Department of History
University of California, Irvine
Instructor: Dr. Barbara J. Becker
Week 8. Cosmological Questions.
Natural History: General and Particular, vol I (1749)
Georges Louis Leclerc, Comte de Buffon (1707-1788)
translated by William Smellie (1781)
ARTICLE I. Of the Formation of Planets.
NATURAL history being our subject, we would willingly dispense with astronomical observations. But, as the Earth is so nearly related to the heavenly bodies, and as observations of this kind illustrate more fully those doctrines we have already advanced, it is necessary to give some general ideas of the formation, motion, and figure of the earth, and other planets.
The earth is a globe of about 3000 leagues in diameter; it is situated 30 million of leagues from the sun, round which it revolves in 365 days. The annual revolution is the effect of two forces; the one may be considered as an impulse from right to left, or from left to right; the other as an attraction from above downwards, or from below upwards, to a common centre. The direction and quantity of these forces are combined, and so nicely adjusted, that they produce a uniform motion in an ellipse approaching to a circle. Like the other planets, the earth is opaque, throws out a shadow, and reflects the rays of the sun, about which it revolves in a time proportioned to its relative distance and density. It likewise revolves about its own axis in 24 hours; and its axis is inclined to the plane of its orbit 66 1/2 degrees. Its figure is that of a spheroid, the two axes of which differ from each other about 165th part; and it revolves round the shortest axis.
These are the principal phaenomena of the earth, the results of discoveries made by means of geometry, astronomy, and navigation. It is unnecessary here to enumerate the proofs and observations by which these facts have been established. We shall continue our remarks to such objects as are still doubtful; and shall therefore proceed to give our ideas concerning the formation of planets, and the changes they have undergone, previous to their arriving at the state in which we now perceive them. To the many systems and hypotheses that have been framed concerning the formation of the earth, and the different states it has passed through, we may be allowed to add our own conjectures, especially as we are determined to support them with a superior degree of probability; and we are the more encouraged to deliver our notion on this subject, because we hope to enable the reader to distinguish between an hypothesis which consists only of possibilities, and a theory supported by facts; between a system, such as we are about to give, of the formation and primitive state of the earth, and a physical history of its real condition, which has been giving in the preceding discourse.
Galileo having traced the laws of falling bodies, and Kepler having observed, that the areas which the principal planets describe in moving round the sun, and those of the satellites round the planets to which they belong, were proportioned to the periods of their revolutions, and that these periods were as the square roots of the cubes of their distances from the sun, or from the principal planets. Newton discovered that the power of gravity extended to the moon, and retained it in its orbit; that the force of gravity diminished in exact proportion to the squares of the distances, and, consequently, that the moon is attracted by the earth; that the earth, and all planets, are attracted by the sun; and, in general, that all bodies which revolve about a centre, and describe areas proportioned to the periods of their revolution, are attracted by that luminary. Gravity, therefore, is a general law of nature. The planets, comets, the sun, the earth, are all subject to its laws; and it is the force of that harmony which prevails in the universe. Nothing in physics is better established than the existence of this power in every material body. Repeated experience has confirmed the effects of its influence, and the labour and ingenuity of geometers have determined its quantity and relations.
This general law being once discovered, the effects of it would be easily explained, if the action of those bodies which produce them were not too complicated. A slight view of the solar system will convince us of the difficulties which attend this subject. The principal planets are attracted by the sun, the sun by the planets, the satellites by their principal planets, and each planet attracts all the others, and is attracted by them. All these actions and re-actions vary according to the quantities of matter and the distances, and give rise to great inequalities, and even irregularities. How are so many relations to be combined and estimated? Among such a number of objects, how is it possible to trace any individual? These difficulties, however, have been surmounted; the reasonings of theory have been confirmed by calculation; every observation has produced a new demonstration; and the systematic order of the universe is now laid open to every man who is able to distinguish truth from error.
