When ten years ago man stepped on the moon, even the sceptics must have sensed that the step was only the start of man's march into the realm of planets. An extraordinary realm, to be sure, which unlike things and processes on earth, appeared to man since ancient times as the realm of exactness. The regularity and precision of the motion of planets became ultimately, in the hands of Newton, the clue to a physics which has not ceased unfolding more and more of the exactness that after all rules this globe of ours as well. One of the great triumphs of that physics was the demonstration by Laplace that the system of which our earth is a principal member has a very large measure of stability, though not an absolute one. The solar system, like anything else, is a child of time. It was born and has grown into its present frame which is destined to collapse in the long run. Indeed, it was a famous hypothesis submitted by Laplace that first assured scientific respectability to an evolutionary approach to the system of planets. In almost two centuries following the first publication of Laplace's nebular hypothesis, and especially during the last three generations, much effort has been spent on elucidating the history of the planetary system. That history is still to be traced out in an essentially satisfactory manner.
Such may seem relatively little progress but even less has been made in what should seem a far easier task, namely, the historiography of those efforts aimed at tracing out that history, or the history of theories of the origin of planetary systems. In fact, there seems to exist on this topic no monograph sufficiently detailed and steeped in the reading of the original documents. While some essay-reviews of the state of the art covering the last few decades are of distinct help to the historian for a primary orientation, he should be rather suspicious of accounts concerning the first three decades of this century. Suspicion should yield to distrust as he tries to orient himself from texts and histories of astronomy about the 19th-century part of the story. Finally, he should not be unwilling to take the attitude of plain disbelief with respect to most secondary information about what had been really proposed during the 18th and 17th centuries on the question. This sad record is largely the making of those who in proposing theories on the evolution of the solar system offered a review of what had been said previously, and of those, also mostly scientifically trained men, who drafted histories of such theories. Their cavalier handling of the facts of scientific history may seem a small matter. Yet a reasonably correct presentation of those facts is of paramount importance with respect to the cultural impact of science. A scientific explanation influences culture in the measure in which it is believed, rightly or wrongly, to be the crowning of past developments. As such, it is apt to be invested with the aura of being the long-sought solution to an age-old problem.
It seems unlikely that there should result beneficial cultural effects from a long chain of unfounded assertions that the final solution had been secured. The many theories already proposed on the evolution of the solar system form such a chain. It hardly ever happened over almost four centuries that a theory on the origin of planets was proposed with a genuine touch of diffidence. This diffidence should have been all the more proper as all theories were held together not so much by solid scientific considerations as by the wish to have tile solution. No wonder that all theories were plagued either by the dubious technique of assuming what was to be explained or by a baffling oversight of the bearing on the question of some basic laws of physics. That first-rate scientists could be guilty on either or both counts should give pause to anyone still fancying that a scientific mind, or any mind using the scientific method, is easily safe from errors, let alone from some elementary mistakes in plain logic.
The search for a satisfactory explanation of the evolution of the system of planets has for its most obvious motivation the urge to know. The system of planets is like an extraordinary mountain in the vast array of the phenomena of nature and it exerts as much an overpowering attraction for scientists as do high peaks for mountaineers. The parallel can be extended further. The conquest of a mountain turns it into a well explored terrain, into one of the various types mountaineers contend with. The explanation of the origin of the system of planets also implies finding in it a typical feature. Typically enough, the preference has always been in favour of finding an explanation which provides assurance that systems of planets are typical, that is, regularly occurring features of the universe. Theories, on the basis of which our own planetary system had to be viewed as an exceedingly rare product of the forces of nature, produced, as a rule, considerable uneasiness.
This difference in appraisals derives from an instinctive commitment to the idea of plenitude, according to which living forms of all grades should be present everywhere in the universe. Since planets are the only known possible abodes for life, theories which promised planets in great numbers found not only ready acceptance, but were also, time and again, embellished with copious remarks, even with lengthy sections, on denizens of other planets. Whether this close tie between theories on the evolution of planetary systems and wishful vistas about planetarians helped or obstructed rigour in scientific reasoning should not be difficult to guess. At any rate, as long as such theories are being constructed with an a priori desire to assure high frequency for planetary systems, the chances remain meagre for an objective evaluation of the possibility that a planetary system like ours may be after all an extremely rare phenomenon, a product of a long chain of interactions of very small probability. A careful survey of the history of the question, and particularly of its most recent phase, may suggest precisely this, especially in view of the fact that although a very wide range of mechanisms have been tried, none of them worked.
[SLJ, introduction to Planets and Planetarians]