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The origin of the solar system
Tectonophysics - Elsevier Printed in The Netherlands
THE
ORIGIN
Publishing
OF THE SOLAR
Company,
Amsterdam
SYSTEM
M.M. WOOLFSON’ Physics
Department,
(Received
College
of Science
and Technology,
Manchester
(Great
Britain)
July 7, 1965)
SUMMARY theory of the origin of the solar system must satisfy a number of constraints imposed by the known properties of the system. Early theories were all simple in form, invo!ving either the contraction of a gaseous nebula or the interaction of two bodies. The difficulties encountered by these early theories, particularly with regard to the distribution of angular momentum in the system, eventually led to the suggestion of more complex systems involving turbulence in a rotating nebula or the interaction of more than two bodies. More recently theories have been favoured in which the sun is assumed to have captured planetary material from a cloud of gas and dust through which it passed. Finally the author describes a theory of his own where the interaction of two stars lead to material from one being captured by the other to form a planetary system. A plausible
INTRODUCTION
Speculation about the origin of the earth is probably as old as man himself. From the legends of primitive tribes to the beliefs of great religions stories are legion concerntng the creation of the earth. However, when the Copernican theory of the solar system was established, it became clear that the origin of the earth could not be considered separately from that of the whole solar system. As the science of astronomy progressed so theories of the origin of the solar system began to be advanced which, even if eventually found to be untenable, were at least based on scientific principles.
THE PROBLEM
TO BE SOLVED
What is required from a theory that it should be considered as plausible? That a cosmologically possible sequence of events should be described which, starttng from some acceptable initial condition, would terminate tn a system with the properties of our own solar system. The most tmportant of these IPresent address: Physics Department, University of York, Great Britain. Tectonophysics,
2 (4) (1965) 333-340
334
M.M. WOOLFSON
properties are listed as follows: (1) The existence of the planets themselves. (2) The planarity of the system. (3) The predominantly direct rotations. (4) The distribution of angular momentum between the planets and the sun. (5) The progression of the orbital radii of the planets. (6) The planetary satellites. (7) The difference in the chemical composition of the earth and the sun. (8) The retrograde rotations of some members of the system. (9) The presence of asteroids, meteorites and comets. Some features are so crucial - e.g., (l), (4) and (7) - that the failure of a theory to give a satisfactory explanation may debar it completely from further consideration. Other features are often treated as only of secondary importance; while some theorists treat them in very general terms,others maintain a discreet silence. To say that a theory should explain a number of the grosser features of the solar system is not to say that the system originated in its present state; a different, even quite random, initial state would be feasible if it could be shown that, by perturbation or other effects, the system would evolve to its present highly-ordered state. Any theory which explained the main features would inspire even greater confidence if it could also explain some of the more subtle features of the solar system or even some’of the anomalies. It is not possible here to give an exhaustive account of theories of the origin of the solar system and of the criticisms which they met. On no other problem can so much imagination have been expended and in no other field can one find so may knowledgeable critics willing, nay even anxious, to repudiate the other man’s ideas. We shall therefore restrict ourselves to describing in outline some of the main theories which have been advanced together with the arguments which have led to their non-acceptance.
THEORIES
OLD AND NEW
One of the most popular of the early theories was the nebula hypothesis of Laplace (1796) which proposed that the planets and the sun originated as a rotating gaseous nebula. As the nebula contracted with constant. angular momentum so conditions were reached where some material acquired sufficient energy to separate from the main body of the nebula. Such material, escaping spasmodically, produced a series of concentric gaseoulf rings from which the planets eventually condensed. The remainder of the nebula, the greatest part of its mass, condensed to form the sun. This theory suffers from two important faults: firstly, it was shown by Maxwell (1859) that a gaseous ring would not condense into a planet and, perhaps of even greater importance, the theory does not explain how 98% of the angular momentum of the system is associated with the planets which contain only a tiny fraction (less than 0.2%) of the total mass. An attempt was made by Roche (1854) to overcome the angular momentum difficulty. He assumed that the nebula had a high central condensation and was rotating as a rigid body. Such a distribution of material could result in a large proportion of the angular momentum of the system being posTectonophysics, 2 (4) (1965) 333-340
THE ORIGIN OF THE SOLAR SYSTEM
335