The force of impulsion, or what is commonly called the centrifugal force, is still unknown; but it affects not the general theory. It is evident, that, as the attractive force continually draws all the planets towards the sun, they would fall in a perpendicular line into that luminary, if they were not kept at a distance by some other power, forcing them to move in a straight line. If, again, this impulsive force were not counteracted by that of attraction, all the planets would fly off in the tangents of their respective orbits. This progressive or impulsive force was unquestionably at first communicated to the planets by the Supreme Being. But, in physical subjects, we ought, as much as possible, to avoid having recourse to supernatural causes; and, I imagine, a probable reason may be assigned for the impulsive force of the planets, which will be agreeable to the laws of mechanics, and not more surprising than many revolutions that must have happened in the universe.
The sphere of the sun's attraction is not limited by the orbits of the planets, but extends to an indefinite distance, always decreasing according as the squares of the augmented distances. The comets, it is evident, which escape our sight in the heavenly regions, are, like the planets, subject to the attraction of the sun, and by it their motions are regulated. All these bodies, the directions of which are so various, move round the sun, and describe areas proportioned to their periods, the planets in ellipses, more or less circular, and the comets, in narrow ellipses of vast extent. The motions, therefore, both of planets and comets, are regulated by impulsive and attractive forces continually acting upon them, and obliging them to describe curves. But it is worthy of remark, that comets run through the system in all directions; that the inclinations of the planes of their orbits are so very different, that though, like the planets, they be subject to the law of attraction, they have nothing in common with regard to their progressive or impulsive motions, but appear, in this respect, to be absolutely independent of each other. The planets, on the contrary, move round the sun in the same direction, and nearly in the same plane.... This similarity in the position and motion of the planets indicates, that their impulsive or centrifugal forces must have originated from one common cause.
May we not conjecture, that a comet falling into the body of the sun might drive off some parts from its surface, and communicate to them a violent impulsive force, which they still retain? This conjecture appears to be as well founded as that of Mr [Gottfried Wilhelm] Leibnitz [1646-1716], which supposed the earth and planets to have formerly been suns; and his system ... would have been more comprehensive, and more consonant to probability, if it had embraced the above idea. We agree with him, that this effect was produced at the time when God is said by Moses to have separated the light from darkness; for, according to Leibnitz, the light was separated from the darkness when the planets were extinguished. But, on our supposition, there was a real physical separation; because the opaque bodies of the planets were detached from the luminous matter of which the sun is composed.
This notion concerning the cause of the centrifugal force of the planets will appear to be less exceptionable, after we have collected the analogies, and estimated the degrees of probability by which it may be supported. We shall first mention, that the motion of the planets have one common direction, namely, from west to east. By the doctrine of chances, it is easy to demonstrate, that this circumstance makes it as 64 to 1, that the planets could not all move in the same direction, if their centrifugal forces had not proceeded from the same cause.
This probability will be greatly augmented, if we take in the similarity in the inclinations of the planes of their orbits ... for, by calculations it has been discovered, that it is 24 to 1 against any two planets being found, at the same time, in the most distant parts of their orbits; and, consequently, 245, or 7692624 to 1, that this effect could not be produced by accident; or, what amounts to the same, there is this great degree of probability, that the planets have been impressed with one common moving force, from which they have derived this singular position.
But nothing could bestow this common centrifugal motion, excepting the force and direction of the bodies by which it was originally communicated. We may, therefore, conclude, that all the planets have probably received their centrifugal motion by one single stroke. Having established this degree of probability, which almost amounts to a certainty, I next inquire what moving bodies could produce this effect; and I can find nothing but comets capable of communicating motion to such vast masses.
Upon examining the course of comets, it is easy to believe that some of them must occasionally fall into the sun. The comet [of] 1680 approached so near, that, at its perihelion, it was not more distant from the sun than a sixteenth part of its diameter; and, if it returns, which is extremely probable, in the year 2255, it may then fall into the sun. This must depend upon the accidents it meets with in its course, and the retardations it suffers in passing through the sun's atmosphere.
