speculation while the others are based on observation. None of these theories is considered wholly satisfactory or widely accepted. Most of these hypotheses agree that all the planets of the solar system seem to have originated broadly at the same time and in a similar manner. It is indicated by the fact that all planets revolve around the sun in the same direction and all of them also rotate on their axes. But there are still many questions to which no satisfactory answer is available. For one, the sun is composed mainly of hydrogen and helium but the core and the mantle of the earth are composed mainly of ferrous and silicate minerals and lack in hydrogen. Also gases like methane, ammonia and hydrogen predominate in the bigger planets while the smaller planets are all deficient in these. This fact suggests a number of possibilities like the planets having been formed by the separation of mineral fragments from the sun. It is also believed that the earth and the smaller planets were once in a molten state. A number of widely accepted conclusions have been arrived at regarding the origin of the earth and the solar system on the basis of various studies and scientific evidence. It is now believed that the stars came into existence about seven billion years ago and the earth perhaps came into existence some 4.5 to five billion years ago. The scientific hypotheses trying to explain the origin of the earth and the solar system can be broadly grouped into two categories:-
• the monistic or the parent hypotheses
• dualistic or the bi-parental hypotheses
The former group of theories is based onthe assumption that the solar system has originated from a single star or nebula. The process of origin according to these theories is called the evolutionary process of origin of the solar system. The theories put forth by Kant, Laplace, Hoyle, Lockyer and Weizsacker are examples of such hypotheses. The hypotheses constituting the second category explain the origin of the earth and the solar system as a result of the coming together of two stars or nebulae. Theories put forth by Chamberlain and Moulten, Jeans and Jeffreys and Russel are examples of such hypotheses. A brief outline of the major hypotheses explaining the origin of the earth is provided here.
(i) Gaseous Hypothesis of Kant
Kant was a German scholar and he put forth his hypothesis in 1755. This hypothesis was based on Newton's law of gravitation. According to this view, the earth has originated from hard, cold and motionless particles of primordial matter scattered in the space. These particles start colliding and falling on one another due to the gravitational attraction. In due course of time the original cold matter was converted into a hot and rotating nebula. The speed of rotation of the nebula kept increasing with its increasing temperature and a bulge started developing in its middle part due to the centrifugal force. As the centrifugal force became still stronger, the central bulge separated from the nebula in the form of a ring that changed into a planet in course of time. A series of nine rings came out of the nebula forming nine planets and the remaining part of the original nebula formed the sun. According to this theory the earth has been formed through aggregation and solidification of the materials separated from the nebula in the form of a ring due to a strong centrifugal force. Circular rings came out of the planets in the same manner and formed satellites. Criticism Kant's hypothesis suffers from a number of shortcomings. This hypothesis has been found to be against mathematical laws. The theory assumes that the mutual collision of particles of the primordial matter resulted in the generation of rotational movement. This assumption is against the principle of conservation of angular movement. The assumption of increasing rotational velocity of the nebula with its increasing size is also against the laws of mathematics. Normally the speed of rotation should decrease with an increase in the size of the nebula. However, in spite of the basic assumptions of this hypothesis being untenable, this hypothesis has been important being the first hypothesis based on the law of gravitation, and it paved the way for the famous nebular hypothesis of Laplace.
