What are the Merits and Demerits of Nebular Hypothesis?

The first Nebular Hypothesis was advanced by the German philosopher Kant (1755) and then by the French mathematician Laplace (1796). According to this hypothesis, the planetary system is believed to have been evolved from a large, hot, gaseous nebula rotating in space (Latin ‘nebula’ means mist).

The rotating nebula, according to the law of universal gravitation, became more compressed and compact with an increase in the speed of rotation. Gradual cooling with contraction in size and increasing concentration of mass towards the centre of the nebula led to an increase in the rate of rotation and a growth of centrifugal force.

With the increased velocity of rotation, the centrifugal force around the equator of the mass eventually became equal to the gravitational attraction between the material at the outer rim of the disc and the central mass .

As a result a ring of material was left while contraction of the remaining material continued. When the centrifugal force exceeded the force of gravity in the equatorial zone of the nebula a ring of matter began to spin off along the whole periphery of the rotating disc. Thus successive rings of matter were formed and left behind the contracting mass.

Further cooling and coalescence of the rings led to the formation of planets and their satellites, while the remnant of the pre-existing nebula formed the central incandescent mass of the solar system and is known as the Sun. According to the Kant-Laplace hypothesis, the earth was originally incandescent and in the process of its development it became cooled and contracted.

Merits of the Nebular Hypothesis

(i) This hypothesis is able to explain for similar directions of rotation of all the planets and the Sun.

(ii) It also accounts for the same plane of rotation of the planet.

Demerits of the Hypothesis

(i) It does not satisfy the principle of conservation of angular momentum in the solar system. This theory was rejected when it was learnt that the angular momentum of the solar system is concentrated in the planets and not in the Sun. This is not compatible with the idea that the mass of the matter rotated more rapidly as it condensed.

(ii) Certain planets, for e.g. Venus and some satellites of Mars and Jupiter etc. rotate in a direction opposite to that of the other planets and the Sun.

(iii) If this hypothesis is assumed to be correct then probably there would have been the formation of another ring or a planet.

(iv) The process of condensation of highly rarified gas into rings rather than its dissipation into space is questioned.

(v) The hypothesis fails to explain the observed differences in the density, size and mass of the planets of the solar system.