5. The rings of Saturn revolve about the planet in the same direction.
6. The planetary orbits are all nearly circular.
7. The cometary is distinguished from the planetary portion of the system by several striking characteristics: the orbits of comets are very eccentric and inclined to each other, and to the ecliptic at all possible angles. The motions of a large proportion of comets are from east to west. The physical constitution of the latter class of bodies is also very different from that of the former; the matter of which comets are composed being so exceedingly attenuated, at least in some instances, that fixed stars have been distinctly visible through what appeared to be the densest portion of their substance.
None of these facts are accounted for by the law of gravitation. The sun's attraction can have no influence whatever in determining either the direction of a planet's motion, or the eccentricity of its orbit. In other words, this power would sustain a planetary body moving from east to west, as well as from west to east; in an orbit having any possible degree of inclination to the plane of the sun's equator, no less than in one coincident with it; or, in a very eccentric ellipse, as well as in one differing but little from a circle. The consideration of the coincidences which we have enumerated led Laplace to conclude that their explanation must be referred to the mode of our system's formation—a conclusion which he regarded as strongly confirmed by the contemporary researches of Sir William Herschel. Of the numerous nebulæ discovered and described by that eminent observer, a large proportion could not, even by his powerful telescope, be resolved into stars. In regard to many of these, it was not doubted that glasses of superior power would show them to be extremely remote sidereal clusters. On the other hand, a considerable number were examined which gave no indications of resolvability. These were supposed to consist of self-luminous, nebulous matter—the chaotic elements of future stars. The great number of these irresolvable nebulæ scattered over the heavens and apparently indicating the various stages of central condensation, very naturally suggested the idea that the solar system, and perhaps every other system in the universe, originally existed in a similar state. The sun was supposed by Laplace to have been an exceedingly diffused, rotating nebula, of spherical or spheroidal form, extending beyond the orbit of the most distant planet; the planets as yet having no separate existence. This immense sphere of vapor, in consequence of the radiation of heat and the continual action of gravity, became gradually more dense, which condensation was necessarily attended by an increased angular velocity of rotation. At length a point was thus reached where the centrifugal force of the equatorial parts was equal to the central attraction. The condensation of the interior meanwhile continuing, the equatorial zone was detached, but necessarily continued to revolve around the central mass with the same velocity that it had at the epoch of its separation. If perfectly uniform throughout its entire circumference, which would be highly improbable, it would continue its motion in an unbroken ring, like that of Saturn; if not, it would probably collect into several masses, having orbits nearly identical. "These masses should assume a spheroidal form, with a rotary motion in the direction of that of their revolution, because their inferior articles have a less real velocity than the superior; they have therefore constituted so many planets in a state of vapor. But if one of them was sufficiently powerful to unite successively by its attraction all the others about its center, the ring of vapors would be changed into one spheroidal mass, circulating about the sun, with a motion of rotation in the same direction with that of revolution."[33] Such, according to the theory of Laplace, is the history of the formation of the most remote planet of our system. That of every other, both primary and secondary, would be precisely similar.
In support of the nebular hypothesis, of which the foregoing is a brief general statement, we remark that it furnishes a very simple explanation of the motions and arrangements of the planetary system. In the first place, it is evident that the separation of a ring would take place at the equator of the revolving mass, where of course the centrifugal force would be greatest. These concentric rings—and consequently the resulting planets—would all revolve in nearly the same plane. It is evident also that the central body must have a revolution on its axis in the same direction with the progressive motion of the planets. Again: at the breaking up of a ring, the particles of nebulous matter more distant from the sun would have a greater absolute velocity than those nearer to it, which would produce the observed unity of direction in the rotary and orbital revolutions. The motions of the satellites are explained in like manner. The hypothesis, moreover, accounts satisfactorily for the fact that the orbits of the planets are all nearly circular. And finally, it presents an obvious explanation of the rings of Saturn. It would almost seem, indeed, as if these wonderful annuli had been left by the Architect of Nature, as an index to the creative process.
The argument derived from the motions of the various members of the solar system is not new, having been forcibly stated by Laplace, Pontécoulant, Nichol, and other astronomers. Its full weight and importance, however, have not, we think, been duly appreciated. That a common physical cause has determined these motions, must be admitted by every philosophic mind. But apart from the nebular hypothesis, no such cause, adequate both in mode and measure, has ever been suggested;—indeed none, it seems to us, is conceivable. The phenomena which we have enumerated demand an explanation, and this demand is met by the nebular hypothesis. It will be found, therefore, when closely examined, that the evidence afforded by the celestial motions is sufficient to give the theory of Laplace a very high degree of probability.
A comparison of the facts known in regard to comets, falling-stars, and meteoric stones, seems to warrant the inference that they are bodies of the same nature, and perhaps of similar origin; differing from each other mainly in the accidents of magnitude and density. The hypothesis of Laplace very obviously accounts for the formation of planets and satellites, moving in the same direction, and in orbits nearly circular; but how, it may be asked, can the same theory explain the extremely eccentric, and in some cases retrograde, motions of comets and aerolites? This is the question to which we now direct our attention.
After the nuclei of the solar and sidereal systems had been established in the primitive nebula, and when, in consequence, immense gaseous spheroids had collected around such nuclei, we may suppose that about the points of equal attraction between the sun and neighboring systems, portions of nebulous matter would be left in equilibrio. Such outstanding nebulosities would gradually contract through the operation of gravity; and if, as would sometimes be the case, the solar attraction should preponderate, they would commence falling toward our system. Unless disturbed by the planets they would probably move round the sun in parabolas. Should they pass, however, near any of the large bodies of the system, their orbits might be changed into ellipses by planetary perturbation. Such was the view of Laplace in regard to the origin of comets.
It seems probable, however, that many of these bodies originated within the solar system, and belong properly to it. The outer rings thrown off by the planets may have been at too great distances from the primaries to form stable satellites. Such masses would be separated by perturbation from their respective primaries, and would revolve round the sun in independent orbits. Again: small portions of nebulous matter may have been abandoned as primary rings, at various intervals between the planetary orbits. At particular distances such rings would be liable to extraordinary perturbations, in consequence of which their orbits would ultimately assume an extremely elliptical form, like those of comets, and perhaps also those of meteors. It was shown in Chapter XIII. that several such regions occur in the asteroid zone between Mars and Jupiter. We may add, in confirmation of this view, that there are twelve known comets whose periods are included between those of Flora and Jupiter. Their motions are all direct; their orbits are less eccentric than those of other comets; and the mean of their inclinations is about the same as that of the asteroids. These facts certainly appear to indicate some original connection between these bodies and the zone of minor planets.
The nebular hypothesis, it is thus seen, accounts satisfactorily for the origin of comets, aerolites, fire-balls, shooting-stars, and meteoric rings; regarding them all as bodies of the same nature, moving in cometary orbits about the sun. In this theory, the zodiacal light is an immense swarm of meteor-asteroids; so that the meteoric theory of solar heat, explained in a previous chapter, finds its place as a part of the same hypothesis.