Introduction

The origin of the earth “blue planet” includes many events such as the origin of the universe, the evolution of many stars and galaxies. The age of the planet earth on a geological time scale is considered to be 4.54 billion years ago (BYA). One hypothesis also stated that earth has approximately one third the age of the universe. Earlier cosmic dust of the universe concentrates to form the particulate matter, these matter converted to gravel, the gravels became tiny balls, and these tiny balls combine to form big balls such as planets or planetesimals. The blue planet arises out of a mixture of rocky debris and gases which encircle the young star (Sun).  There are many theories that explain the origin of the earth planet, let’s study some of these theories.

One of the famous hypotheses proposed by Immanuel Kant and Laplace is the Nebular hypothesis of the origin of the earth, which states that the formation of the earth is the result of condensation of a flat nebular disk encircling the sun. Another theory given by Schmidt suggests that the formation of earth is the result of the accretion of solid bodies which were already present in the universe.  One more hypothesis, the binary hypothesis proposed by Chamberlain and Moulton, suggests that rock-solid bodies from the universe are attracted towards the sun and form a cigar-shaped body, and material from this body separated and further condensed to form the rock-solid planets. 

Nebular Hypothesis

The nebula was the cloud dust composed of light elements such as H, He etc. which wander around the sun and further flattened to form a protoplanetary disk. This nebula played a major role in the origin of the earth. 

• The nebular disk formed from early elements circled the sun
• This disk is further condensed to form the solar system with the star and its orbiting planets. It also suggests that wandering nebulae collected its majority mass of elemental material in the centre and formed the sun which accounts for the majority of mass (around 99%) in the solar system
• The floating particulate matter of nebula attracts to each other by the force of charge and gravity to form a bigger mass
• These masses in the nebula collide; stick with each other to form the proto-planets. And these protoplanets further evolve to form planets of both types eg. Gaseous and rock-solid planets, both types of planets have rock-solid core material
• The nebular masses are scattered into particles on the basis of different temperature zones. High-temperature debris remains near the sun and allows condensation of metallic elements with high melting points, and lowers the temperature and farther from the sun allows condensation of gaseous molecules such as water vapour, methane, carbon dioxide. This explains the formation of our solar system including inner rocky planets (Mercury, Venus, Earth, Mars) and outer gaseous planets (Jupiter, Saturn, Uranus, Neptune).
• During the course of time, rocky planets add solid material to their core and gaseous planets add gas to their core that is why the gas planets such as Saturn and Jupiter are composed of light elements such as Hydrogen and Helium
• As the Nebula coalesced into planetesimals, the gravity of Jupiter fastened the movements of nearby debris, and further destructive collisions happened, again and again, resulting in the formation of Asteroid belts; these asteroids are the main source of meteorites on the inner planets

Origin of the Universe: The Big Bang Theory

The Big Bang theory was proposed by Friedmann and Lemaitre in 1920. According to them, the event of the Big Bang occurred around 13.8BYA. This is also known as expanding universe theory. The idea of expansion of the universe was given by Hubble. The big bang was an explosion of highly condensed material that had a very high temperature and density and scattered further in the universe. And results in the formation of planets with decreased density and temperature. Following are the stages of Big Bang events:-

• The primordial matter of high temperature and density with an infinite volume exploded, and these events resulted in the expansion of the material, even this expansion continues till today and will happen till infinite time 
• As the expansion happens some energy from the universe forms matter called Dark Matter
• The big bang happens within some minutes, it is believed that within three minutes of the big bang first atom started to form
• Further the temperature of matter decreased to 4500K (Kelvin) and formed certain nuclei, then a sufficient amount of lighter elements such as H, He, Li etc was produced
• Then matter further cools down to form atoms

Formation of Stars and Planets

• After the formation of galaxies through the Big bang events, the clumps of gas, dust and light elements further condensed to a smaller size. The event was sponsored by the lower temperature 
• As the condensation happens, tiny molecules are produced with high density, then these tiny molecules with high density have a centre with increasing temperature (Approx 10 million degree celsius)
• This high-temperature results in initiating the process of nuclear fusion and further leads to the formation of the stars such as the sun (For Sun this reaction starts 5 bya) 

(Process – Dark cloud -> Prestellar core formation-> Protostar formation-> Pre main Sequence star -> Main star Formation)

• The leftover material after the formation of the star further collided and condensed to form the planetesimal. And these planetesimals further add the matter to form the planets.  

Evolution of Earth

• The barren, rocky, hot and condensed object having the atmosphere of light elemental gases in a highly volatile state leads to the formation of a highly dense and high-temperature planet earth. Planet earth formed by the debris condensation results in the formation of rock-solid core and evolution of earth further continues around 120 mya to 150 MYA through the bombardment of tiny or big planetesimals
• The material in the core of the earth further separated differentially on the basis of their density. A heavy element sinks in the core and light elements float. After the formation of the core of the earth, the core creates a strong magnetic field which further begins the formation of the atmosphere
• As time passes this hot ball cools down and solidifies in condensed size leads to the formation of the outer crust of the earth
• The gaseous atmosphere around this object forms water by condensation which further leads to the formation of early cells and atmospheric layers of our planet

Origin of Life

This is the last event in the evolution of the earth. There is water and elements which react and create organic cells. These reactions are supported by the temperature of the early earth. Chemical reaction results in the formation of organic material which further forms complex materials. The assembly of molecules and conducive reaction environment leads to duplication of such molecules which convert the chemicals into living matter or early primordial cells. These cells are called bacterial cells which further evolve the earth by adding gases such as oxygen to the environment. The Blue-green algae add a sufficient amount of biological material to the atmosphere to evolve life from water to land.  

Conclusion 

The theory explaining the origin of the Earth, the origin of the universe and evolution of life on earth clearly suggest that lighter elements convert to form the more complex and higher material. Many researchers are currently engaged in the study of the core of the earth by using the study of meteorites that strike on earth. Countries like Japan and China sent their probe on the asteroids to extract and bring back samples of the asteroid to further reveal the mystery of the universe and our solar system.