We may, therefore, presume, with the great Newton, that comets sometimes fall into the sun. But they may fall in different directions. If they fall perpendicularly, or in a direction not very oblique, they will remain in the body of the sun, serve the purposes of feuel [sic], and, by their impulse, remove the sun from his place, in proportion to the quantity of matter they contain.
But, if a comet falls in a very oblique direction, which will most frequently happen, it will only graze the surface, or penetrate to no great depth. In this case, it may force its way past the sun, detach certain portions of his body, to which it will communicate a common impulsive motion; and these portions pushed off from the sun, and even the comet itself, may turn planets, which will revolve round this luminary in the same direction, and nearly the same plane.
A calculation, perhaps, might be made of the quantity of matter, velocity, and direction, a comet ought to have, in order to force from the sun masses equal to those which compose the six planets and their satellites. But it is sufficient here to observe, that the whole planets, with their satellites, make not a 650th part of the sun's mass; for, although the density of Saturn and Jupiter be less than that of the sun, and though the earth be four times, and the moon near five times more dense than the sun; yet they are only atoms when compared to his immense volume.
It must be acknowledged, that, although a 650th part of a whole may seem inconsiderable, it would require a very large comet to detach this part from the sun. But, if we consider the prodigious rapidity of comets in their perihelion, the near approach they make to the sun; the density and the strong cohesion of parts necessary to sustain, without destruction, the inconceivable heat they undergo; and the solid and brilliant nucleus which shines through their dark atmospheres; it cannot be doubted that comets are composed of matters extremely dense and solid; that they contain, in small limits, a great quantity of matter; and, consequently, that a comet of no enormous size may remove the sun from its place, and give a projectile motion to a mass of matter equal to the 650th part of his body.
This remark corresponds with what we know concerning the respective densities of the planets, which always decrease in proportion to their distances from the sun, having less force of heat to resist.... Now, supposing the quantity of matter in this comet to be equal to a ninth part of the sun, or, allowing it to be only 100dth part of the bulk of the earth, its quantity of matter would still be equal to a 900dth part of the sun: Hence a body of this kind, which would be but a small comet, might push off from the sun a 900dth or a 650th part, especially when the amazing rapidity of comets, in their perihelion, is taken into the calculation.
The correspondence between the density of the whole planets, and that of the sun, deserves also to be noticed.... We may, therefore, conclude, that, in general, the matter of the planets is very nearly of the same kind with the solar matter, and, of course, that the former may have been separated from the latter.
To this theory, it may be objected, that, if the planets had been driven off from the sun by a comet, in place of describing circles round him, they must, according to the law of projectiles, have returned to the same place from whence they had been forced; and, therefore, that the projectile force of the planets cannot be attributed to the impulse of a comet.
I reply, that the planets issued not from the sun in the form of globes, but in the form of torrents, the motion of whose anterior particles behoved [sic] to be accelerated by those behind, and the attraction of the anterior particles would also accelerate the motion of the posterior; and that this acceleration, produced by one or both of these causes, might be such as would necessarily change the original motion arising from the impulse of the comet, and that, from this cause, might result a motion similar to what takes place in the planets, especially when it is considered, that the shock of the comets removes the sun out of its former station.
This reasoning may be illustrated by an example. Suppose a musket-ball discharged from the top of a mountain, and that the force of the powder was sufficient to push it beyond a semidiameter of the earth, it is certain that this ball would revolve round the earth, and return at every revolution to the place from whence it had been discharged. But, instead of a musket-ball, if a rocket were employed, the continued action of the fire would greatly accelerate the original impulsive motion. This rocket would by no means return to the same point, like the ball; but ... would describe an orbit, the perigee of which would be more or less distant from the earth in proportion to the greatness of the change produced in its direction by the accelerating force of the fire. In the same manner, if the original projectile force impressed by the comet on the torrent of solar matter was accelerated, it is probable, that the planets formed by this torrent acquired their circular or elliptical movements around the sun....
