Nebular Hypothesis

At the beginning of the 17th century, Johannes Kepler was able to illustrate that the planets tended to move elliptically. Subsequently, the publication of Isaac Newton's laws of motion and gravitation in 1687 marked the first systematic scientific approach to examining the origin of the solar system. Then, in 1755, the German philosopher Immanuel Kant proposed the theory that the solar system had its beginnings as a cloud of dispersed particles of both dust and gas and that it is a product of centrifugal and centripetal forces. In 1796, [Page 905]the French mathematician Marquis Pierre-Simon de Laplace refined the theory further. He described the original state of the solar system as a hot, rotating nebula. As the mass cooled and contracted, the nebula assumed a flattened shape. The sun was formed at the center, with rings of gaseous material surrounding it. Planets then condensed from the rings. By the same process, moons formed around planets. This theory seems to explain why planets generally move in the same plane and direction.

It was at this point in time that the theory first became known as the “nebular hypothesis.” It has since sometimes been referred to as the Kant/ Laplace nebular theory, because Kant apparently arrived independently at the modified version of the hypothesis about the same time that Laplace did so. Laplace thought that the theory supported the predictability of the universe, while Kant believed that it indicated the universe was likely to change through time.

Though the nebular hypothesis has been examined and modified through the subsequent years by the scientific community, Kant's original brilliant concept can still be said to serve as an important component of current theories on planet formation.

Early in the 20th century, several British and American scientists pointed out definite deficiencies in the nebular hypothesis and proposed that planets were formed by a rare encounter of a star and the sun. In the mid-20th century, these star encounters were shown to be impossible, as the gaseous material involved would naturally dissipate rather than condense as planets. Therefore it was generally concluded that the formation of planets and stars must take place during the same process. Scientists have indeed noted that planets tend to form around newborn stars, and they now refer to the disks of dust and gas they have observed around these stars as “protoplanetary disks.”

Naturally, scientists have always been interested in determining how long it took the universe and specific planets to form. A common theory has been that there are two stages in the formation. It is during the first stage of accretion that small, rocky planets such as Earth form. Solids collide and stick together, with gases forming atmospheres around these smaller planets.

A smaller planet must have the time to grow large enough for the second stage to begin and for larger, gaseous planets to form. There is more limited opportunity for these larger planets to form, because the gas itself might disappear in a few million years. On the other hand, the formation of the smaller planets can continue more slowly for up to 10 or hundreds of million years.

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