The origin of the planets constitutes one of the central problems of astronomy, perhaps the more so as it includes the origin of the Earth itself. The problem has remained a permanent challenge down the years, and has stimulated many attempts which even if not yet entirely successful have led to important new lines of study. Thus the most famous equation in physics, V2(p = 0, was first come upon in 1787 by Laplace in a study of the rings of Saturn, while the theory of rotating gravitating liquids, which has application to the planets, has led to important advances in both dynamics and pure mathematics.

Many lines of evidence show inescapably that not only the Earth but the Sun itself cannot be infinitely old, but we do not yet know if the same holds for the Universe. Radioactivity sets a limit to the age of the Earth of not more than 5000 million years. The question has always been of wide interest whether other solar systems exist moving round other stars, but even if such systems existed in abundance there is but the slenderest possibility of detecting other planets telescopically, even if as large as Jupiter. Only if the mechanism of formation of planets were fully understood might it become possible to know if other stars possessed planetary systems. Yet another reason for study of the problem is to make sure that the Solar System can happen within the laws of science, as we understand them.

We cannot yet be certain that existing laws of nature and knowledge of the contents of space itself are adequate for the problem to be solved. For generations, it was widely believed that the whole history of the world could be encompassed in a time-span that went back only to the year 4004 BC, and the possibility raised by Newton’s work of tracing the planets backwards in time to that date must have given many the thrilling prospect of deter¬mining the circumstances of Creation itself! But it soon became clear how doomed to failure this was, and increasingly so amid the deeper and deeper knowledge of the astronomical world since attained.

The celebrated NEBULAR HYPOTHESIS of Laplace (1749-1827) in some of its features has proved the most enduring perhaps of all, for, after almost a century of excursions into hypotheses of other kinds, most recent theories bear considerable resemblance in broad appearance to the Laplacian scheme. In this hypothesis, it is assumed that the planets condensed from a nebula that itself finally contracted into the primitive Sun. Unbeknown to Laplace, it seems, similar ideas to this had been proposed verbally in less precise form by Swedenborg as early as 1734, and also by the great Kant in 1755 who assumed that such a nebula would heat up as a result of contraction. But he incorrectly supposed that it would develop rotation of its own accord, and do so increasingly rapidly for the nebula to shed a succession of rings at its outer rim that would collect lengthwise into the several planets. Laplace under¬stood the need for endowing the nebula with rotatory momentum |from the outset, and he also assumed that it would take on the form of a lens-shaped distribution distended by the high initial temperature, with subsequent cooling bringing about the necessary contraction and increasing rotational speed. Although these developments were not established with much rigour, the theory obviously held out the possibility of explaining at least the closely circular and co-planar character of the planetary orbits.

The seeming successes of the hypothesis must for a time have shielded it from much criticism, and first serious misgivings seem to have arisen from a study by Maxwell (1831-1879) of the rings of Saturn, a study that showed that a fluid or gaseous ring could not aggregate into a single mass, as had been supposed it could. A wider question, impossible to meet at the time of Laplace or indeed of Maxwell, was that of the origin of the postulated nebula itself, and it is to the answering of this that most modern theories have in the first place been addressed. Another serious difficulty is that if any such nebula extended out to the distance of Neptune and were rotating sufficiently quickly at that stage to have become unstable at its outer edge, the angular momentum of the whole mass would have been so great that the central end-product, the Sun, would have been left rotating very much more rapidly than it does, an unsatisfactory implication that any hypothesis must avoid if it is to be acceptable, as the Sun contains well over 99 per cent of the mass of the whole system.

Following these criticisms, attention began to return to so-called CATASTROPHIC HYPOTHESES. The earliest known attempt on such lines had already been made far earlier by Buffon (1770), who suggested that some great body, inappropriately termed a comet, had collided with the Sun to generate the planets in an unspecified way. In 1880 Bickerton revived the idea, but perceived that only an object of stellar mass could be expected to be capable of removing material from the powerful gravitational field of the Sun. From the appearance of spired nebulae (whose nature was then unknown), Bickerton conjectured that the Sun underwent grazing impact with another star for each to acquire a tongue of gaseous material so heated by the collision as to extend out into the requisite nebula. The orbits of the planets were to be associated with the plane of motion of the two stars in their encounter. A fatal defect attends any such process, however, for although material might be removed from the two stars, such an encounter cannot endow it with more than a minute fraction of the rotatory momentum associated with the great extent of the actual system com¬pared with the size of the Sun: Jupiter moves at 1000 solar radii, and Neptune at about 6000.

