The vast space surrounding us is called universe. It is mostly empty space. The universe includes everything that exists: the most distant stars, planets, satellites, as well as our own earth and all the objects on it. Nobody knows how big the universe is or whether it has any limits.



This theory was developed by Immanuel Kant, a philosopher of Germany in 1755 and modified by Pierre Laplace, a scientist of Germany in 1796..

I. Kant Hypothesis

Kant has put forth his Nebular theory based on the gravitational laws of Newton.
Salient features:
  1. Entire universe was covered with a cloud of matter.
  2. It was cold and stagnant without any movements.
  3. He called it ‘Nebulae.’
  4. Hard particles of matter collided with each other due to gravitational force. Thus, heat was produced with the collision and friction of the particles. The heat generated started movements in the Nebulae.
  5. The Cold Nebulae slowly becomes very hot and its velocity also increased.
Nebular Hypothesis
6. The rotation of the Nebulae increased and its material at the center was thrown away in the form of rings.
7. Several rings of matter were thrown away. The remaining central part of the Nebulae is our present Sun.
8. The material thrown out in the form of rings were later cooled and condensed to form planets.
9. In the same process of cooling the planets have thrown material to form satellites.

II. Laplace Hypothesis

  1. Laplace independently advocated nebular theory in 1796.
  2. He assumed that the Nebulae was very hot and revolving around a centre. It was cooling and as a result shrinking.
  3. Shrinking has increased its velocity. As a result of great velocity, the material at the equator was throughout in the form of rings one after the other which were condensed into planets. In the same manner, satellites were also formed from the material thrown out from the planets.


The nebular theory was widely accepted for over 100 years and it was popular because:
  1. The structure and composition of all planets is uniform because the material was derived from the same source.
  2. All the planets revolve around the sun in the same direction in the same plane and in circular orbits.
  3. The planets are at different distances from the sun.
  4. The sun, as well as the interior of the earth is very hot because the nebulae was very hot.

Criticisms of Nebular theory

  1. The material thrown out in the form of rings could not get condensed to form planets which are spheroid in shape.
  2. The rotational velocity of the sun is very low whereas one the planets are moving fast. The Nebular theory fails to explain this contradictory.
  3. Some of the satellites are moving against their planets. Example: Uranus and Neptune. There is no explanation for this anomaly in the Nebular theory.


The most popular argument regarding the origin of the universe is the Big Bang Theory. It is also called expanding universe hypothesis. This theory was proposed by Abbe Georges Lemaitre, a Belgium astronomer in 1927. Later Edwin Hubble, in 1929, provided evidence that the universe is expanding. As time passes, galaxies move further and further apart.
Big Bang Theory


The tidal hypothesis was advocated by Sir James Jeans and Prof. Herald Jeffreys in 1919 and 1926 respectively. According to this hypothesis, in the beginning, the sun was a gaseous mass during which a star which was many times larger than the sun came nearer to it. Due to the gravitational pull, it has raised tides in the gaseous matter of the sun. As the star approached more nearer to the sun. The ejected material was in the shape of a cigar, with a large mass at the centre and thinner at both ends. In course of time, the gaseous mass was cooled and condensed to form planets. As a result, large planets were formed at the centre and smaller planets at the end. In addition, large planets have given rise to more satellites like Jupiter and Saturn. Small planets either have few or no satellites.


Binary star
It was proposed by H.N. Russel and R.A. Littleton in 1936. In the beginning, the sun had a companion star like the Binary Star system. Another star of huge size approached the companion star which produced a huge tide and the material was later detached. The companion star was also moved away. The detached material was condensed to form planets and satellites.