A number of theories have been propounded by various scientists and philosophers across the world regarding the origin of the Earth. The earlier theories given by scientists are as follows:
A. Monoist Theories:
1. Immanuel Kant’s Gaseous Hypothesis:
- According to Kant, there existed a primordial matter scattered in the universe. It contained a slowly rotating cloud of gas (nebula) in which particles collided with each other due to the gravitational force. Due to collision heat was generated, and thus, the temperature of the primordial matter started rising.
- The increase in the temperature changed the state of primordial matter from solid to gaseous particles. With the continuous increase in temperature and the rising rate of rotatory motion, the nebula started expanding in size.
- The size of this gaseous cloud increased continuously, which in turn increased the rotation speed of the particles due to which centrifugal force became greater than the centripetal force. This resulted in the formation of concentric rings, and a residual central mass was created, which was considered as the sun.
- The irregularity of the rings caused the development of the cores for the formation of the corresponding planets.
2. Nebular Hypothesis of Laplace:
- Laplace’s theory is the modification to Kant’s theory. • Laplace assumed that there was a huge and hot gaseous Nebula in the space which was rotating on its axis from the very beginning.
- The continuous cooling of the nebula was due to loss of heat from its outer surface through the radiation. It was continuously reduced in size due to the contraction of cooling.
- To sum up, this hypothesis considered that the planets were formed out of the rapidly spinning nebula. The nebula condensed, cooled and solidified into celestial bodies such as stars, planets etc.
B. Binary Theories:
3. Planetesimal theory of Chamberlain and Moulton:
- Chamberlain & Moulton considered that there was a proto-sun which was intruded upon by a wandering star (companion star).
- The companion star passed very close to the proto-sun which was cold, solid and rocky and infinite number of small particles (planetesimals) were detached from the outer surface of the proto-sun due to massive gravitational pull exerted by the giant intruding star.
- Numerous small planetesimals accreted together and grew into the planets, and with the passage of time, the remaining proto-sun changed into the present-day sun.
4. Tidal Hypothesis of Jean & Jeffery:
- James Jeans and Harold Jeffrey supported the theory of Chamberlain and Moulton.
- This hypothesis considers the proto-sun to be a hot burning mass of gas, unlike
Chamberlain’s theory which considered the proto-sun to be cold, solid and rocky.
- 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.
- The tidal force of the intruding star had a great impact on the surface of the protosun.
Revised Nebular Hypothesis of Otto Schmidt and Carl Weizascar: They considered that the sun was surrounded by solar nebula containing mostly the hydrogen and helium along with what may be termed as dust. Due to the friction and collision of particles, accretion took place, and a disk-shaped cloud and the planets were formed.
C. Trihybrid Theory:
6. The Binary Star Hypothesis of Russel:
- According to Russell, there were two stars near the primitive sun in the universe
(binary star system).
- A giant star named approaching star came near the companion star, and a large amount of matter of the companion star was attracted towards the giant approaching star because of its massive tidal force.
- As the approaching star came nearer to the companion star, tidal force and the
gravitational force continued to increase.
- When the giant approaching star came nearest to the companion star, a large amount
of matter was ejected from the companion star due to maximum gravitational force exerted by the giant approaching star.
- From the ejected matter, planets were formed.
The Big Bang Theory
- Initially, all matter constituting the universe existed in the form of a singular atom. This singular atom (tiny ball) had an unimaginably small volume, infinite temperature and infinite density.
- At the event of the Big Bang, nearly 13.7 billion years ago, the tiny ball (singular atom) exploded violently, which resulted in a huge expansion. The expansion occurred rapidly within fractions of a second after the bang after which it had slowed down. The expansion continues even to the present day.
- As a result of expansion, some energy was converted into matter. The first atom began to form within the first three minutes from the Big Bang event. After the first three minutes, the protons and neutrons had assembled into hydrogen and helium nuclei. The abundance of helium is a key prediction of big bang theory, and it’s been confirmed by scientific observations.
- Within 300,000 years from the Big Bang, the temperature came down to 4,500 K, and the atomic matter was formed. With the passage of time, the matter cooled and more diverse kinds of particles began to form, which eventually condensed into the stars and galaxies of our present universe. The universe became transparent.
Formation of the Star
- The formation of stars is believed to have taken place nearly 5-6 billion years ago.
- The matter and energy were distributed unevenly in the early universe. Initially, the differences in density gave rise to differences in gravitational forces. As a result, the matter got drawn together. This led to the formation of galaxies.
- A galaxy contains a large number of stars. The formation of a galaxy starts with the accumulation of hydrogen gas in the form of a very large cloud called nebula.
- The growing nebula developed localised clumps of gas. The clumps continue to grow into further denser gaseous bodies, eventually giving rise to the formation of stars.
Formation of the Planets
- The planets were formed nearly 4.6 billion years before the present.
- The localized lumps of gas within a nebula started forming a core to the gas cloud due to the gravitational force.
- As a result of gravitational force and formation of core, a huge rotating disc of gas and dust developed around the gas core.
- The condensation started within the gas cloud, and the matter around the core developed into small rounded objects. The process of cohesion developed between these small rounded objects which developed them into planetesimals. Planetesimals are a large number of smaller bodies.
- As a result of the collision, larger bodies started forming and gravitational attraction resulted in the materials sticking together.
- Finally, the large number of small planetesimals accreted together due to cohesion to form fewer large bodies in the form of planets.
Formation of the Moon
- The moon is the only natural satellite of the Earth which was formed nearly 4.4 billion years before the present. The scientists have made a number of attempts to explain the formation of the moon.
- Sir George Darwin, in 1838 suggested that initially, the Earth and the moon formed a single rapidly rotating body. The whole rotating mass became a dumb-bell shaped body and broke eventually. Sir Darwin also suggested that the material forming the moon was separated from what we have at present the depression occupied by the Pacific Ocean. However, these explanations were not accepted by the present scientists.
