Early theories
Origin of the Earth
• Mathematician Laplace in 1796 gave theory known as Nebular Hypothesis which considered that the
planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating.
• Later in 1900, Chamberlain and Moulton considered that a wandering star approached the sun. As a
result, a cigar-shaped extension of material was separated from the solar surface. As the passing star
moved away, the material separated from the solar surface continued to revolve around the sun and it
slowly condensed into planets.
• This argument was supported by Sir James Jeans and later Sir Harold Jeffrey. At a later date, the
arguments considered of a companion to the sun to have been coexisting called binary theories.
Modern theories Origin of the Universe
• Big Bang Theory is also called expanding universe hypothesis given by Edwin Hubble, in 1920. In the
beginning, all matter forming the universe existed in one place in the form of a “tiny ball” (singular atom)
with an unimaginably small volume, infinite temperature and infinite density.
• At the Big Bang the “tiny ball” exploded violently which led to a huge expansion 13.7 billion years ago.
The expansion continues even to the present day.
• Within first three minutes from the Big Bang event, the first atom began to form. Within 300,000 years
from the Big Bang, temperature dropped to 4,500 K (Kelvin) and gave rise to atomic matter. The universe
became transparent.
• The expansion of universe means increase in space between the galaxies. An alternative to this was
Hoyle’s concept of steady state. It considered the universe to be roughly the same at any point of time.
However greater evidence is available for the expansion/ big bang theory.
The Star Formation
• A galaxy contains a large number of stars. Galaxies spread over vast distances that are measured in
thousands of light-years. The diameters of individual galaxies range from 80,000-150,000 light years. A
galaxy starts to form by accumulation of hydrogen gas in the form of a very large cloud called nebula.
• Eventually, growing nebula develops localised clumps of gas which continue to grow into even denser
gaseous bodies, giving rise to formation of stars. The formation of stars is believed to have taken place
some 5-6 billion years ago.
Formation of Planets
Formation of starts follow the given stages
(i) The stars are localised lumps of gas within a nebula. The gravitational force within the lumps leads to
the formation of a core to the gas cloud and a huge rotating disc of gas and dust develops around the gas
core.
(ii) In the next stage, the gas cloud starts getting condensed and the matter around the core develops into
small rounded objects.
(iii) These small-rounded objects by the process of cohesion develop into what is called planetesimals.
(iv) Larger bodies start forming by collision, and gravitational attraction causes the material to stick together.
(v) In the final stage, these large number of small planetesimals accrete to form a fewer large bodies in the
form of planets.