sessed by a small fraction of the total mass but Jeans (1916) showed that the density of material at the periphery of such a nebula would be so small that planetary condensations could not have formed. There seemed to be no escape from the angular momentum quandary and the idea that the.solar system originated in a nebula form was abandoned for about eighty years. An interesting new idea was introduced in a theory suggested by Birkeland (1912). He proposed that ions emitted by the sun and being under the influence of the solar magnetic field would pursue spiral paths to limiting circles. The radii of these circles would depend on the ratio of ionic charge to mass and thus rings of different ions would be set up. There are many objections to this idea; the accepted compositions of the planets do not agree with the pattern suggested by this mechanism and, in any case, it can be shown that even the ~nermost rings would lie outside the known limits of the solar system . Alfven (1942, 1943) also invoked the solar magnetic field and Berlage (192’7, 1930a) the solar electric field to describe how ionic particles could have produced the planets but it is generally true to say that most astronomers expect the process to have been mechanical and not to have involved fields other than those due to gravitation. The failure of the nebula hypothesis led to the in~o~ction of a number of so-called “tidal theories”. Such a theory, somewhat elemental in form, had been advanced by Buffon (1745) who thought in terms of the collision of a “comet” with the sun. However, the first tidal theory to be seriously considered was that of Chamberlin (1901) and Moulton (1905) who proposed that the sun was disrupted by the tidal action of a passing star. The material so removed acquired angular motion round the sun due to the sideways pull of the star and later small condensed portions of this material (called planetisimals by Chamberlin and Moulton) coalesced to form the planets. Another variety of tidal theory was advanced separately by Jeffreys (1918) and Jeans (1919). Both these workers proposed that the planets were formed more-or-less directly from the removed solar material. Jeans pictured the material as removed in the form of a long cigar-shaped filament which later condensed at various points along its length to form the planets. The shape of the filament has the merit of providing a rough explanation of the relative sizes of the planets. However where Jeans envisaged that disruption of the sun took place when the sun extended to beyond the orbit of Neptune, Jeffreys considered the sun to have been much more highly condensed and approximately in its present state. With Jeans’s model it is difficult to understand how the inner planets could have been produced; either the initial orbits had to intersect the body of.the sun or the orbits had to reduce in dimensions by large and almost inexplicable amounts. On the other hand, it was shown by Russell (1935) and, with the aid of an electronic computer, by Lyttleton (1960) that if the sun had been disrupted while in its present state (as assumed by Jeffreys) then it would have been impossible to produce planets at distances from the sun of more than a few, say four, solar radii. Another very serious objection to tidal theories was advanced by Spitzer (1939) who showed that if sufficient material had been ejected from the sun to form the planets it would have been at such a high average temperature (ca. 10s OK ) that it would have dissipated violently into space rather than condensed into planets. This objection was so discouraging that Tectonophysics,
2 (4) (1965) 333-340
336
M.M.
WOOLFSON
tidal theories were no longer seriously considered. More recently an even Stronger reason has been advanced for discounting any theory which takes hot solar material as the origin of planets. Lithium, beryllium and boron are consumed by nuclear reactions at high temperatures and these elements are fairly abundant in the earth’s crust. There followed a number of what can be termed “catastrophic theories” which were advanced to overcome Russell’s objections. Lyttleton (1936) suggested that the sun was originally one member of a binary pair, the other member of which was struck by an invading star. The colliding stars could both have left the vincity of the sun leaving behind enough material to form the planets. Later Lyttleton (1941) considered the possibility that the sun could have had two companions which collided and left the sun’s vicinity. Hoyle (1946) suggested a super-nova explosion as a means of removing a companion of the sun and leaving behind potential planetary material. These catastrophic theories would all seem to be open to the objections which are due to the high temperature of the original planetary material. Other difficulties, regarding the masses of the planets so formed, have been raised by Jeffreys (1952).’ In any case such theories seem, at best, to explain only the presence of material to form the planets and very little else. There has been a recent consolidation of opinions in favour of the idea that planetary systems are common and a natural consequence of the evolution of some types of stars. This is supported by some observational evidence it has been found for example that the star 61 Cygni moves in a way which shows that it has a dark companion with mass about fifteen times that of Jupiter. Many astronomers have therefore concluded that, on the basis of the density of stars in our galaxy, stellar endounters would be so rare that to attribute the formation of planetary systems to such-events would be unreasonable There have been several recent attempts to revive nebula theories. These mostly consider the nebula in the form of a rotating disk and all have as their main feature some explanation of