- Presently, it is believed that the formation of the moon, as a satellite of the Earth, is a result of ‘giant impact’ or the phenomenon described as ‘the big splat’.
- A body of the size of one to three times that of mars collided into the Earth shortly after the formation of the Earth. It separated a large part of the Earth into space which then continued to orbit the Earth and eventually formed into the present moon about 4.44 billion years ago.
- Our solar system consists of the sun, eight planets, a number of moons, millions of smaller bodies such as comets and asteroids and a huge quantity of gases and dust particles.
- The nebula from which our Solar system is believed to have been formed started its core formation nearly 5-5.6 billion years ago, and the planets were formed about 4.6 billion years ago.
- Terrestrial planets: Mercury, Venus, Earth and Mars, are called the inner planets or the terrestrial planets. Terrestrial means Earth-like. They lie between the sun and the belt of asteroids. They are called Terrestrial planets because they are made up of rock and metals, and have relatively high densities.
- Jovian planets: Jupiter, Saturn, Uranus and Neptune are called the outer planets or Jovian or Gas Giant planets. Jovian means Jupiter-like. These planets are much larger than terrestrial planets and have a thick atmosphere made mostly of helium and hydrogen.
- The difference in the formation and composition of terrestrial and jovian planets are attributed to the few reasons. The terrestrial planets were formed in the close vicinity of the parent star. It was too warm there for the gases to condense into solid particles. Jovian planets, on the other hand, were formed at quite a distant location. The intense solar wind blew off lots of gas and dust from the terrestrial planets. The intensity of solar wind was not that high to cause similar changes in Jovian planets. Also, the smaller size and lower gravity of the terrestrial planets did not allow them to hold the escaping gases.
- All the planets were formed in nearly the same period around 4.6 billion years ago.
Evolution of the Earth
- Initially, the Earth was a rocky, barren and hot object having a thin atmosphere of helium and hydrogen. This was very different from the present-day Earth.
- There have been some events and processes, which caused this change from rocky, barren and hot Earth to the present day earth being a beautiful planet with ample amount of water and conducive atmosphere favouring the existence of life.
- The Earth has a layered structure and matter is distributed on the basis of density. The atmospheric matter has the least density.
- The Earth’s interior has been divided into different zones, and each of these contains materials with different characteristics.
Development of Lithosphere:
- During its primordial stage, the Earth was mostly in a volatile state. The gradual increase in density resulted in an increase of the inside temperature.
- Due to this differentiation, the material inside started getting separated depending on their densities. The heavier materials (like iron) sank towards the centre of the Earth and the lighter ones to moved towards the surface.
- Further, cooling started with the passage of time, and it solidified and condensed into a smaller size. This led to the development of the outer surface in the form of a crust.
- During the formation of the moon (the giant impact), the Earth was further heated up. The process of differentiation led the materials getting separated into different layers in terms of density.
- Differentiation of different layers resulted in the formation of the crust, mantle, outer core and inner core with density increasing from the crust to the core.
Evolution of Atmosphere and Hydrosphere:
- Nitrogen and oxygen dominate the present composition of Earth’s atmosphere. However, the early atmosphere was nowhere near the present-day composition of the atmosphere. The present-day atmosphere is the result of three stages in the evolution of the atmosphere. In the first stage, the loss of the primordial atmosphere took place. The hot interior of the Earth contributed to the evolution of the atmosphere in the second stage. In the final stage, the composition of the atmosphere was modified by the living world through the process of photosynthesis.
- The early atmosphere contained a large amount of hydrogen and helium which was stripped off as a result of the solar winds. This happened in case of all the terrestrial planets, including the Earth. Due to the intense solar winds, the terrestrial planets were supposed to have lost their primordial atmosphere.
- When the cooling of the Earth started, the gases and the water vapour were released from the interior of the Earth. The process of the outpouring of gases from the interior is known as ‘degassing’. This started the evolution of the present atmosphere.
- The early atmosphere largely contained nitrogen, carbon dioxide, water vapour,
methane, ammonia and very little amount of free oxygen.
- Water vapour and gases were getting added to the atmosphere due to the continuous volcanic eruptions. As the Earth cooled, the water vapour released started getting condensed, and the carbon dioxide in the atmosphere got dissolved in rainwater. The temperature further decreased, causing more condensation and more rains.
- The rainwater falling onto the surface got collected in the depressions and gave rise to oceans. The Earth’s oceans are supposed to have been formed within 500 million years from the formation of the Earth. Thus, the oceans are nearly 4,000 million years old, and life was confined to the oceans for a long time.
- The evolution of life began nearly 3,800 million years ago. However, the process of photosynthesis evolved nearly around 2,500-3,000 million years before the present.
- The process of photosynthesis led the oceans to have the contribution of oxygen. When the oceans were saturated with oxygen, the oxygen began to flood the atmosphere nearly 2,000 million years ago.
Origin of Life:
- The last phase in the evolution of the Earth is related to the origin and evolution of life. The evolution of life began nearly 3,800 million years ago.
- Since the initial atmosphere of the Earth was not conducive for the development of life, the origin of life must have been started through the complex chemical processes.
- According to modern scientists, the origin of life started as a kind of chemical reaction, which first generated complex organic molecules and assembled them.
- The assemblage of the organic molecules was such that they could duplicate themselves converting inanimate matter into living substance. Fossils found in rocks are the source of information about the record of life that existed on this planet in different periods. The microscopic structures closely related to the present form of blue algae have been found in geological formations that are much older than these were some 3,000 million years ago. Thus, it can be assumed that life began to evolve nearly 3,800 million years ago.