Some appreciation of this defect may have moved Chamberlain (1843-1928) and Moulton (1872-1952) to develop their famous PLANETESIMAL THEORY in which the star passes much further out from the Sun, but disturbs it by strong tidal action causing it to erupt a succession of so-called bolts of material to distances comparable with those of the great planets on the side nearer the star, and less violent ejections on the remoter side carrying the bolts to the distances of the terrestrial planets. The inner remains of these bolts went to form the initial cores of the planets, while the outer parts would expand and cool into a vast swarm of infinitesimal solid particles spread out into a disc-like distribution rotating round the Sun in a plane and direction determined by the motion of the passing star. The cores, by gradually gathering in the planetesimals, would round up their initially highly eccentric orbits. The main objection is of the same nature as before in that for serious tidal action to take place the star would need to pass far too close to the Sun to supply the angular momentum requisite for the vast scale of the Solar System.

Not long before this, Jeans (1877-1947) investigated purely tidal action as a means of removing material from the Sun, examining the mechanism mathematically rather than merely asserting verbal conclusions. It was shown that, if a similar star passed within a few diameters of the Sun, so gigantic would be the tidal wave on the solar surface that it would more or less constitute a gaseous arm reaching out towards the other star. From the outer-most tip of this arm would become detached successive blobs of gas, mere droplets compared with the Sun, and these primitive planets would be drawn sideways into orbits about the. Sun by the pull of the receding star. Such planets would inevitably be at high temperature and gaseous to start with, and so only large planets could be formed: planets with as little mass as Mercury and Mars, and even more so the Moon, could not survive in gaseous form but would dissipate into space.

A more general objection was raised by Jeffreys who pointed out that bodies drawn off the Sun by gravitational forces could not rotate much more rapidly than the Sun itself, whereas the majority of planets still do so, some even 50 or 60 times as fast. To overcome this difficulty, a grazing collision was reintroduced, which would lead to a ribbon-like stream stretching out between the stars. In producing this from the outer layers, sliding past each other at hundreds of kilometers a second, turbulent viscosity would generate strong vorticity in the mingling material, and when the stream segmented into embryo planets through internal gravitation, this vorticity would lead to the necessary rapid rotations.

Much invalid criticism was levelled at both the tidal and collision theories on the quite erroneous grounds that the prior probability of so close an encounter was too small to permit such a postulate. But the first person to perceive clearly an insuperable difficulty common to all mechanisms of encounter with the Sun was H.N.Russell (1877-1957). His objection, so simple that it is amazing that it had so long been overlooked, is hi essence the fol-lowing. On any theory, the planetary material must acquire angular momentum far greater than would correspond to any object circling the Sun just outside its surface. But if a particle were so moving, and its velocity doubled by some means, that would certainly double its angular momentum, but it would endow it with such energy that it would escape altogether from the Sun, and not remain bound to it in a planetary orbit. In fact Jupiter possesses over thirty times the angular momentum it would have if in a circular orbit skimming the Sun, and Neptune nearly eighty times as much. Even Mercury could not be put into its present orbit by any single mechanism that removed mass from the Sun.

A simple means of escape from this difficulty was not long & coming with the proposal that, before the existence of the planets the Sun may have been a binary star, examples of which abound in the Galaxy. If the scene of encounter with a third passing star is transferred to the companion star already moving at the distance of the great planets, any material removed from it and from the colliding star will automatically have the necessary angular momentum from the outset. It can be shown too that the en- counter can cause the companion star to escape from the Sun for ever. Whether material so removed by a grazing collision of two stars could immediately aggregate into planets is a question that has never been settled, though the extreme violence of the process would seem to render it doubtful. But it would be sufficient if an amount of order somewhat greater than the total planetary mass were captured in gaseous form, for this on cooling down would surround the Sun with a Laplacian disc of dust and gas. It is to be noticed also that the intrinsic angular momentum would prevent the material falling into the Sun to any appreciable extent, and thus the solar rotation would not be speeded up.

Variants on this theory were later proposed. First, that the companion star to the Sun was itself a very close binary which result of addition of interstellar material, merged into a single star that would automatically be rotationally unstable and undergo such violent break-up as to throw the resulting components apart at speeds far above those needed for escape from the Sun. At the same time, between the two separating components there would be left a mere wisp of material captured by the Sun – less than one per cent would be quite enough – that again would give a nebula from which planets could develop. Another suggestion was that a companion much more massive than the Sun developed to the supernova stage to throw off vast amounts of material at high temperature and containing the heavy elements needed for most of the planets.

