Origin of the earth

This theory was postulated by Lemaitre Georges and Edwin Hubble in 1920s. The Big bang theory explains that about 15 billion years ago a tremendous explosion occurred which started the expansion of the universe. This explosion is known as the big bang. At the point of this event all matter and energy of space were contained at one point. The explosion pushed matter and energy exploding in all directions. As the universe expanded, matter collected into clouds that began to condense and rotate, forming the forerunners of galaxies. Within galaxies, including our own Milky Way galaxy, changes in pressure caused gas and dust to form distinct clouds. In some of these clouds, where there was sufficient mass and the right forces, gravitational attraction caused the cloud to collapse. Where the mass of material in the cloud was sufficiently compressed, nuclear reactions began and a star was born. Some proportion of stars, including the Sun, formed in the middle of a flattened spinning disk of materials. In case of the Sun, gas and dust within this disk collided and aggregated into small grains, eventually formed larger bodies called planetesimals (very small planets). Some of these planetesimals or protoplanets reached diameters of several hundred kilometers. In successive stages these planetesimals coalesced into the current eight planets and their numerous satellites.

While the theory provides some insights on how the universe evolved about 13-20 billion years ago, there are arguments put against the theory. Firstly, the theory propounds that the universe emerged from the enormous atomic nuclear, it remained silent on how, and through which mechanisms and processes the atomic nuclear was composed, evolved and developed prior to its accumulation and explosion. Secondly, based on the theory, it appears that all galaxies emerged at the same time and therefore could properly have at least the same age, but this has not been the case. In fact, the current astronomic researches found that while other galaxies are too old, there are many young galaxies around and far from the Earth.

Nebular hypothesis it is also known as Kant's gaseous hypothesis was proposed by Immanuel Kant and published in his Universal Natural History and Theory of the Heavens in 1755 and modified by Pierre Laplace in 1796. Kant's nebular hypothesis states that the solar system was formed from a cold spinning cloud of gases called the solar nebular that resulted from uneven distribution of gases throughout the universe in the Milky Way Galaxy. As the gravitational pull began to condense the gas towards the center, the speed of the rotation increased. This caused the cloud to flatten, and create an accretion disk. Matter continued to collect as the force of gravity toward the center increased. Eventually, the gas warmed from increasing pressure. As the mass further increased, also the gravity increased and the temperature continued to rise. A ball of hot gas formed in the center of accretion disk creating a protostar also known as the Sun. Finally, when enough gas gathered in the center of the protostar the pressure generated enough heat to fuse the atoms to form a star. Outside the star matter was forming into clumps of gas, dust and rocks which created protoplanets. These protoplanets continued to grow as they trapped materials in their gravitational fields. As the protoplanets were formed from the same cloud of gas and dust, they travelled around the sun in the same direction and the same plane. Also, the Nebular hypothesis explains how the planets are arranged. The heat and wind caused by the Sun swept the lighter gases further out developing the solar system. That is why the rock terrestrial planets such as Mercury, Venus, Earth and Mars are located closer to the sun while the gas giants are formed in the cooler outer region of the solar system.

Like many other theories, Kant's Nebular hypothesis was challenged by Laplace in 1796 in his book entitled 'Explosion of the World System'. Laplace showed three defects of Kant's nebular hypothesis as follows:

1. Large amount of heat cannot be generated due to the collision of cold particles of primordial matter as claimed by Kant.
2. Mutual collision of particles cannot generate motion in the primordial matter and the random motion of the particles cannot generate circular motion in the primordial matter.
3. The angular velocity of rotary speed of the nebular cannot increase due to increase in size of the nebular as assumed by Kant.

Pierre Laplace expanded on Kant's hypothesis. In 1796 Laplace hypothesized that there was a huge and hot gaseous nebula in space which was rotating on its axis. Through continuous circular motion of the nebular and the continuous cooling due to loss of heat from its outer surface through the process of radiation, the nebular turn into continuous reduction in its size because of cooling. Thus, reduction in size and volume of the nebula increased its circular velocity (rotary motion). As the size of the nebula continued to decrease, the velocity of rotatory motion continued to increase. Thus, the nebula started spinning at very fast speed and consequently the centrifugal force became so great that it exceeded the centripetal force. When this stage was reached the materials at the equator of the nebula became weightless. Consequently, the outer layer was condensed due to excessive cooling and contracting central nucleus of the nebula, thus the outer ring (layer) was separated from the remaining part of the nebula. This separated ring of material started moving around the nebular. Further, Laplace maintains that the original ring was divided into nine rings and each ring moved away from the other ring. All the materials of each ring condensed at a point or knot in the form of a 'hot mass of gases". Each mass of gases was later on cooled and condensed to form a planet. Thus, nine planets were formed from nine rings and the remaining central nucleus of the nebula became the sun.

