Earlier Theories
Nebular Hypothesis (1755)
Planetesimal Hypothesis (1904)
Gaseous Tidal Hypothesis (1925)
Double Star Hypothesis (1938)
Weizsacker’s Hypothesis (1944)
Meteorite Hypothesis (1944)
Proto-planetary Hypothesis (1951)
Fred Hoyle’s Magnetic Hypothesis (1958)
Planetesimal Hypothesis
Proposed by Chamberlin and Moulton in 1904
According to this hypothesis:
The Sun existed before the formation of planets. Due to the disruptive forces of the sun and the powerful gravitational pull of the passing star, giant masses of gas were torn from the surface of the pre-existing sun. These giant masses of gas broke into various small chunks which on cooling gave rise to solid particles called Planetesimals.
Gaseous Tidal Hypothesis
It was proposed by Jeans and Jeffreys in 1925
It states that planets were formed from a gaseous filament that was torn out from the sun due to the gravitational pull of a huge star that happened to approach very close to the sun. This extremely unstable gaseous filament ejected into space and immediately got split into several fragments. These fragments gradually cooled, condensed, and gave rise to planets, and thus the solar system came into existence.
Modern theory
The Big Bang Theory
It is a modern theory of the Origin and the Evolution of the Earth
A. B. Lemaitre, a Belgian astronomer, was the first to provide a theory on the origin of the Universe in 1927. Later, Edwin Hubble in 1929, propounded the Big Bang theory with evidence of the expanding universe. It is also known as the “Expanding Universe Hypothesis”.
According to this theory, all matter that formed the universe existed at one point (tiny ball) called a singularity or Singular atom having an unimaginable small volume, extremely high temperature, and density.
Around 13.7 billion years ago, there was a big explosion (BIG BANG!). The ‘tiny ball’ exploded violently which led to a huge expansion and this expansion continues even today. There was a rapid expansion within fractions of a second after the bang. Thereafter, the expansion slowed down. With the expansion some of the energy was converted into matter, and within the first three minutes from the big bang event, the first atom began to form.
Within 300,000 years from the Big Bang, the temperature dropped down to 4500 Kelvin and gave rise to atomic matter. The majority of atoms formed were hydrogen, along with helium and traces of lithium. Huge clouds of these elements fused through gravity to form stars and galaxies.
Formation of stars and planets According to Big Bang Theory
Initially, the distribution of matter and energy was uneven in the universe. This initial density difference gave rise to differences in gravitational forces and caused the matter to get drawn together. Thus, the galaxies start to develop. Galaxies started to form by the accumulation of hydrogen gas in the form of a large cloud called Nebula. Eventually, this growing nebula developed localized clumps of gas that grew continuously into an even denser gaseous body and gave rise to the formation of stars. The star formation is believed to have taken place 5-6 billion years before.
The gravitational forces within the lumps of gases of the nebula lead to the formation of a core and huge rotating discs of gases. And dust developed around the core.
In the next stage, the 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 is 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. In the final stage, these large numbers of small planetesimals accrete to form large bodies in the form of planets.
