The theories of the origin of the solar system can be divided into 2 groups, viz., Evolutionary theories, & Catastrophic theories. The theories which suggest, that the planets formed during the evolution of the Sun are called Evolutionary theories, e.g. The Nebular Hypothesis. Whereas, the Catastrophic theories are those which state that planets are formed by some special accident or catastrophe, such as the close approach of two stars or by the collision of two stars, e.g., Planetesimal & gaseous tidal hypothesis.
1. Evolution of Earth & earlier theories:
The origin of Earth took place around 4.54 billion years ago. The earlier theories regarding the origin & evolution of Earth are the Nebular Hypothesis, Planetesimal Hypothesis, Gaseous Tidal Hypothesis, Binary Star Hypothesis, & Gas Dust Cloud Hypothesis. Whereas, the modern theories of the origin of the Earth are the Big Bang Theory, Star Formation Theory, Planet Formation Theory, etc. The evolution of Earth denotes how the Lithosphere, Atmosphere, & Hydrosphere of the Earth were created. It also describes the evolution of life on Earth, which took place around 3.5 billion years ago.
1.1. Nebular hypothesis:
• This hypothesis is put forward by German Philosopher Immanuel Kant in 1755 and restudied by Pierre Laplace in 1796, who named it the ‘Solar Nebular Disk Model’.
• This hypothesis states that the sun & planets including Earth have formed from a disc- shaped rotating nebula (a vast cloud of hot gas).
• Initially, there was a hot gaseous cloud, rotating along its axis, which lost its energy by radiation and became cooler. As a result, the nebula gets contracted inward and its rotation speed increases to conserve angular momentum. Due to this, the centrifugal force in the equatorial zone also increases thereby causing the nebula to bulge out in the equatorial zone.
• Due to the continuous cooling & contraction of the nebula, the centrifugal force becomes greater than the gravitational attraction acting inward. As a result, the gaseous ring was separated out.
• Due to the reputation of the above process, successive rings of gaseous material were thrown out from the central mass.
• Finally, the rings condense into planets. Planetoids (a body resembling a planet) form when such rings get broken into many small fragments.
• The central mass of the nebula continues to shrink & finally formed the Sun. Terrestrial planets like Mercury, Venus, and Earth, were formed near the centre of the disc. They had metallic components like iron & nickel, whereas, the planets like Jupiter, Neptune, and Saturn, were formed on the cooler edges.
Demerits:-
• It could not explain the energy distribution and doesn’t satisfy the principle of conservation of angular momentum in the solar system.
• This theory was rejected when it is learnt that the angular momentum of the solar system is concentrated in the planets and not in the sun.
• The process of condensation of highly rarified gas into rings rather than its dissipation into space is questioned.
• It fails to explain the observed differences in the density, size & mass of the planets.