To a large extent Laplace's nebular hypothesis appears to share the common features of those found in Kant's gaseous theory. It is a modified version which tries to correct the weak points raised in Kant's theory. In doing so, the theory attained popularity and respect for decades. However, the Laplace's nebular theory was later challenged by the following arguments.

1. Where did the heat and motion in that nebula come from?
2. What was the reason behind the formation of certain fixed number of planets from the irregular ring?
3. Why did only 9 rings come out from the irregular ring detached from the nebula?
4. Why there were not more or less than 9 rings?
5. The small degree of cohesion between the particles of the nebula would make the formation continuous, not an intermittent process?
6. If Laplace's hypothesis is accepted that the planets were formed from the nebula, then the planets must have been in liquid state in their initial stage. But the planets in liquid state cannot rotate and revolve around the sun on rings.

Sir James Jeans was a British scientist. He propounded his 'tidal hypothesis' to explain the origin of the earth in the year 1919. Jeans postulated that the solar system was formed from the sun and another intruding star. He furthermore postulated that due to massive gravitational force of the intruding star, a huge amount of matter was ejected from the primitive sun, which later on became the building material of future planets. It is argued that, in the beginning the sun was a big incandescent gaseous mass of matter and besides the sun, there was another star termed as 'intruding star' in the universe. This intruding star was much bigger in size than the primitive sun. The primitive sun was stationary and was rotating on its axis while the 'intruding star' was moving along such a path in such a way that it was destined to come nearer to the primitive sun. Jeans claims that there was a great impact of the tidal force of the intruding star on the surface of the primitive sun. The tidal hypothesis was modified by Harold Jeffreys, a British scientist in 1929, by presenting his concept as Collision Hypothesis. According to Jeffreys there were three stars in the universe before the origin of our solar system. One was our primitive sun, the second one was its 'companion star' and the third was 'intruding star' which was moving towards 'companion star. Thus, the intruding star collided against the 'companion star'. Due to the collision, the companion star was completely smashed and shattered. Some shattered portions were scattered in the sky while remaining debris started revolving around the primitive sun. However, the impact of collision and explosion enabled the intruding star to clear itself off from the gravitational attraction of the primitive sun and gradually vanished in the universe. The planets of our solar system were formed from the remaining debris of the companion star.

According to B. Levin the universe is infinite in space and time and the stars are so distant from each other such that a close encounter between them is a remote possibility. James Jeans did not explain the destiny of the intruding star. Also, the tidal hypothesis fails to explain the real distances between the sun and the planets in our present solar system.

The Interstellar dust hypothesis was proposed by Otto Schmidt, a Russian scientist in 1943 to explain the complex problems of the origin and characteristics of the solar system and the Earth. According to the 'Inter Stellar dust hypothesis' the Sun during its 'galactic revolution' captured the dark matter of the universe. The dark matter of gaseous cloud and dust particles had their own angular momentum. The dark matter after being attracted by the sun during its 'galactic revolution' started revolving around the primitive rotating sun. Schmidt called these dark matters 'InterStellar dusts'. The dust particles after being combined and condensed were changed into a flat disc which started revolving around the sun. It may be pointed out that the flat disc of captured dark matter started revolving around the sun under the combined impacts of three types of motions namely;

1. The rotational motion of the sun itself.
2. Gravitational force exerted by the sun on the disc of dark matter.
3. The angular momentum of dark matter of the disc.

The particles having larger amount of centrifugal force were thrown out towards the margins of the rotating disc of dark matter around the sun. The particles having small amount of centrifugal force were attracted towards the band of the disc nearer to the sun. The intense heat from the sun dispersed the gaseous particles towards the margins of the disc. Collision among the dust particles started the process of aggregation and accretion around the bigger particles which became the essentials of the future planets. With the passage of time these embryos captured more and more matter and thus grew in size to become asteroids. These asteroids further grew in size due to continuous acceleration of nearby matter around them and thus they became planets.

