CHAPTER V
FORMATION OF PLANETS
IN our first two chapters we saw what sign-posts in the sky there are pointing to the course evolution of a solar system probably follows, and secondly, what evidence there is that our system took this road. We now come to a question not so easy to precise,—the actual details of the journey. It is always difficult to descend from a glittering panoramic survey to particular path-finding. The obstacles loom so much larger on a near approach.
Most men shy at decisions and shun self-committal to any positive course, but when it comes to constructing a cosmogony, few at all qualified hesitate to frame one if the old does not suit. The safety in so doing lies in the fact that nothing in particular happens if it refuses to work. Its absurdity is promptly shown up, it is true, by some one else. For there is almost as good a trade in exposing cosmogonies as in constructing them. But no special opprobrium attaches to failure, because everybody has failed, from Laplace down, or up, as you are pleased to consider it. Besides it is really not so easy to do, as one is tempted to believe before his book is published. Then only does the difficulty dawn, with a speed and clarity inversely proportional to the previous relation of the critic to the author. For the author himself is apt to be blind. With the fatal fondness of a parent for his offspring it is rare for the defects to be so glaringly apparent to their perpetrator. At the worst he considers them venial faults which can be glossed away.
Attacking the subject in this judicial spirit, the reader can hardly expect me to satisfy him with a cosmogony entirely home-made, but at best to pursue a happy middle course between creator and critic, advocating only such portions as happen to be my own, while sternly exposing the mistakes of others.
In undertaking the hazardous climb toward the origin of things two qualities are necessary in the explorer: a quick eye for possibilities and a steady head in testing them. Without the discernment to perceive relations no ascent to first principles is possible; and without the support of quantitative criterion, one is in danger of becoming giddy from one’s own imagination. Congruities must first hint at a path; physical laws then determine its feasibility.
An eye for congruities is the first essential. For congruity alone accuses an underlying law. It is the analogic that with logic leads to great generalizations. Certain concords of the sort in the motions of the planets were what suggested to Laplace his system of the world. With the uncommon sense of a mathematician he perceived that such accordances were not necessitated by the law of gravitation, and on the other hand, could not be due to chance. The laws of probability showed millions to one against it. One of these happy harmonies was that all the large planets revolved about the Sun in substantially the same plane; another that they all travelled in the same sense (direction). Had they been unrelated bodies at the start, such agreement in motion was mathematically impossible. Their present consensus implied a common origin for all. In other words, the solar system must have grown to be what it is, not started so.
This basic fact we may consider certain. But from it we would fain go on to find out how it evolved. To do so the same process must be followed. Considering, then, our solar system from this point of view, one cannot but be struck by some further congruities it presents. These are not quite those that inspired Laplace, because of discoveries since, and demand in consequence a theory different from his.
The out about constructing a theory is that fresh facts will come along and knock for admission after the door is shut. They prove irreconcilables because they were not consulted in advance. The consequence is that since Laplace’s time new relations have come to light, and some supposed concords have had to be given up; so that were he alive to-day he would himself have formulated some other scheme. Two, however, are still as true: that the planets all revolve in the same plane and in the same sense, and that sense that of the Sun’s rotation. But so general a congruity as this points only to an original common moment of momentum and is equally explicable however that motion was brought about. It seems quite compatible with an original shock. To say that it was caused by a disruption is simply to go one step farther back than Laplace. If, then, such a catastrophe did occur as the meteorites aver, we may perhaps draw some interesting inferences about it from the present state of the system. In a very close approach such as we must suppose for the disruption, one within Roches’ limit of 2.5 diameters, the stranger, supposing him of equal size, would sweep from one side of the former Sun to the other in about two hours, and the brunt of the disrupting pull occur within that time. That the former Sun was rotating slowly seems established by the time, twenty-eight days, it now takes to go round. In which case the orbits of the masses which were to form the planets would all lie in about the same plane,—the plane of the tramp’s approach. If there were exceptions, they should be found in the innermost. For such should partake most largely of the Sun’s own original rotation and travel therefore most nearly in its plane. And as a fact Mercury, the Benjamin, does differ from the others by revolving in a plane inclined some 7° to their mean, agreeing in this with the Sun’s own rotation, with whose plane it was probably originally coincident (digression from it now being due to secular retrogression of the planets’ nodes) [[see NOTE 4]].
From the relations which advance has left unchanged we pass to those phenomena which seemed to present congruities in Laplace’s day, but which have since proved void owing to subsequent detection of exceptions. Time prevents my making the catalogue complete, but the reader shall be shown enough to satisfy him of the problem’s complexity and to whet his desire for further research—on the part, preferably, of others.