Nebular Hypothesis

• Immanuel Kant, a German philosopher gave this theory.Continue Reading Geography Notes For W.B.C.S Examination-The Origin and Evolution of the Earth .
• 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

• As per these theories, the sun had a companion.

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.

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.

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 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.