(ii) Nebular Hypothesis of Laplace
Laplace, the famous French scholar, presented his nebular hypothesis in 1789. Views of Laplace about the origin of the earth have been influenced by two major facts: (i) existence of nebula in the universe, and (ii) presence of a ring around the Saturn. He starts his theory with the existence of a hot and rotating nebula in the space. This nebula was very extensive in size. Due to a very high temperature of the nebula explosions were taking place in the nebula and the matter thrown away from the nebula formed the atmosphere around it. Formation of atmosphere resulted in increased heat loss from the nebula that resulted in reduction of its size. Contraction and shrinkage of the nebula caused an increase in its velocity of rotation. Increased velocity led to an increase in the centrifugal force until a stage was reached when there was an exact balance between the centrifugal force and the gravitational force at the equator of the nebula. As a result of this a ring of the matter near the equator would have no weight and this part will start bulging out. This part of the bulging matter could not remain with the still cooling and shrinking nebula and separated in the form of a ring that started revolving around the parent nebula in the direction of its rotation. In the same manner successive rings, each of comparatively smaller size were thrown away from the nebula. These rings continued approaching star went away on its journey. The tidal protuberances jutting out of the sun were left behind. However, this rising mass could not go back to the sun as it had been pulled too much away from the parent body. The cigar-shaped protuberances sepa-rated from the sun began to rotate around the sun under its gravitational influence. These spiral arms gradually condensed into several masses, thus forming the planets. The satellites according to this theory were formed in the same manner under the sun's gravitational pull upon these planets. This theory explains a number of characteristics of the solar system. The tidal filament drawn from the sun would be the thickest in the middle part and would taper towards its ends. Thus, among the planets formed due to breaking up of this filament, the larger planets would be formed in the middle and smaller planets at its two ends. The actual arrange-ment of the planets in the solar system is in conformity with this hypothesis. A similar sequence in the size of the satellites is also found. In case of planets having several satellites, the larger satellites are in the middle and the smaller ones towards the edges. According to this theory the bigger planets remained in a gaseous state for a longer time as they took a longer time to cool. Consequently, a larger number of satellites of relatively smaller size were formed around them. The medium-sized planets gave rise to fewer satellites but the sizes of these satellites are comparatively larger. The smaller planets cooled quickly and therefore satellites did not develop from these. According to this hypothesis the orbits of the planets should not synchronize with the orbit of the sun but they should be inclined at a definite angle due to the gravitational influence of the sun. The fact that the orbits of various planets are inclined at different angles also supports this theory.
(v) Modifications of the Tidal Hypothesis by Jeffreys
Unlike James Jeans, Harold Jeffreys has postulated an actual collision of the, sun with the passing star. According to him the collision led to a portion of the solar material breaking away from the sun, and the planets have been formed of this material. This modified version of the hypothesis provides a simpler explanation of the rotation and revolution characteristics of the planets. According to this hypothesis the smaller planets and satellites were not formed by slow condensation from the gaseous state. Partial liquefaction or solidification of the material was necessary for holding it together. If the material remained in a gaseous state for long, it would have diffused into space. In case of these bodies a liquid core must have immediately been formed through cooling. The earth is supposed to have cooled down until it was completely liquid and thereafter to have solidified through further cooling. During the process of solidification the materials got concentrated in zones in the order of their densities. Jeffreys has emphasised that all these changes took place during a relatively short time. Although this theory has a number of points in its favour, it is not free from shortcomings. Firstly, it has pointed out that the stars lie at such long distances from each other that the possibility of their coming close together or their mutual encounter can easily be ruled out. Secondly, the gaseous matter ejected from the sun would be so hot and there would be so much momentum in the gaseous particles that they would be soon lost in space. According to this theory the sun and all planets should be made of the similar type of material. But in reality, the sun is made up pre-dominantly of lighter constituents such as hydrogen and helium while the planets are composed mainly of heavier elements such as iron, silicon, aluminium, etc. Scholars have also tried to show by mathemati-cal calculations that this hypothesis fails to explain the distances between the sun and the different planets.
(vi) Binary Star Hypothesis of Russel
The great distance of many of the planets from the sun indicates that they have not been born from the sun. The binary star theory of Russel is also based on this assumption. HN Russel has suggested that there was another companion star of the sun and the two together formed a twin-star system. A third star happened to pass close to the companion star of the sun and it resulted in the ejection of gaseous matter from the latter in the form of a filament that ultimately separated from it. The planets were formed from this gaseous filament in course of time. In the beginning the planets were close together and the satellites were born due to the mutual gravitational attraction between them. This theory also believes that the third star was too far away from the sun to have any impact on the latter. The idea of the primitive sun being a binary star is supported by the fact that at least 10 per cent of the stars in the universe are binary stars. This hypothesis also helps to explain the great distances of the planets from the sun and also their high angular momentum. Criticism The most serious drawback of this theory is its inability to account for the removal of the sun's companion from its control and for the retention of the tidal filament that is later supposed to have condensed into planets revolving around it. The hypothesis also fails to explain the present position of the planets. According to this hypothesis all planets should have been formed at roughly an equal distance from the sun.