Almost all recent discussions adopt the view that the first requirement of any theory is to endow the Sun with a primitive nebula by some orderly rather than catastrophic process. If this happens as part of the mechanism of formation of the Sun itself, then a rapidly rotating primitive Sun as end-product seems inescapable, though the suggestion has been made that this can be dealt with by postulating subsequent enormously strong magnetic coupling to condensations within the nebula. On the other hand, material for such a nebula could simply be captured by the Sun as it moves through interstellar space. Since the Galaxy contains a great many vast clouds of gas and dust, occupying something like 15 per cent of its volume, planetary systems if initiated in this way may be very numerous, even if only a small proportion of stars have successfully undergone such an event.

There is no possibility of planets condensing under gravitational forces within the gaseous component of such a nebula for the simple reason that the volume-density would be far too low. About one per cent of a nebula so derived would consist of dust particles ,or come to contain this proportion if originally entirely gaseous,and this solid material would quickly settle into a thin disc as a result of collisions between the particle. The rings of Saturn are an example of (his: it any particle were not moving strictly in the plane of the ring, its motion perpendicular to that plane would be rapidly damped out by collisions. If the whole of the mass of the four inner planets were spread out into a circular disc extending out just beyond the orbit of Mars, it would be a few centimeters thick. The vital point is that because of the high density of the material of the disc, aggregations can form within it and continue to grow despite the shearing motion of circulation round the Sun. Clumps of material would form almost everywhere in the disc and so great would be the congestion that only when bodies of order 100km in radius had grown could they be spaced out in separate circular orbits. But through perturbations, strictly circular orbits would not be possible, so collisions would continue to occur at low relative speeds and give rise to larger bodies still. The process would end, though never quite completely, with a few large bodies each dominating the original zone of the disc from which its mass had been collected.

Up to the final stage, there would necessarily remain some of the original dust and debris of collisions, always in the form of a disc. This material would provide a kind of dissipative medium tending to round up any non-circular orbits of the large aggregations, and it would provide precisely the conditions in which a body such as the Moon, at first moving as an independent small planet near the orbit of the Earth, could come to be permanently captured. Purely as a result of the combined action of the Sun and Earth, the latter could temporarily capture the Moon in an orbit much larger than the present orbit, but the slightest addition of mass to the Earth or Moon or both at this stage, which would occur from the material of the residual disc, would draw them together and render them permanently bound gravitationally, getting closer as more material showered down on their surfaces. Evidently the material of the disc was effectively exhausted before the Moon was drawn into the Earth completely.

As for the great outer planets, they too could only start to form by growth within a dust-disc. But once a primitive planet had accumulated, at or beyond the distance of Jupiter, and attained a mass about equal to that of the Earth, it would then have a sufficiently strong field to collect in hydrogen and helium and any other gaseous components of the nebula. The temperature of the material at these distances would be much lower than at those where the terrestrial planets formed, and so the thermal speeds would be sufficiently small for such bodies to collect in even the lightest gases and continue to do so until the whole nebula had become exhausted.

The rotations of the planets are readily explicable on any such theory by the vorticity inherent in the material of the disc, and the surrounding gaseous nebula, resulting from the orbital motion about the Sun. This vorticity or spin is in the same sense as the orbital motions, and would mean that the rotation of any aggregation would be about an axis perpendicular to the plane of the disc. Belated collisions and coalescence of large planets could of course act to disturb to some extent the precise directions of the rotation axes. The curious direction of the axis of Uranus, which lies almost in the orbital plane of the planet, suggests that the final stage of formation of this particular planet was by the collision and elision of two bodies, their relative orbital angular momentum of en¬counter, which might be in almost any direction, necessarily becoming part of the rotatory momentum of the combined single mass, and probably the greater part.

An important feature of the process of growth is that, because of the extreme thinness of the disc, material will fall to the surface of a growing planet only in a very narrow great-circle band, and not at all parts of its surface. Only when the mass had reached a value about that of the Moon at present would the energy of infall be enough to liquefy the particles and the surface material stopping them. Up to this point, a growing planet would remain sufficiently cool always to be solid. Even when it reached more than lunar mass, and infalling material caused melting, such material would flow polarwards away from the plane defined by the disc, and spread thinly over the surface. Any theory of the present kind for the origin of planets would imply that the terrestrial planets, and in particular the Earth, began their existence in entirely solid form, the very opposite of what was for so long believed.

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