Nebular hypothesis of Laplace – Geography Optional Notes

One of the earliest theories on the origin of the Earth is the “Nebular Hypothesis of Laplace,” proposed by the French scholar Laplace. In this hypothesis, Laplace sought to revise Kant’s Gaseous Hypothesis.

Introduction to the Nebular Hypothesis: What Is It?

Laplace’s nebular hypothesis differs from Kant’s with respect to their assumptions about primordial matter. According to Kant, the primordial substance consisted of a cloud or nebula of cold, static matter. In contrast, Laplace proposed that nebulae were made up of hot primordial matter. The Nebular Hypothesis was first suggested by Pierre Simon de Laplace in 1796.

What were the key elements of Laplace’s theory?

Assumptions

1. The assumption was made that there existed a massive and fiery gaseous nebula in space.
2. Right from the start, this colossal and heated nebula spun on its axis.
3. Through the process of radiation, the nebula constantly released heat from its outer surface, resulting in continuous cooling and subsequent contraction, leading to a reduction in size.

Mechanics of the Nebular Hypothesis: How Did It All Come Together?

• Laplace believed that, based on the above-mentioned assumptions, the nebula was composed of gases rather than solids. According to this theory, everything from the sun to stars, planets, and asteroids formed from the nebula cloud.
• At the beginning, there existed a nebula cloud consisting of Helium, Hydrogen, and dust particles, with a size similar to that of the present-day solar system.
• Due to the gradual loss of heat from the outer surface of the nebula through radiation, the nebula continuously decreased in size.
• Consequently, the reduction in the size and volume of the nebula increased the circular velocity of the nebula’s rotatory motion.
• As the velocity increased, the nebula began to spin rapidly, and the centrifugal force eventually overtook the centripetal force.
• As a result, the outer surface, which had cooled excessively, could not continue to rotate with the central nucleus of the still-cooling and contracting nebula.
• The outer ring was thus separated from the rest of the nebula.
• This ring began to revolve around the main nebula.
• Laplace believed that the original ring was divided into nine separate rings, each of which moved away from the outer ring.
• Consequently, nine planets were formed from these rings, while the central nucleus of the original nebula became the Sun.

Evaluating the Nebular Hypothesis: What Evidence Supports and Challenges It?

• The origin of the nebula was not specified in his description.
• The reason for only nine rings emerging from the irregular ring separated from the nebula was not explained.
• If the sun is indeed the remaining nucleus of the nebula, as claimed by Laplace, there should be a small bulge around its midsection that would indicate the probable separation of the irregular ring from the sun. However, no such bulge is present in the sun’s midsection.
• According to the nebular hypothesis, all satellites should revolve in the same direction as their parent planets. However, a few satellites of Saturn and Jupiter revolve in the opposite direction of their parent planets.
• The peculiar distribution of present-day angular momentum in our solar system cannot be explained by the nebular hypothesis.

Legacy of the Nebular Hypothesis

The theory’s strength lies in its ability to provide the most plausible explanation for the layered structure of the Earth’s interior.

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