The earth is but one of nine planets which make up our solar system. But to humans Earth is by far the most important of all planets, for it is our home. It is also, so far as we know, the only planet which supports any type of life. Whence came the earth? How and when did it all begin? People have speculated on such questions since the beginning of recorded history. This problem, one that is still unsolved, has resulted in the development of a number of hypotheses-none of which is entirely satisfactory.
The Nebular Hypothesis. This hypothesis suggests that the solar system developed from a nebula-a vast disc-shaped cloud of gas. This idea 
was first proposed in 1755 by the German philosopher Immanuel Kant. Later, in 1796, the French mathematician Pierre Laplace developed this theory more fully and stated it in more scientific terms. It is interesting to note that these men arrived at similar conclusions, although Laplace was not aware of Kant’s earlier work. This hypothesis assumes, in brief, that at some period in the distant past, a great nebula-its diameter reaching beyond the orbit of Pluto (our most distant planet)-was slowly rotating in space. As this gaseous mass cooled, it shrank and rotated more and more rapidly. Eventually the outermost part of the nebula rotated so fast that centrifugal force overcame gravitational force and a ring of gas separated itself from the equatorial region of the parent body. The nebula continued
to contract and the speed of rotation increased until a total of ten rings had been thrown off. Nine of these rings slowly condensed to form our planets. The sixth ring, rather than condensing into a single body, broke up into many small masses. These small bodies formed the planetoids, and the central mass of the nebula
later condensed to form the sun (Fig. 116). The Laplacian hypothesis was quite popular and gained much scientific support in the nineteenth century. Unfortunately, later research proved this theory to be untenable and it was abandoned in the early twentieth century. There are numerous objections to this hypothesis, but
the most important is that this mechanism would not work because the sun rotates too slowly in comparison with the rest of the planets.
The Planetesimal Hypothesis. The planetesimal hypothesis was proposed in 1900 by Thomas C. Chamberlin, a geologist, and Forest R. Moulton, an astronomer, both from the University of Chicago.
According to this hypothesis, the sun was originally a star which existed without planets.
At some time in the remote past, another star passed very close to the sun, exerting a gravitational force great enough to tear great masses of material from opposite sides of the sun. As the matter pulled out from the sun it cooled and condensed into solid particles called planetesimals. The largest of these planetesimals acted as nuclei which attracted other planetesimals, and by accretion the planets slowly grew to their present sizes-each pursuing its own orbit around the sun. It is believed that the five major planets were produced from the sun matter tom from the side closest to the passing star, and the minor planets and planetoids were formed from smaller masses of sun material derived from the opposite side. The satellites formed from small clusters of planetesimals located near the nuclei from which the planets originated.
Although this hypothesis was rather widely accepted for several decades, a number of geological and astronomical objections have been raised to this idea. For example, much of what we know of the structure of the earth suggests that it was originally in a molten condition. However, Chamberlin and Moulton postulated an originally solid planet. In addition, there is some doubt that the planetesimals could have gathered together by accretion-the collision of these particles in outer space would probably have destroyed them.
The Tidal or Gaseous Hypothesis. This hypothesis, like the planetesimal hypothesis which preceded it, also involves an original sun that had a close encounter with a passing star. Known also as the tidal disruption or tidal filament hypothesis, this conception of how our solar system formed was proposed in 1918 by two British scientists, Sir James Jeans and Sir Harold Jeffreys. Jeans, an astronomer, and Jeffreys, a geophysicist, offered this proposal to counteract some of the objections that had been raised to the planetesimal hypothesis. They accepted the supposed near-collision between the sun and another star but believed that the material pulled out of the sun came out as a long spindle or cigar-shaped filament of solar gases. This gaseous filament later broke up into units which condensed to a molten and finally a solid stage, thus forming the planets. Astronomers have shown that a gaseous filament of this sort would not form solid bodies such as our planets; it would instead simply disappear in space. For this and many other reasons, this hypothesis is no longer acceptable to most scientists.
The Protoplanet Hypothesis. Within recent years scientists have taken a closer look at a possible nebular origin of the earth and devised a new theory in the light of newer findings. This idea has replaced the original nebular hypothesis of Kant and Laplace as well as the later planetesimal hypothesis. The revised theory, known as the protoplanet hypothesis, was first proposed in 1944 by C. F. von Weizsacker and modified by Gerald P. Kuiper. It has been found that rapidly rotating nebulas will develop large whirlpools or vortexes at various places on the disk of nebular material. Each of these great whirlpools might then have collected the surrounding material by gravitational attraction, thus forming a protoplanet (Fig. 118). It is believed that nine protoplanets –one for each of the present-day planets—were formed, and these were originally much larger than the finished planet. Smaller whirlpools developed inside some of the larger vortexes, and these gave rise to spinning discs that became the satellites, or moons, of the planets. Many astronomers support this theory because observations through large telescopes have revealed numerous true nebulas between the stars. Equally significant is the fact that some of these massive swirls of gas and dust are actually condensing to form new stars. The protoplanet hypothesis is generally acceptable to most scientists because it explains many of the known facts about the solar system. It is nonetheless far from complete, and the origin of the solar system and Earth is still largely in the realm of speculation.
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