THE ORIGIN AND EVOLUTION OF THE EARTH:-
EARLY THEORIES-

Nebular Hypothesis- by German philosopher Immanuel Kant. Mathematician Laplace revised it in 1796.

• The hypothesis considered that the planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating. Later in 1900, Chamberlain and Moulton considered that a wandering star approached the sun.
• As a result, a cigar-shaped extension of material was separated from the solar surface. 
• As the passing star moved away, the material separated from the solar surface continued to revolve around the sun and it slowly condensed into planets

Binary theories- the arguments considered of a companion to the sun to have
been coexisting. 

Revised Nebular Hypothesis- By Otto Schmidt in Russia and Carl Weizascar in Germany

• They considered that the sun was surrounded by solar nebula containing mostly the hydrogen and helium along with what may be termed as dust.
• The friction and collision of particles led to formation of a disk-shaped cloud and the planets were formed through the process of accretion.

MODERN THEORIES

Origin of the Universe:-

1. Big Bang Theory- also called expanding universe hypothesis. 

• In the beginning, all matter forming the
  universe existed in one place in the form of a “tiny ball” (singular atom) with an unimaginably small volume, infinite temperature and infinite density. 
• At the Big Bang the “tiny ball” exploded violently. This led to a huge expansion. It is now generally accepted that the event of big bang took place 13.7 billion years before the present. The expansion continues even to the present day. As it grew, some energy was converted into matter. There was particularly rapid expansion within fractions of a second after the bang. Thereafter, the expansion has slowed down. Within first three minutes from the Big Bang event, the first atom began to form.
• Within 300,000 years from the Big Bang, temperature dropped to 4,500 K (Kelvin) and gave rise to atomic matter. The universe became transparent.
• The expansion of universe means increase in space between the galaxies. An alternative to this was Hoyle’s concept of steady state. It considered the universe to be roughly the same at any point of time. However, with greater 
• evidence becoming available about the expanding universe, scientific community at present favours argument of expanding universe. 

The Star Formation

• The distribution of matter and energy was not even in the early universe. 
• These initial density differences gave rise to differences in gravitational forces and it caused the matter to get drawn together. These formed the bases for development of galaxies.
• A galaxy contains a large number of stars. Galaxies spread over vast distances that are measured in thousands of light-years.
• A galaxy starts to form by accumulation of hydrogen gas in the form of a very large cloud called nebula.
• Eventually, growing nebula develops localized clumps of gas.
• These clumps continue to grow into even denser gaseous bodies, giving rise to formation of stars. The formation of stars is believed to have taken place some 5-6 billion years ago.
• A light year is a measure of distance and
  not of time.
• Light travels at a speed of 300,000 km/second. Considering this, the distances the light will travel in one year is taken to be one light year. This equals to 9.461 1012 km. The mean distance between the sun and the earth is 149,598,000 km. In terms of light years, it is 8.311minutes. 

Formation of Planets:- 

The following are considered to be the stages in the development of planets : 

• The stars are localised lumps of gas within a nebula. The gravitational force within the lumps leads to the formation of a core to the gas cloud and a huge rotating disc of gas and dust develops around the gas core. 
• the gas cloud starts getting condensed and the matter around the core develops into small- rounded objects. These small-rounded objects by the process of cohesion develop in the same period sometime about 4.6 billion years ago.
• Planetesimals are a large number of smaller bodies.
• Till recently (August 2006), Pluto was also considered a planet. However, in a meeting of the International Astronomical Union, a decision was taken that Pluto like other celestial into what is called planetesimals objects
• these large number of small planetesimals accrete to form a fewer large bodies in the form of planets.

OUR SOLAR SYSTEM 

• Our Solar system consists of eight planets.

• The nebula from which our Solar system is supposed to have been formed, started its collapse and core formation some time 5-5.6 billion years ago and the planets were formed about 4.6 billion years ago.
• Our solar system consists of the sun (the star), 8 planets, 63 moons, millions of smaller bodies like asteroids and comets and huge quantity of dust-grains and gases.
• Out of the eight planets, mercury, venus, earth and mars are called as the inner planets as they lie between the sun and the belt of asteroids the other four planets are called the outer planets.
• Alternatively, the first four are called Terrestrial, meaning earth-like as they are made up of rock and metals, and have relatively high densities.
• The rest four are called Jovian or Gas Giant planets. Jovian means jupiter-like. Most of them are much larger than the terrestrial planets and have thick atmosphere, mostly of helium and hydrogen.
• All the planets were formedmay be called ‘dwarf planet’. Some data regarding our solar system are given in the box below. 

The difference between terrestrial and jovian planets can be attributed to the following conditions:

(i) The terrestrial planets were formed in the close vicinity of the parent star where it was too warm for gases to condense to solid particles. Jovian planets were formed at quite a distant location.

(ii) The solar wind was most intense nearer the sun; so, it blew off lots of gas and dust from the terrestrial planets. The solar winds were not all that intense to cause similar removal of gases from the Jovian planets.

(iii) The terrestrial planets are smaller and their lower gravity could not hold the escaping gases 

The Moon 

• only natural satellite of the earth. 

Early theory:- 

• In 1838, Sir George Darwin suggested that initially, the earth and the moon formed a single rapidly rotating body.
• The whole mass became a dumb-bell-shaped body and eventually it broke.
• It was also suggested that the material forming the moon was separated from what we have at present the depression occupied by the Pacific Ocean. 

Giant impact’ “the big splat Theory :-

• A body of the size of one to three times that of mars collided into the earth sometime shortly after the earth was formed. It blasted a large part of the earth into space.
• This portion of blasted material then continued to orbit the earth and eventually formed into the present moon about 4.44 billion years ago.

EVOLUTION OF THE EARTH:-

• the planet earth initially was a barren, rocky and hot object with a thin atmosphere of hydrogen and helium.
• The earth has a layered structure.
• From the outermost end of the atmosphere to the centre of the earth, the material that exists is not uniform.
• The atmospheric matter has the least density. 
• From the surface to deeper depths, the earth’s interior has different zones and each of these contains materials with different characteristics.

Evolution of Lithosphere 

• The earth was mostly in a volatile state during its primordial stage.
• Due to gradual increase in density the temperature inside has increased.
• As a result the material inside started getting separated depending on their densities.
• This allowed heavier materials (like iron) to sink towards the centre of the earth and the lighter ones to move towards the surface.
• With passage of time it cooled further and solidified and condensed into a smaller size. This later led to the development of the outer surface in the form of a crust.
• During the formation of the moon, due to the giant impact, the earth was further heated up. It is through the process of differentiation that the earth forming material got separated into different layers.

Starting from the surface to the central parts, we have layers like the crust, mantle, outer core and inner core. From the crust to the
core, the density of the material increases.