Short essay on The Nebular Hypothesis of Laplace

Marquis de Laplace propounded his well-known nebular hypothesis in 1796. In fact, it is a modified version of Kant’s hypothesis. He rectified Kant’s error by assuming that the primordial matter present in the space was already a hot and rotating gaseous nebula.

Because of traditional cooling there was contraction in the size of the nebula. Because of contraction there was decrease in the size of nebula which resulted in the increase of the speed of its rotation.

With the subsequent increase in the cooling and contracting nebula, there was consequent increase in the centrifugal force of the outer rim in its equatorial region which separated from the main body.

This ring may be compared with the present of ring of the Saturn planet. At a later stage this ring is assumed to have collected as a planet. Further cooling of the rotating nebula would result in its contraction.

Its speed of rotation would increase. So the centrifugal force would continue to increase and the centripetal force or the force of gravity would ultimately be overcome by the centrifugal force. Another rim would separate and form another planet. This process would continue and cause the formation of different planets.

The same process would be repeated on different planets and would cause the formation of their satellites. Thus the remaining part at the nucleus of the nebula became the sun.

The above-mentioned process caused the rotation of the planets and their revolution in the same sense and their location roughly in the same plane.

Laplace offered an explanation of the fact that with a few exceptions among the satellites e.g. Uranus and Neptune, each celestial body possesses acquired motions of rotation and revolution in one and the same sense and that several planetary orbits are almost in the same plane,.

However, the fact that some of the satellites have motions opposed to that of most of the planets and their satellites goes against this hypothesis.

The nebular hypothesis assumes that the planets were in the initial stage a gaseous mass, and on cooling became in liquid condition, and because of radiational cooling there developed a solid crust.

The evolution of a solid earth from an initially gaseous and later liquid mass is supported by a solid crust on the upper part of the earth and the evidences of a liquid molten interior as proved by vulcanicity.

Criticism:

The criticism cited against Laplace’s hypothesis are based on the following grounds –

(i) There appears to be every possibility that the separation of matter from the original nebula was a continuous phenomenon. It cannot be an intermittent process as postulated by Laplace,.

The particles of the gaseous nebula were loose and lacked cohesion among themselves. It is, therefore, more likely that instead of breaking away intermittently they might have developed into one mass and then separated from the main body of the nebula.

(ii) It seems probable that cooling and contraction resulting in increased speed of rotation would have developed into two stars of sub-equal masses, describing orbits about each other and not into the solar system like ours.

(iii) Laplace’s assumption that a separated ring would aggregate into one planet has not been fully discussed. It is rather more probable that there would have developed a number of more or less smaller planets in the same orbit.

(iv) The fact that the satellites of Uranus and Neptune move in the opposite direction of the motion of the other bodies of the solar system is sufficient to show that the solar system did not originate in the way as assumed by Laplace.

(v) Laplace could not explain why only nine planets were formed and not more.

(vi) Considering the masses, distances and motions of the bodies of our solar system, it is clear that their present total angular momentum initially found in the nebula is not sufficient for the break up of the nebula by excess of rotation.

It is to be noted that excess of rotation would not throw away a ring until the nebula had shrunk within the innermost planet i.e. Mercury. According to the law of the conservation of angular momentum the angular momentum of parts must be equal to the angular momentum of the whole.

It has been discovered that the angular momentum of the original nebula was not adequate to cause such rapid rotation and so much centrifugal force as to cause the separation of rings of matter. In fact, the angular momentum of all the planets is 98%, while that of the sun is only 2%.

(vii) The present size of the planets is a clear proof of the fact that each planet represents a very small fraction of the total mass of the sun, the parent body. The original angular momentum of individual planets, therefore, must have been a very small fraction of that of the sun.

Thus, the concentration of 98% of the angular momentum in the planets cannot be comprehended in this way also, and this is the most serious objection to the nebular hypothesis of Laplace.