[Several objections to this conjecture are presented and answered.]
I acknowledge that I cannot determine which of the causes above assigned has actually produced an alteration in the projectile force of the planets; but they at least show that such a change is not only possible, but probable; and this is enough for my present purpose.
Without farther insisting on the objections that may be made against my hypothesis, or the analogical proofs that might be brought in support of it, I shall prosecute my subject, and draw the proper conclusions. Let us first examine what might happen to the planets, and particularly to the earth, when they were impressed with their projectile forces, and what was their state after their separation from the body of the sun.
A projectile motion having been communicated by the stroke of a comet, to a quantity of matter equal to 650th part of the sun's mass, the light particles would separate from the dense, and, by their mutual attractions, form globes of different solidities. Saturn being composed of the largest and lightest part, would be removed to the greatest distance from the sun; Jupiter, being denser than Saturn, would have a nearer station; and so of the rest. The largest and least solid planets are most distant, because they received a greater projectile force than the smaller and denser; for the projectile force being proportioned to the surfaces to which it is applied, the same stroke would make the larger and lighter parts of the solar matter move with more rapidity than the smaller and heavier....
The comet, by falling obliquely on the sun, as mentioned above, must have forced off from his surface a quantity of matter equal to a 650th part of his body. This matter being in a liquid state, would at first form a torrent, of which the largest and rarest parts would fly to the greatest distances; the smaller and more dense, having received only an equal impulse, would remain nearer the sun; his power of attraction would operate upon all the parts detached from his body, and make them circulate round him; and, at the same time, the mutual attraction of the particles of matter would cause all the detached parts to take on the form of globes, at different distances from the sun, the nearer moving with greater rapidity in their orbits than the more remote.
But, to this it may be objected, that, if the planets had been detached from the sun, they must have been burning and luminous, not cold and opaque bodies; nothing can have less resemblance to a globe of fire than a globe composed of earth and water; and, by comparison, the matter of the earth is totally different from that of the sun.
It may be replied to this objection, that the matter changed its form upon its separation, and that the fire, or light, was extinguished by the projectile motion communicated by the stroke. Besides, may it not be supposed, that the sun, or a burning star, moving with a rapidity equal to that of the planets, would soon be extinguished; and that this may be the reason why all the luminous, or burning stars, are fixed, and with out motion; and why those called new stars, which have probably changed their stations, are frequently extinguished and disappear? To confirm this remark, comets, when in their perihelia, ought to be inflamed even to their center; but they never become luminous stars; they only emit a burning vapour, a considerable portion of which they leave behind them in their course.
In a medium which has little resistance, I acknowledge, that fire may subsist, although the burning body be moved with great rapidity. It must likewise be acknowledged, that what I have said applies only to those stars which disappear for ever, not to those that appear and disappear at stated intervals, without changing their situations in the heavens. Of these M. [Pierre Louis Moreau] de Maupertuis [1698-1759], in his discourse on the figure of the stars, has given a most satisfactory account. But those which have appeared, and then vanished forever, must unquestionably have been extinguished either by the quickness of their motion, or some other cause. There is not a single example of a luminous star revolving round another; and not one of the sixteen planets which revolve round the sun have any light in themselves.
Farther, fire, in small masses, cannot subsist so long as in large ones. The planets would burn a considerable time after they issued from the sun; but, at length, would extinguish for want of combustible matter. For the same reason, the sun itself will be extinguished; but at a period as much beyond that which extinguished the planets, as the quantity of matter in the sun exceeds that of the planets. However this may be, the separation of the planets from the sun, by the shock of a comet, appears sufficient to account for their extinction.
