Our planet, accordingly, is attached to a safe and steady luminary, one subject, not to destructive spasms, but to vicissitudes so mild as to evade distinct meteorological recognition.[102] It is, moreover, governed by a polity settled on a broad basis of tranquillity and permanence. All this is as it should be. The conditions specified were a pre-requisite to the unfolding of human destinies. Nor can it be confidently asserted that they have been realized anywhere else. Our system may be unique; while, on the other hand, replicas of it might, imperceptibly to us, be profusely scattered through the wide realms of space. It is certain that a telescopic observer on Sirius or α Centauri would see our sun unattended; not even Jupiter could be brought into view by optical appliances in any degree comparable to those at our disposal.
There are, nevertheless, strict limitations to the possible diffusion of planetary worlds like those that wander amid the zodiacal constellations. We have become aware of incapacitating circumstances, by which a multitude of stars are precluded from maintaining retinues of subordinate globes. Spectroscopic discoveries have compelled a revision of ideas as to cosmical arrangements. Especially the large proportion established by them of binary to single stars makes it impossible any longer to regard the solar system as a pattern copied at large throughout the sidereal domain. We cannot, then, compare it with any other; the mechanism of which the earth forms part must, perforce, be studied in itself and by itself, and it may, for aught that appears, be the outcome of special and peculiar design.
The machine in question is self-sustaining and self-regulating; no extraneous power noticeably affects its working. This immunity from disturbance is the fortunate consequence of its isolation. A great void surrounds it. The span of Neptune's orbit is but a hand-breadth compared with the tremendous unoccupied gulf outside—unoccupied, that is to say, by bodies of substantial mass. The feebleness of starlight relatively to sunlight affords some kind of measure of the impotence of stellar attractions to compete with the over-ruling gravitational power that sways the planetary circulation. This it is which gives to it such remarkable stability. The incomparable superiority of the sun over his dependent orbs not only safeguards them against foreign interference, but reduces to insignificance their mutual perturbations. Hence the strong concentration of force exemplified in our system—the absolutely despotic nature of the authority exercised—makes for a settled order by excluding subversive change.
The organization of the solar kingdom, as disclosed by modern research, is greatly more varied and complex than Laplace took it to be. His genetic scheme was, indeed, no sooner promulgated than deviations from the regularity and unanimity of movement upon which it was based began to assert their inconvenient reality. They have since multiplied; and, emerging to notice under the most unlikely aspects, they occasion incongruities which tax, for their explanation, all the resources and audacities of the most inventive cosmogonists. Let us briefly consider their nature.
The swarm of asteroids that bridge the gap between Mars and Jupiter revolve, it is true, with the general swirl of planetary movement; but they use a large license as regards the shape and lie of their orbits. And their partial exemption from the rules of the road becomes entire for comets and meteors, which have nevertheless proved themselves to be aboriginal in our system by their full participation in its proper motion. Finally, several of the major planets set convention at defiance in the arrangement of their several households, and thereby intimate departures from the supposed normal course of development so frequent and so considerable as to shake belief even in its qualified prevalence. Thus, the anomalously short period of the inner satellite of Mars, besides throwing doubt over its own mode of origin, tends to obscure the history of its more sedately circulating associate. Deimos cannot have been thrown off from its primary under conditions materially different from those attending the birth of Phobos.
The sub-systems of Uranus and Neptune exhibit, moreover, eddies of retrograde movement suggesting primitive disturbances of a fundamental kind; while the surprising disclosures connected with Saturn's firstborn and furthest satellite, photographically detected by Professor W. H. Pickering in 1898, have added one more knotted thread to the tangled skein we would fain unravel. Until acquaintance was made with Phœbe, counterflows of revolution within the same satellite-family were unknown, and, if contemplated at all, would have been scouted as impossible. One ternary star, to be sure—ξ Scorpii—had been recognised as probably owning an immediate and a more remote attendant, in oppositely directed orbital movement;[103] but the cases are in many ways disparate, and the analogy, though instructive, is imperfect.
If the ninth Saturnian moon is to be regarded as sprung from the condensing mass of the planet, a total change in the state of the parent body must have supervened during the long interval between its separation and that of its successor, Iapetus. The change, in Professor W. H. Pickering's opinion,[104] was nothing less than a reversal of axial movement. The nebulous spheroid destined to develop into the wonderful Saturnian system had, presumably, when Phœbe became detached from it, a diameter of sixteen million miles, and gyrated tranquilly from east to west, in a period of about a year and a half. But the action of sun-raised tides availed first to destroy and finally to invert this movement; for the natural outcome of tidal friction is synchronism, and this implies agreement, both in period and direction, between the rotation and revolution of the body acted upon. Acceleration through contraction did the rest; and by the time another satellite was ready to separate, the originating globe span normally in seventy-nine days, the actual revolutionary period of Iapetus. The view that such was the course of events is plausible at first sight; yet the doubt remains whether the cause alleged was adequate to the effect produced. At the distance of Saturn, solar tidal friction exerts only about 1/20000 its power on the earth;[105] its efficacy would, on the other hand, be greatly enhanced by the distension of the mass subjected to it; but approximately to what extent, our powers of calculation are impotent to determine.
This is not all. Exhaustive photographic research promises to unfold intricacies of construction in secondary systems demanding the patient industry of many generations for their complete unravelment. The families of the great planets will perhaps be found to include crowds of inferior members which pay slight heed in their circulatory arrangements to the trammels of convention. In those of both Jupiter and Saturn the phenomenon has lately been brought to light of 'asteroidal' satellites, as they may be termed, minute bodies travelling round their primaries at nearly the same mean distances, each group evidently representing the unagglomerated materials of a single full-sized satellite. The pigmy components of such groups doubtless exist in multitudes; each great planet, most likely, is encompassed by at least one zone of moonlets; but so far only specimen-objects have been picked up. The tenth Saturnian satellite, discovered, like its predecessor, by Professor W. H. Pickering, is thus associated, by its period and locality, with Hyperion, the seventh and least prominent of Saturn's visual train, the apparent insignificance of which suggested to Sir John Herschel that it might have many co-occupants of the wide gap between Titan and Iapetus.[106] But the surmise had to await verification until methods were intensified beyond what seemed possible in the middle of the nineteenth century.
The corresponding Jovian pair found by Professor Perrine circulate far outside the boundaries of the original Galilean realm, in orbits which interlock as a consequence of their marked difference in eccentricity.[107] They are mutually inclined at an angle of 27 degrees, nor are they supposed actually to intersect, so that collisions are apparently out of the question. Direct movement is indicated, but cannot yet be claimed to belong quite certainly to both objects. We are only beginning to make acquaintance with the submerged populations of the Saturnian and Jovian kingdoms; they are perhaps multitudinous; they are certainly peculiar, and we await impatiently and curiously the further developments of their remarkable behaviour.
The one certain inference derivable from the diversity of facts ascertained within the last hundred years is that our world is not (so to speak) machine-made. The modus operandi employed to disengage the planets from their nebulous matrix was not of cast-iron rigidity; it was adaptable to circumstances; it left room for the display of boundless inventiveness in details. This was made, nevertheless, to consist with the perfect preservation of the main order, both in design and operation. The general plan is broadly laid down and unmistakable; the springs of the machine are undisturbed in their free play, and for the primary reason that departures from regularity, which might, in any way, prove a menace to stability, affect bodies of negligible mass. The great swing of settled movement goes on irrespectively of them. 'De minimis non curat lex.'