the observed distribution of angular momentum. One of the most widely known of these theories is attributable to Weizsacker (1944). In this it is assumed that the rotation of the disk nebula would give rise to vortices which would act as centres of planetary formation. Jefffeys (1952) doubts whether turbulent motion could arise in the way described. In any case the angular momentum difficulty is only overcome if the original planets were very large compared with their present size and if the excess material was lost from the solar system altogether. Jeffreys (1952) has pointed out that this material could not possibly have left the system completely and that if it had been reabsorbed into the sun then the angular momentum problem is brought back all over again. An ingenious explanation of how the angular momentum difficulty can be overcome has been advanced by Hoyle (1955). He suggested that as the central part of the nebula condensed in a spherical form and as the outer part, in the shape of an annular disk, expanded, so a gap was formed between the two parts. The action of magnetic field across this gap is presumed to have permitted the transfer of angular momentum and Hoyle describes how this might have occurred in such a way that the ring gained angular momentum at the expense of the central nebula. Hoyle admits that the theory leaves a number of difficulties to be resolved; Lyttleton (1961) asserts that it is very doubtful whether magnetic fields of the type required actually occur. The next and final class of theories that we shall consider may be Tectonophysics, 2 (4) (1965) 333-340
THE ORIGIN OF THE SOLAR SYSTEM
337
called “accretion theories”. Schmidt (1958) described a process whereby if a star has passed near the sun whilst they were both moving through a cloud of dust and gas then the sun could have captured material from the cloud, Lyttleton (1961) has suggested that the presence of the star was not necessary for the capture process if the relative speed of the sun and cloud had been sufficiently small. These theories might be open to the objection that there seems to be no reason for the captured material to coagulate into planets; however, Lyttleton considers that non-elastic collisions between “sticky” particles could bring about this process., This type of theory does have the advantage of postulating a cold origin of the planetary material. The above survey of theories is by no means comprehensive but it does indicate the main lines of approach to the problem. It is probably fair to say that at present there is no completely acceptable and accepted theory of the origin of the solar system. Theories which attempt to answer one problem are usually overcome by another and the consideration of increasingly more complex and less understood physical processes does not seem to have ameliorated the position. A theory which has been advanced by the author (Woolfson, 1964) appears to solve the most pressing of the problems while not invoking complicated physical processes to do so. Explanations of many .of the finer details of the solar system also seem possible in terms of the proposed model. This theory is now briefly described.
THE CAPTURE
THEORY
This theory describes the process by which a light diffuse star may be tidally disrupted by the sun to provide the material for the planets. The manner of the distortion and break-up of the light star is considered both theoretically and by computation with a model star. It is concluded that the star would have ejected material in the form of a filament, that condensations within the filament could have produced planets and that these planets would have been captured by the sun. A schematic representation of the process, based on the computed results, is shotin in Fig.1. Many of the snags which beset tidal or indeed other types of theory do not apply to this one. We may note that: (1) Russell’s and Lyttleton’s objections concerning the maximum orbital radii no longer apply. Planets are protluced naturally within the required range of distances from the sun; in the case of the detailed computation given in the author’s paper (Woolfson, 1964) planets are produced in orbits with perihelia between 1.4 and 38.4 astronomical units. (2) There is no difficulty concerning the distribution of angular momentum. That of the planets is derived directly from the orbital motion of the star while that of the sun is derived mainly from a comparatively small quantity of stallar material which it captures. (3) The star was in a cool state. This obviates Spitzer’s objection and also satisfies the chemical requirements that the earth has a cpld origin. ’ (4) The approach and recession of the star would have given an increasing and then decreasing rate of loss of stellar material thus giving the largest planets in the middle of the system. The difficulty which appears to remain is that of probability and the Tectonophysics, 2 (4) (1965) 333-340
338
M.M. WOOLFSON
Protoplanets r\
C
