The earth, just like other planets was probably formed by aggregation of large number of smaller bodies called planetesimals. In the region occupied by inner planets, Mercury, Venus, Earth and Mars the planetesimals were made up of mostly iron, and rocky substances. In addition to iron, these planetesimals also contained elements such as silicon, oxygen, small amounts of magnesium, aluminium and calcium. These elements were produced by the massive stars of the galaxy. In the region of outer planets, Jupiter, Saturn, Uranus and Neptune, the planetesimals were largely made up of hydrogen, water, carbon dioxide and methane.

The planetesimals collided with each other and began to collect to form the earth. As the earth grew in size, the force of its gravitational attraction increased in proportion to the mass of the accumulated material. Under this strong gravitational pull more planetesiamals were attracted by the earth.

These bodies collided with the earth with great force and stuck to it. During these collisions kinetic energy of the 0planetesimals was converted into heat. This resulted in the rise in temperature of the earth. In addition to the heat produced during bombardment of the elements present in the earth’s interior.

These elements — uranium, thorium and potassium— have unstable nuclei and decompose without the help of any external agency. During this disintegration the particles were liberated at high speed. These particles on colliding with surrounding atoms of solid rock lost their kinetic energy to the rocks and heated them. The combined heating effect of these two processes melted the planet. This melting of the earth took place about 800 million years after its formation. This destroyed all surface records of the earth for the first 800 million years of its existence. After melting, the earth began to re-organise under the influence of gravity. This process led to the partitioning of the earth into core, mantle and crust. This process is called differentiation.

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Life on Planets

Earth is probably the only planet in the solar system having life on it.

For the existence of life on a planet, it should have proper conditions which are necessary for the if to originate and flourish. These conditions are:

1. The planet should have elements like carbon, hydrogen, nitrogen and oxygen which a re the building blocks for the synthesis of more complicated molecules such as amino acids, proteins, nucleotides, etc.

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2. The planet should have optimum temperature so as to permit the life sustaining reactions.

3. The planet should have some medium such as water to transport nutrients.

4. The planet should have a protective blanket around it so as protect the living bodies from the harmful radiations.