Universe

Many scientists and experts believe that the origin of the universe is the Big Bang Theory. The event of the big bang took place 13.7 billion years ago.
Some scientists highlight this as expanding universe hypothesis.

At the beginning of the Big bang theory, all matter forming the universe existed in one place in the form of a “tiny ball” (singular atom). This singular atom was of extremely small volume, infinite temperature, and infinite density.

With The Big Bang the “tiny ball” exploded violently and led to a huge expansion. As it grew, some energy was converted into matter. There was particularly rapid expansion within fractions of a second after the bang.

Scientists believe that within the first three minutes of the Big Bang event, the first atom began to form.

Within 300,000 years from the Big Bang, the temperature dropped to 4,500K (Kelvin) and gave rise to atomic matter. The universe became transparent.

Formation of galaxies and stars

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.

Nebula and stars

A galaxy starts to form through the accumulation of hydrogen gas in the form of a very large cloud called a nebula. Eventually, the growing nebula develops localized clumps of gas. These clumps continue to grow into even denser gaseous bodies, giving rise to the formation of stars. The formation of stars is believed to have taken place 5-6 billion years ago.

Galaxy

A collection of stars forms a galaxy. Galaxies spread over vast distances that are measured in thousands of light-years. The diameters of individual galaxies range from 80,000-150,000 light years.

Formation of planets

The stars are localized lumps of gas within a nebula. The gravitational force within these lumps leads to the formation of a core to the gas cloud. These huge rotating disc of gas and dust develops around the gas core.

Planetesimals

Such 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 into what are called planetesimals. Larger bodies start forming by collision, and gravitational attraction causes the material to stick together. Planetesimals are a large number of smaller bodies.
During the final stage of this process, these large numbers of small planetesimals accumulate to form fewer large bodies in the form of planets.

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 sometime 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 a huge quantity of dust grains and gases.

Terrestrial or inner planets 

Mercury, Venus, earth, and Mars are called the inner planets as they lie between the sun and the belt of asteroids the other four planets are called the outer planets.

The first four are called Terrestrial, meaning earth-like as they are made up of rock and metals, and have relatively high densities.

Jovian or Gas planets 

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 atmospheres, mostly of helium and hydrogen. 

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

The solar wind was most intense nearer the sun. This solar wind blew off lots of gas and dust from the terrestrial planets. 

The Origin of Earth

The earth broke off about 4.5 billion years ago with an explosion. The earth was a  burning hot white mass of gas and dust.

Over a long period of time, dust and gas gradually condensed to form solid rock. Such condensation and shrinking made the earth heat up so much that the rock melted into a liquid.

After millions of years,  the earth’s crust cooled and formed hard rock again.

At that time the interior of the earth was very hot. The crust of the earth was formed from the cooling and hardening of molten matter and hot gases. With the cooling of the earth, the crust hardened and formed the land. Cooling of the earth also condensed water vapor into liquid water creating seas.

The planet earth initially was barren. It was a rocky and hot object with a thin atmosphere of hydrogen and helium. 

Evolution of Lithosphere

During the primordial stage, the earth was mostly in a volatile state.

Due to a 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 toward the center of the earth and the lighter ones to move toward the surface.

With time earth became cooler and solidified and condensed into a smaller size. This developed the outer surface in the form of a crust.

Evolution of Atmosphere and Hydrosphere

The present composition of the earth’s atmosphere is mainly composed of nitrogen and oxygen.

There are three stages in the evolution of the present atmosphere.

• The first stage is – the loss of the primordial atmosphere.
• The second stage – the hot interior of the earth contributed to the evolution of the atmosphere.
• The third stage –  the composition of the atmosphere was modified by the living world through the process of photosynthesis.

During the cooling of the earth, gases and water vapor were released from the interior solid earth. This started the evolution of the present atmosphere.

The early atmosphere largely contained water vapor, nitrogen, carbon dioxide, methane, ammonia, and very little of free oxygen.

The process through which the gases were outpoured from the interior is called degassing.

Water vapor

Continuous volcanic eruptions contributed water vapor and gases to the atmosphere.

As the earth cooled, the water vapor  started getting condensed. The carbon dioxide in the atmosphere got dissolved in rainwater and the temperature further decreased causing more condensation and more rain.

The rainwater falling onto the surface got collected in the depressions to give rise to oceans.

The earth’s oceans were formed within 500 million years of the formation of the earth. This tells us that the oceans are as old as 4,000 million years.

Photosynthesis

Scientists believe that around  3,800 million years ago, life began to evolve. However, around 2,500-3,000 million years before the present, the process of photosynthesis got evolved.

Life was confined to the oceans for a long time.

Oceans began to contribute oxygen through the process of photosynthesis.

Eventually, oceans were saturated with oxygen, and 2,000 million years ago, oxygen began to flood the atmosphere.