(vii) Fission Hypothesis of Ross Gun
Ross Gun has tried to explain the origin of the solar system on the basis of the process of origin of binary stars. A binary star is created through a division of a star by fission when its speed of rotation increases due to cooling and contraction. Ross Gun believed in a chance encounter between a rapidly rotating star and another star when the former was in a state of fission. There was a great tidal impact on the disinte-grating star leading to removal of a filament from this. The tidal material drawn from the star experiencing fission broke into several parts and started revolving around the first star in the form of planets.
(viii) Cepheid Hypothesis of Banerji
A.C. Banerji put forth his theory in 1942. According to this theory the sun and the planets have originated from a cepheid. A cepheid is a type of star 5 to 20 times larger in size than the sun and there is a regular pulsation in it. According to Banerji another star came close to the cepheid due to which the pulsation in the cepheid increased and it became unstable. This resulted in ejection of a large amount of material through tidal action that ultimately condensed to form the sun and its planets. The velocity of the tidal ejection was so high that the sun and the planets were freed from the gravitational influence of the cepheid. According to this hypothesis the revolution of the planets is related to the other star which in course of time receded away into the space.
(ix) Nova Hypothesis of Hoyle and Lyttleton
F. Hoyle, a British astronomer, put forth his theory in 1939. According to this hypothesis the solar system has originated through the explosion of a supernova. This theory believes in the existence of a companion star of the sun. This star was much bigger than the sun and the two constituted a binary system. The companion star of the sun was experiencing explosive activity. In the opinion of Hoyle and Lyttleton, the planets have originated from the expanding gases btained from the explosion of a supernova. The expansion of the gases was more in the direction of the sun as a result of which this gaseous matter came within the gravitational influence of the sun. This matter later on condensed to form the planets and the satellites. As a result of the explosion, the supernova recoiled and started receding away from the sun and ultimately separated. This hypothesis explains the high angular velocity of the planets but is unable to offer a satisfactory explanation for the rotation of the planets and the origin of their satellites.
(x) Electromagnetic Hypothesis of Hannes Alfven
Most of the scholars have employed the force of gravitation to explain the origin of the planets. Alfven on the other hand has tried to explain this phenomenon with reference to electromagnetism. There is a magnetic field surrounding the sun and in the early times the magnetic attraction of the sun was greater than its gravitational attraction. The speed of rotation of the sun was also faster than at present. Due to its gravitational attraction the sun attracted the atoms of a nebula present in the space. These atoms got surrounded by extensive clouds of atoms whose extent was roughly equal to the extent of all planets taken together. According to the laws of motion of charged particles in a magnetic field, the ionized atoms collected in the equatorial plane of the sun at a distance approximately equal to that of the Jupiter and the Saturn from the sun and started revolving around the sun. Due to the presence of the revolving atomic matter around it, sun's speed of rotation decreased. The atomic matter revolving around the sun in time condensed to form different planets. The magnetic field of different planets also attracted other atoms thus forming satellites. However, this hypothesis fails to explain the formation of the inner planets-Mercury, Venus, Earth and Mars.
(xi) Interstellar Dust Hypothesis of Otto Schmidt
The Soviet scientist Otto Schmidt proposed this hypothesis in 1943. This theory is based on the views of Kant and Laplace. According to this hypothesis large amount of gases and dust particles are scattered in the universe. In the beginning the sun was able to attract some gases and dust particles due to its gravitational attraction. This material formed a sheet of cloud and dust revolving around the sun. In the initial stage the cloud particles of different sizes were revolving around the sun in an unorganised manner. Later on the heavier particles got concentrated near the bottom of the cloud and the cloud heap attained the shape of a vast flat disk. Mutual collision of particles led to their consolidation forming embryos of planets that later on took the form of asteroids. These asteroids started revolving around the sun within the dust disk in the direction of the latter. The asteroids grew in size by collecting and absorbing the scattered matter by gravitational attraction. They thus turned into planets. Even after the formation of the planets enough material remained scattered in space in attenuated form which starting revolving around the planets. This material in course of time condensed and formed the satellites.