The earth and planets, when they issued from the sun, were totally composed of liquid fire; in which state they would continue no longer than the violence of the heat that kept them in fusion. But this heat would gradually decay from the moment they left the sun. During their fluid state, they necessarily assumed circular figures; and their diurnal motion would elevate their equators, and flatter their poles. I agree with M. Leibnitz, that this figure corresponds so exactly with the laws of hydrostatics, that the earth and planets must necessarily have bee nonce in a state of fluidity occasioned by fire; and consequently, that the interior parts of the earth must be composed of vitrified matter, of which sand, free-stone, granite, and perhaps clay, are fragments, or scoriae.
It is therefore extremely probable, that the planets were originally parts of the sun separated by a stroke which communicated to them a projectile motion; and that their different distances proceeded solely from the difference of their densities. To compleat this theory, it only remains to account for the diurnal motion of the planets, and the origin of their satellites; and this, instead of adding fresh difficulties, will tend greatly to confirm my hypothesis: For rotation, or what is called diurnal motion, entirely depends on the obliquity of the stroke; an oblique impulse on the surface of a body necessarily gives it a rotatory motion. If the body which receives the impulse be homogeneous, the rotatory motion will always be equal and uniform; but it will be unequal, if the body consist of heterogeneous parts, or of parts different in density. Hence we may conclude, that the matter of each planet is homogeneous, because the diurnal motion of each is uniformly performed in the same time; and this circumstance is an additional proof, that portions of different densities were originally separated from the sun.
But the obliquity of the stroke might be so great as to throw off small quantities of matter from the principal planet, which would necessarily move in the same direction. These parts, by mutual attraction, would reunite, according to their densities, at different distances from the planet, follow its course round the sun, and at the same time revolve about the body of the planet, nearly in the plane of its orbit. It is easy to perceive that the portions we mean are the satellites: Thus the formation, position, and motion of the satellites correspond, in the most perfect manner, with our theory; for they all move in the same direction, and in concentric circles round their principal planets, and nearly in the plane of their orbits. All these common effects, depending on an impulsive force, must have proceeded from a common cause, which was a projectile force communicated to them by the same oblique stroke. This account of the motion and formation of the satellites will be strongly supported, if the other circumstances and phaenomena attending them be duly weighted. Those planets that are furnished with satellites, move quickest round their axes. The revolution of the earth is quicker than that of Mars, in the proportion nearly of 24 to 15; the earth has a satellite, and Mars has none; Jupiter, whose diurnal motion is 500 or 600 times more rapid than that of the earth, has four satellites; and it is extremely probable, that Saturn, who has five satellites and a ring, revolves much more quickly than Jupiter.
We may even conjecture, with some probability, that the plane of the equator of Saturn's ring is nearly the same with that of the planet; for, supposing, according to the preceding theory, the obliquity of the impulse which put Saturn in motion to have been very great, his diurnal motion would at first be in proportion to the excess of the centrifugal force above that of gravity, and, of course, a considerable quantity of matter would be thrown off from his equatorial regions, and necessarily assume the figure of a ring, the plane of which would be nearly the same with that of his own equator. This quantity of matter detached from the equatorial regions of Saturn, must have flattened the equator of that planet; which is the reason why, notwithstanding him to revolve round his axis, the diameters of Saturn are not so unequal as those of Jupiter, which differ from each other more than an eleventh part.
Though this theory of the formation of the planets and their satellites appears to be extremely probable; yet, as every man has his own standard of estimating probabilities of this nature, and as this standard varies according to the different capacities of combining analogies more or less remote, I pretend not to convince those who are unwilling to believe. I have offered those ideas to the public, not only because I thought them rational, and calculated to unravel a subject upon which, however important, nothing has hitherto been written; but because the impulsive motion of the planets gives rise to numberless phaenomena in the universe, which admit not of an explanation by gravity alone. To those who may be disposed to deny the possibility of my theory, I would propose the following queries:
1. Is it not natural to imagine, that a moving body has received its motion from the impulse of some other body?