In the tidal theory the direction of the planets' movements depended on the star that passed: it passed in the plane in which the planets now revolve and in a direction which determined their circling from west to east. But why should the satellites of Uranus revolve perpendicularly to that plane and some moons of Jupiter and Saturn in reverse directions? This the tidal theory fails to explain.
According to all existing theories, the angular velocity of the revolution of a satellite must be slower than the velocity of rotation of its parent. But the inner satellite of Mars revolves more rapidly than Mars rotates.
Some of the difficulties that confront the nebular and tidal theories also confront another theory that has been proposed in recent years.8 According to it, the sun is supposed to have been a member of a double star system. A passing star crushed the companion of the sun, and out of its debris planets were formed. In further development of this hypothesis, it is maintained that the larger planets were built out of the debris, and the smaller ones, the so-called "terrestrial" planets, were formed from the larger ones by a process of cleavage.
The birth of smaller, solid planets out of the larger, gaseous ones is conjectured in order to explain the difference in the relation of
7 Laplace. Theorie analytique des probabilites (3rd ed., 1820), p. lxi; cf. H. Faye, Sur I'Origine du monde (1884), pp. 131-132.
8 By Lyttleton and, independently, by Russell.
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weight to volume in the larger and smaller planets; but this theory is unable to explain the difference in the specific weights of the smaller planets and their satellites. By a process of cleavage, the moon was born of the earth; but since the specific weight of the moon is greater than that of the larger planets and smaller than that of the earth, it would seem to be more in accord with the theory that the earth was born of the moon, despite its smallness. This confuses the argument. The origin of the planets and their satellites remains unsolved. The theories not only contradict one another, but each of them bears within itself its own contradictions. "If the sun had been unattended by planets, its origin and evolution would have presented no difficulty."
•
The Origin of the Comets
The nebular and tidal theories endeavor to explain the origin of the solar system but do not include the comets in their schemes. Comets are more numerous than planets. More than sixty robin-bobin
comets are known to belong definitely to the solar system. These are the comets of short periods (less than eighty years); they revolve in stretched ellipses and all but one do not go beyond the line marked by the orbit of Neptune. It is estimated that, besides the comets of short periods, several hundred thousand comets visit the solar system; however, it is not known for certain that they return periodically. They are seen presently at an approximate rate of five hundred in a century, and are said to have an average period of tens of thousands of years.
A few theories of the origin of comets have been proposed, but aside from one attempt to see in them planetesimals that did not receive a side pull sufficiently strong to bring them into circular orbits,1 no scheme has been developed that explains the origin of the solar system in its entirety, with its planets and comets; yet no cosmic theory can persist which limits itself to the problem of either planets or comets exclusively.
9 Jeans, Astronomy and Cosmogony, p. 395.
1 An attempt to explain the comets, in the frame of the planetesimal theory, as scattered debris of a great wreck, was made by T. C. Chamberlin, The Two Solar Families (1928).
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One theory sees in the comets errant cosmic bodies arriving from interstellar space. After approaching the sun, they turn away on an open (parabolic) curve. But if they happen to pass close to one of the larger planets, they may be compelled to change their open curves to ellipses and become comets of short period.2 This is the theory of capture: comets of long periods or of no period are dislodged from their paths to become short-period comets. What the origin of the long-period comets is remains an unanswered question.
The short-period comets apparently have some relation to the larger planets. About fifty comets move between the sun and the orbit of Jupiter; their periods are under nine years. Four comets reach the orbit of Saturn; two comets revolve inside the circle described by Uranus; and nine comets, with an average period of seventy-one years, move within the orbit of Neptune. These comprise the system of the short-period comets as it is known at present. To the last group belongs the Halley comet, which, among the comets of short periods, has the longest period of revolution—about seventy-six years. Then there is a great gap, after which there are comets that require thousands of years before they return to the sun, if they return at all.
The distribution of the short-period comets suggested the idea that they were "captured" by the large planets. This theory has for its support the direct observation that comets are disturbed on their path by the planets.
Another theory of the comets supposes their origin to have been in the sun, but in a manner unlike that conceived of in the tidal theory of the origin of planets. Mighty whirls on the surface of the sun sweep ignited gases into great protuberances; these are observed daily. Matter is driven off from the sun and returns to the sun. It is calculated
2 That planets are able to change the path of a comet is not only known from observation but has even been calculated in advance. In 1758 Clairaut predicted the retardation of Halley's comet, on its first return foretold by Halley, for a period of 618 days, because it had to pass near Jupiter and Saturn. It was retarded for almost the computed length of time. Similarly, the orbits of other comets were occasionally distorted. LexelTs comet was disturbed by Jupiter in 1767 and in 1770
by the earth, D'Arest's comet was disturbed in 1860, Wolf's comet in 1875 and 1922. By an encounter with Jupiter in 1886, Brook's comet changed its period from 29 years to 7 years; the period of Jupiter was not altered by more than two or three minutes, and probably less.
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that if the velocity of the ejection were to exceed 384 miles per second, the speed of motion in a parabola, the matter would not return to the sun but would become a long-range comet. Then the path of the ejected mass might become perturbed as a result of its passage near one of the larger planets, and the comet would become one of a short period.
Birth of a comet in this manner has never been observed, and the probability that matter in explosion may reach a speed of 384 miles per second is highly questionable. It was therefore robin-bobin
supposed alternatively that millions of years ago, when the activity of their gaseous masses was more dynamic, the large planets expelled comets from their bodies. The speed required for the ejected mass to overcome the gravitational pull of the ejecting body is less in the case of the planets than in the case of the sun, owing to their smaller gravitational pull. It is calculated that a mass hurled from Jupiter at a speed of about 38 miles per second, or at only a little more than a third of this velocity in the case of Neptune, would become expelled.
This variant of the theory neglects the question of the origin of the long-period comets. However, an explanation was offered, according to which the large planets throw the comets that pass close to them from their short orbits into elongated ones, or even expel them entirely from the solar system.
When passing close to the sun, comets emit tails. It is assumed that the material of the tail does not return to the comet's head but is dispersed in space; consequently, the comets as luminous bodies must have a limited life. If Halley's comet has pursued its present orbit since late pre-Cambrian times, it must "have grown and lost eight million tails, which seems improbable." 3 If comets are wasted, their number in the solar system must permanently diminish, and no comet of short period could have preserved its tail since geological times.