Nebular hypothesis

The first theory of the origin of the earth based upon astronomical observation was proposed by the French astronomer Laplace in 1796. It was probably suggested by the rings now present about the planet Saturn. According to this hypothesis our solar system was originally a vast nebula of highly heated gas, extending beyond the orbit of the outermost planet and rotating in the same direction as the planets now revolve. As this nebula, which was more than five billion miles in extent, lost heat, it contracted. Due to contraction the speed of rotation in-creased and resulted in flattening at the poles and a bulging at the equator. As further contraction continued, the speed of rotation increased until the centrifugal force at the equator of the spheroid was equal to the force of gravity and a ring of particles was left behind. The process continued until 10 successive rings were formed and the central mass became the sun. Each ring revolved as such for a time and then broke up to form a planet and its satellites. From one ring the 1200 or more planetoids between Mars and Jupiter were supposed to have formed.

According to the hypothesis the earth was first a globe of highly heated gas, then it became liquid and with further cooling a crust formed over the liquid interior. From the gas of the original nebula an atmosphere collected around the earth and vapours condensed to form the water of the oceans.

For more than one hundred years this was accepted as the most satisfactory explanation of the earth's origin, regardless of the increasing number of objections arising against it with advance in knowledge of Astronomy and Physics. Many of the objections cannot be stated here, but a few will suffice to show the nature of the difficulties that this simple theory presents.

Laws of Physics indicate that the separation from the gaseous nebula would take place as individual molecules and not as rings. But, granted that rings could form, it is a mystery how contraction of a ring could produce a spheroid or yield other rings to form satellites. Since the parent and its satellites were travelling at the same speed and in the same direction at the time of separation and the parent kept on increasing its rotation by cooling, all the satellites should have a velocity slower than their parents. Some of the satellites move with a velocity too rapid or too slow and one in a direction opposite to that called for by the theory.

Even more serious difficulties are encountered when the moment of momentum is calculated for each stage at which a planet separated or for the entire solar system expanded as a gas beyond the orbit of Pluto. Not only are the masses of the planets out of proportion to the moments of momenta, but also the original nebula with the momentum of the present solar system would not have a rapid enough rotation or a centrifugal force sufficient to form a ring until it had contracted within the orbit of the innermost planet.