(xii) Nebular Cloud Hypothesis of Von Weizsacker
Karl Von Weizsacker was a German physicist and he presented this hypothesis in 1943. This hypothesis explains the origin of the solar system due to condensation of fine particles of dense interstellar clouds of gases and dust of a nebula surrounding the sun. He emphasized that the interstellar space is filled with mixture of gases and fine dust particles having a chemical composition similar to that of the sun and the other stars. When the sun was formed through condensation of this matter, a large part of this interstellar matter remained in the form of a giant cloud around the sun. Another possibility is that the sun entered into an extensive nebular cloud. This nebular cloud composed of hydrogen, helium and dust particles started revolving around the sun like a ring, under the gravitational influence of the sun. Large lumps of matter were formed through collision of dust particles present in the nebular cloud. These lumps growing in size ultimately formed the planets. According to this hypothesis the planets were formed about 100 million years ago. So long as their size kept on growing due to accretion of additional matter, the planets were hot. With the stoppage of the fall of interstellar dust particles, the process of growth of size of the planets also stopped and the planets started getting cooled through radiation and developed a solid crust. Through this hypothesis, Weizsacker has tried to explain the varying distances of the different planets and the formation of different satellites. This hypothesis seems to be highly probable and is quite logical. Even today there are nebular clouds in abundance in space. If this mode of origin of the solar system is accepted, then there should be other planetary systems also in the Milky Way as the passage of a star through a nebula is a normal event. The hypotheses put forth by Otto Schmidt and Weizsacker have some similarity of views and they have generated fresh interest in the nebular hypothesis of Laplace.
(xiii) Protoplanet Hypothesis of Gerald Kuiper
Gerald Kuiper, an American astronomer, suggested modifications in the hypothesis put forth by Weizsacker and suggested his hypothesis in 1951. He believes that condensation in the nebulous ring around the primordial sun took place earlier than the condensation of the sun. In other words, the planets are older than the sun. The primordial sun and the nebula had a similar composition, consisting primarily of hydrogen and helium with a low content of heavier elements amounting to not more than one to two per cent. As the nebula cooled, it contracted and got divided into a number of separate clouds or protoplanets. Each planet originated as a collection of solid particles at the centre around which extended an extensive envelope of gases. The protoplanets were of different size but were bigger than present planets. As the protoplanets contracted, the satellites were formed close to the planets through a similar process. The direction of rotation of the original nebula was anticlockwise and that is the direction of rotation of the planets also.
While the planets and satellites were formed, the central mass was slowly condensing and developing into a star-the sun. Atomic reactions in this star generated heat in its interior part and it started radiate part of the nebula and the ejected particles were driven away into space by the solar wind of radiation. The lighter gases of the protoplanets were also driven away similarly and the heavier parts of the protoplanets assumed the form of the planets. Complete removal of the lighter gases from planets closer to the sun resulted in their higher density. The planets at a greater distance retained part of these lighter gases and thus have a lower density. The two largest planets, the Jupiter and the Saturn have the lowest density. As the mass and the size of the protoplanets decreased, their speed of rotation increased and their gravitational attraction became weaker leading to their satellites moving away from them. The foregoing brief overview of the various theories trying to explain the origin of the earth indicates that the basic assumptions of early scholars like Kant and Laplace are once again finding a support through the modern theories. The generalization that the solar system has not been formed through some special accident or catastrophe, but it has been formed through a systematic contraction and condensation of a solar nebula is being accepted by an increasing number of scholars. Looks bluish white, body due to the presence of ocean and ice-caps. Earth's core is mainly made up of iron and nickel; mantle is mostly solid. Outer core due to its movement gives magnetic field of earth manifested in Van-alien Radiation bell. Van-Allen Radiation belts are two concentric circles. Inner belt is more energetic and situated at 3000 km above the equator. Outer concentric circle is less energetic and found at 16000 km above the earth. Van-Allen radiation belts are formed due to concentration of solar-winds.
