CHAPTER-2 THE ORIGIN AND EVOLUTION OF THE EARTH class 11 NOTES

Early Theories

Nebular Hypothesis

Immanuel Kant, a German philosopher, gave this theory.
In 1796, a mathematician Pierre-Simon Laplace reexamined it.
According to this hypothesis, the planets were moulded out of a cloud of material associated with a young Sun, which was rotating slowly.

Binary theories

Revised Nebular Hypothesis

Revised Nebular Hypothesis was propounded by Carl Weizascar in Germany and Otto Schmidt in Russia.
They regarded that a solar nebula surrounded the sun and that the nebula comprised of chiefly hydrogen, helium and something called dust.
The collision of particles and the friction caused a disk-shaped cloud to be formed and then the planets were created via the accretion process.

Modern theories

Big Bang Theory

Alternatively called the expanding universe hypothesis.
As per this theory, in the beginning, all matter or substance forming this universe existed at one place as a tiny ball. This tiny ball had an extremely small volume, infinite density and temperature.
At the Big Bang, this ball blasted fiercely and forcefully and started a substantial process of expansion which continues to this day.
Now it is accepted that this event took place 13.7 billion years ago.

Formation of Planets

The following are regarded as the stages in the planets’ development:

The stars are localised gas lumps inside a nebula.
A core to the gas cloud as well as a spinning disc of dust and gas are created because of the gravitational force within the lumps.
After this, the cloud of the gas condenses and the matter over the core is changed into tiny rounded objects.
These small round objects develop into what are called planetesimals by a cohesion process.
The smaller objects start forming larger bodies by colliding with one another and they stick together because of gravitational force.
In the last stage, these large number of small planetesimals aggregate to develop into a smaller number of large bodies called planets.

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