The hypothesis has been criticized that gravitational force of the primitive sun was incapable of capturing dark matter scattered in the universe. According to some astrophysicists 'meteorites and asteroids were formed as a result of the disintegration of some planets and not the planets were formed out of meteorites and asteroids. Until now no trace of remnants of dark matter could be discovered either in the archaeological drilling on the earth's surface or on any planet.

Creation theory (theological theory) is the religious belief that the universe and life originated from God commands as written in the Holly Bible (Genesis 1:1-31). The scriptures explain that in the beginning, God commanded, 'let be light' and then it appeared. He separated the light and the darkness. The light was named as 'Day' and the darkness as 'Night'. He then commanded dome to divide the water to be in two separate places that is above and under it. The water below the sky were commanded to come together in one place, so that the land will appear and it was done. He named the land "Earth", and the water that comes together as "Sea". God commanded, "Let lights appear in the sky to separate day from night and it will be the signs of days, years and seasons of the year. God made the two larger lights, the sun to rule over the day and the moon to rule over the night; he also made the stars. The two lights were placed on the sky to shine on the Earth. God created the great sea. God created the great whales and all kinds of creatures that live in the water, and all kinds of birds. God commanded the Earth to produce all kinds of animal life that is domestic and wild, 'large and small" and it was done. He then created human beings who will have power over the fish, birds, and all animals, domestic and wild, 'large and small'. Also, the Holly Quran (Al-Anbiya 21:30) postulates to the theory of creation as it is written, "Do the unbelievers not realize that the heaven and the Earth used to be one solid mass that was exploded into existence? And from water we made all living things. Would they believe?".

As in the case of other theories, the creation theory is criticized by atheists and some scientists for being incompatible with scientific principles.

According to theories of the origin of the universe, it is believed that rocky planets including Mercury, Venus, Earth and Mars, were formed near the Sun, and the gaseous planets which include Jupiter, Saturn, Uranus and Neptune were formed in more distant orbits. The temperature of the early solar system explains why the inner planets are rocky and the outer ones are gaseous. As the gases coalesced to form a protosun, the temperature in the solar system raised. In the inner solar system temperatures were as high as 2000K, while in the outer solar system it was as cool as 50K. In the inner solar system, only substances with very high melting points would have remained solid. All the rest would have vaporized. So the inner solar system objects are made of iron, silicon, magnesium, sulfur, aluminum, calcium and nickel. Many of these were present in compounds with oxygen. There were relatively few elements of any other kind in a solid state to form the inner planets. The inner planets are much smaller than the outer planets, and because of this they have relatively low gravity and were not able to attract large amounts of gas to their atmospheres. In the outer regions of the solar system where it was cooler, other elements like water and methane did not vaporize and were able to form the giant planets. These planets were more massive than the inner planets and were able to attract large amounts of hydrogen and helium. This explains why the solar system and the universe are mainly composed of hydrogen and helium. The four rocky planets namely Mercury, Venus, Earth and Mars are also called terrestrial planets because they are made up of rocks and metals and have solid surfaces, and a core which is mainly made of iron. In many ways, all rocky planets are similar. They all have a solid rocky crust, some form of mantle, and a core. They are the closest four planets to the Sun. They are much smaller than the gas planets and rotate more slowly.

Gas giants are planets that are made of gases. They include Jupiter, Saturn, Uranus and Neptune. Unlike rocky planets, gas giants do not have a well defined surface as there is no clear boundary between the atmosphere and the surface. The gas giants have atmospheres that are mostly hydrogen and helium. They rotate relatively rapidly, for instance, while the Earth spins once on its axis every 24 hours, Saturn spins once every 10 hours. Gas giants may have a rocky or metallic core but the majority of their mass is in the form of gas. Jupiter and Saturn probably have liquid metallic hydrogen interiors (liquid hydrogen conducts electricity). Scientists believe Uranus and Neptune have interiors that contain a mixture (or layers) of rock, water, methane, and ammonia. All four gas giants have rings and moons. Figure 1.2 show the formation process of planets from the planetesimals formed in the solar nebular by collisional coagulation.

Formation of planets in the solar system