Fig.1. A representation of the stages in the process of planetar formation due to the interaction of two stars. A. Time = 0. B. Time = 2.1 d sec. C. Time = 4*10gsec. D. Time = 6.108 sec. idea that interaction theories must lead to a small occurrence of planetary systems. This argument, based on the observed density of stars in our own environment, is not really valid. It is accepted that stars are born in clusters through the fragmentation of large dust-gas clouds, typically of some 1,000 solar-mass units. Some of the larger stars, condensing quickly, will heat up the intersellar medium and generate random motions which will gradually cause the cluster to disperse. However,while the stars are comparatively close together there should be a great deal of interaction between them. This could give rise to planetary systems or, in the many-bodied environment, to binary systems. The binary and planetary systems could then be broken up by a further interaction but nevertheless there will probably be some rough dynamic equilibrium established and when the cluster has finally dispersed a number of binary and planetary systems would persist. The computations referred to have been carried out with many sets of trial parameters and, under a wide variety of conditions, with various masses and densities of star and different interaction orbits, the results have indicated that a planetary system would result. The author proposes to show in future publications how the development of the system leads to many other features such as satellites, asteroids, Tectonophysics,
2 (4) (1965) 333-340
THE ORIGIN
OF THE SOLAR
SYSTEM
339
and some of the anomalies. However this theory, like all others before it is vulnerable to criticism, as any respectable theory ought to be, and the fact that it is unchallenged at this time may be no more than a sign of its recent origin. CONCLUSIONS
It is doubtful whether there will ever be a theory of the origin of the solar system which is universally accepted. A great difficulty is that the system has probably developed so much from its original form, due to perturbation effects and perhaps to the influence of a once-dense interplanetary medium, that one cannot be certain what the original state would have been. If only this was known it would certainly help astronomers to deduce the mechanism of formation of the system. Another difficulty is that there is only one planetary system on which we can make detailed observations. If there were several systems which could be examined than it would be possible to decide which features were the most characteristic and this would be a vital clue as to the mechanis, of their production. The situation with regard to postulating new theories or examining old ones has changed radically in the last two decades. The advent of the electronic computer makes feasible the examination of the properties of proposed mechanisms with a detail hitherto out of question. While it may not be possible to define a set of conditions which would unmistakably indentify a correct theory it is not difficult to define the contrainfs which the correct theory must satisfy. Better observation of the members of the solar system, perhaps made with satellite telescopes or space probes may improve our knowledge of the system to the point where the contraints are so severe that it is improbable that any theory other than the correct one could satisfy them all. REFERENCES Alfven, H., 1942. On the cosmology of the solar system. Ann. Stockholm Obs., 14 (2): l-33. AlfvCn, H., 1943. On the cosmology of the solar system.11. Ann. Stockholm Gbs., 14 (3): 1-3: Berlage, H.P., 1927. Versuch einer Entwicklungsgeschichte der Planeten. Beitr. Geophys., 17, l-68. Berlage, H.P., 1930. On the structure and internal motion of the gaseous disc constituting the original state of the planetary system. Proc. Koninkl. Ned. Akad. Wetenschap., Proc. Ser. B., 35: 553-562. Birkeland, K., 1912. Sur i’origine de planetes et de leurs satellites. Compt. Rend., 155: 89%895. Buffon, G.L.L., 1745. De la formation des plan&es. In: Oeuvres Complets. 1. Adolf Deros. Bruxelles. 1852. rm.126-140. Chamberlin; T.C., 19OI. On a possible function of disruptive approach in the formation of meteorites, comets, and nebulae. Astrophvs. J. 14: 17-40. Hoyle, F., 1946. The synthesis of the elements from-hydrogen. Monthly Notices Roy. Astron. Sot., 106: 34*383. Hoyle, F., 1955. Frontiers of Astronomy. Harper, New York, 360~~. Jeans, J.H., 1916. The part played by rotation in cosmic evolution. Monthly Notices Roy. Astron. Sot., 77: 186199. Jeans, J.H., 1919. Problems of Cosmology. Cambridge Univ. Press, London, 293 pp.
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Jeffreys, H., 1918. Early history of the solar system. Monthly Notices Roy. Astron. Sot., 78: 424-442. Jeffreys, H., 1952. Bakerian lecture. The origin of the solar system. Proc. Roy. Sot. London, Ser. A, 214: 281-291. Laplace, P.S., 1796. Exposition du Systeme clu Monde. Imprimerie Cercle-Social, Paris. Lyttleton, R.A., 1936. The origin of the solar system. Monthly Notices Roy. Astron. Sot. 96: 55’9-568. Lyttleton,‘R.A., 1941. The origin of the solar system. Monthly Notices Roy. Astron. Sot., 101: 216-226. Lyttleton, R.A., 1960. Dynamical calculations relating to the origin of the solar system. Monthly Notices Roy Astron. Sot., 121: 551-569. Lyttleton, R.A., 1961. An accretion hypothesis for the origin of the solar system. Monthly Notices Roy Astron. Sot., 122: 399-407. Maxwell, J.C., 1859. On the stability of the motion of Saturn’s rings. Cambridge Univ Press. London. Moulton. F.R.. 190.5. On the evolution of the solar system. Astrophys. J., 22: 16.ilXl. Roche, E., 1854. La figure dcs atmospheres. Mem. Acad. Montpellier. 2: 39’5-439. Russell, H.N., 193.7. The Solar System and its Origin. Macmillan. Nc\\ York. N.Y., 144 pp. Schmidt, A., 1958. A Theory of the Earth’s Origin.-Foreign Languages Publishing House, Moscow, 133 pp. Spitzer, L., 1939. The dis*Fipation of planetary filaments. Astrophys. J., 90: 675-688. Weizsacker, C.F., 1944. Uber die Entstehung des Planeten systems. Z. Astrophys., 22: 319-355. Woolfson, M.M., 1964. A capture theory of the origin of the solar system. Proc. Roy. Sot. London, Ser. A, 282, 485-507.
