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Nebular Hypothesis

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, 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.

It has been speculated that the disks around smaller planets disappear after 3 to 5 million years. Yet that time period may well be too lim­ited to permit the formation of larger planets, such as Jupiter. A number of scientists now accept the theory that other solar systems with similar sized planets must be fairly common (and planets larger than Jupiter have actually been noted), but more research is needed to attempt to explain exactly how Jupiter—and all the other planets— did form.

Two theories of how the gas giants formed have been proposed. “Core accretion” would result in these giants forming relatively slowly, because a large, solid core would be necessary to attract a large quantity of gas. “Disk instability” proposes that a cold disk could break up on its own if it is cold and dense enough, resulting in gravitational abilities. The latter process could produce protoplanets in only hundreds or thou­sands of years.

The original concept that the nebular hypothe­sis involved a disk forming around a condensed center is still held regarding what is now com­monly referred to as the Solar Nebula. As gas and dust collapsed toward the center, kinetic energy was formed and the temperature rose to the point of producing a nuclear reaction and the subse­quent birth of the sun.

Theories have also been developed about the differences between the inner and outer planets. Their size appears to involve how much water is available and whether planets are far enough away from the sun for ice to form. Planets located at a far enough distance from the sun can acquire more solid mass and attract large amounts of many other elements, including the abundant ele­ments hydrogen and helium. These larger planets formed beyond what is referred to as the “snow line.” As is the case with many credible theories, there have been questions about whether or not this process is inevitable. Discoveries and observa­tions of other solar systems, such as 51 Pegasi, indicate enough definite differences from our solar system to raise further questions about exactly how planets are formed.

Current efforts to verify or modify the body of knowledge about how planets are formed are employing many avenues for research. Some, such as Richard H. Durisen’s attempt to update Laplace’s theory, make use of computer simula­tions while still identifying dense gas rings as the mechanism of planet formation. Other research focuses on areas beyond our own solar system, such as the Hubble Space Telescope’s production of images of protoplanetary disks around stars in the Orion Nebula—about 1,600 light-years away. Continuing observation by astronomers is likely to yield greater understanding of the process of planet formation.

Betty A. Gard

See also Kant, Immanuel; Laplace, Marquis Pierre-Simon de; Planets; Stars, Evolution of; Telescopes

Further Readings

Durisen, R. H. (2005). Rings of creation. Mercury, 34(3), 12-19.

Schilling, G. (1999). From a swirl of dust, a planet is born. Science, 286(5437), 66-68.

Weintraub, D. A. (2000). How do planets form?

Mercury, 29(6), 10.

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