Q. 1. Describe the Nebular Hypothesis regarding the origin of the Planets.
Ans. Nebular Hypothesis. Some Monistic Theories (One Star Theories) explain the origin of planets. A German Philosopher, Immanuel Kant, proposed the Nebular Hypothesis. This Hypothesis was based on Newton’s Laws of motion.
Outlines of Hypothesis :
(i) According to Kant, the primordial matter was scattered in the space.
(ii) This matter was supernaturally created.
(iii) There was a slowly rotating cloud of gas called Nebula.
(iv) The original cold and motionless Nebula became a hot and spinning Nebula under the gravitational attraction. The French mathematician, Laplace proposed more or less, same theory in 1796.
(v) The rotational speed increased under Law of conservation of angular momentum. The centrifugal force then increased.
(vi) Thus, successive rings of gaseous matter were thrown off from the central mass (Equator) by centrifugal force.
(vii) The rings later on condensed to form planets.
The residual central mass remained as the sun.
(i) It was the forerunner of Laplace theory.
(ii) Heat and motion cannot be produced in cold Nebula without some external force.
(iii) Kant, despite criticism went to say ‘‘Give me matter and I can create the earth.”
Q. 2. Describe the collision Hypothesis proposed by Sir James Jeans and Sir Harold Jeffreys.
Ans. This theory belongs to Dualistic (Two Stars) Theories. This theory was forwarded by British scientists Sir James Jeans and Sir Harold Jeffreys in 1926.
According to this theory, planets are bi-parental in origin. It is also known as Tidal Hypothesis.
Outline of Hypothesis:
(i) The sun was originally a very hot gaseous mass in the space.
(ii) Another star, several times bigger than the sun came close to the sun and caused Tidal disruptions.
(iii) Due to gravitational pull of the approaching star, gaseous material was pulled away from the preexisting sun. The ejected matter, called Filament was cigar shaped.
(iv) This filament was broken into small chunks called planetesimals.
(v) By collision and gravitational attraction the larger planetesimals collected the smaller bricks and condensed into planets.
Merits and Demerits: It is the most accepted Hypothesis. The present arrangement of planets confirms this idea. The number of satellites also proves the theory. The smaller planets have less satellites.
But this theory fails to explain the location of the planet Mars. The temperature of the Sun is too high for the formation of planets.
Q.3 Compare the inner planets and outer planets.
Ans. Of the eight planets Mercury, Venus, Earth and Mars are called as inner planets as they lie between the Sun and the belt of asteroids called the asteroid belt. The other four planets, lining up after the asteroid belt are Jupiter, Saturn, Uranus and Neptune are called outer planets. Alternatively the first four are called ‘Terrestrial’, meaning Earth like, as they are made up of rock and metals and have relatively high densities. The rest four are called Jovian or Gas Giant planets. Jovian means Jupiter like. Most of them are much larger than the terrestrial planets and have a thick atmosphere mostly of helium and hydrogen. All the planets were formed in the same period, some time about 4.6 billion years ago.