Tectonophysics,
2 (4) (1965) 333-340
THE
ORIGIN
Publishing
OF THE SOLAR
Company,
Amsterdam
SYSTEM
M.M. WOOLFSON’ Physics
Department,
(Received
College
of Science
and Technology,
Manchester
(Great
Britain)
July 7, 1965)
SUMMARY theory of the origin of the solar system must satisfy a number of constraints imposed by the known properties of the system. Early theories were all simple in form, invo!ving either the contraction of a gaseous nebula or the interaction of two bodies. The difficulties encountered by these early theories, particularly with regard to the distribution of angular momentum in the system, eventually led to the suggestion of more complex systems involving turbulence in a rotating nebula or the interaction of more than two bodies. More recently theories have been favoured in which the sun is assumed to have captured planetary material from a cloud of gas and dust through which it passed. Finally the author describes a theory of his own where the interaction of two stars lead to material from one being captured by the other to form a planetary system. A plausible
INTRODUCTION
Speculation about the origin of the earth is probably as old as man himself. From the legends of primitive tribes to the beliefs of great religions stories are legion concerntng the creation of the earth. However, when the Copernican theory of the solar system was established, it became clear that the origin of the earth could not be considered separately from that of the whole solar system. As the science of astronomy progressed so theories of the origin of the solar system began to be advanced which, even if eventually found to be untenable, were at least based on scientific principles.
THE PROBLEM
TO BE SOLVED
What is required from a theory that it should be considered as plausible? That a cosmologically possible sequence of events should be described which, starttng from some acceptable initial condition, would terminate tn a system with the properties of our own solar system. The most tmportant of these IPresent address: Physics Department, University of York, Great Britain. Tectonophysics,
2 (4) (1965) 333-340
334
M.M. WOOLFSON
properties are listed as follows: (1) The existence of the planets themselves. (2) The planarity of the system. (3) The predominantly direct rotations. (4) The distribution of angular momentum between the planets and the sun. (5) The progression of the orbital radii of the planets. (6) The planetary satellites. (7) The difference in the chemical composition of the earth and the sun. (8) The retrograde rotations of some members of the system. (9) The presence of asteroids, meteorites and comets. Some features are so crucial - e.g., (l), (4) and (7) - that the failure of a theory to give a satisfactory explanation may debar it completely from further consideration. Other features are often treated as only of secondary importance; while some theorists treat them in very general terms,others maintain a discreet silence. To say that a theory should explain a number of the grosser features of the solar system is not to say that the system originated in its present state; a different, even quite random, initial state would be feasible if it could be shown that, by perturbation or other effects, the system would evolve to its present highly-ordered state. Any theory which explained the main features would inspire even greater confidence if it could also explain some of the more subtle features of the solar system or even some’of the anomalies. It is not possible here to give an exhaustive account of theories of the origin of the solar system and of the criticisms which they met. On no other problem can so much imagination have been expended and in no other field can one find so may knowledgeable critics willing, nay even anxious, to repudiate the other man’s ideas. We shall therefore restrict ourselves to describing in outline some of the main theories which have been advanced together with the arguments which have led to their non-acceptance.
THEORIES
OLD AND NEW
One of the most popular of the early theories was the nebula hypothesis of Laplace (1796) which proposed that the planets and the sun originated as a rotating gaseous nebula. As the nebula contracted with constant. angular momentum so conditions were reached where some material acquired sufficient energy to separate from the main body of the nebula. Such material, escaping spasmodically, produced a series of concentric gaseoulf rings from which the planets eventually condensed. The remainder of the nebula, the greatest part of its mass, condensed to form the sun. This theory suffers from two important faults: firstly, it was shown by Maxwell (1859) that a gaseous ring would not condense into a planet and, perhaps of even greater importance, the theory does not explain how 98% of the angular momentum of the system is associated with the planets which contain only a tiny fraction (less than 0.2%) of the total mass. An attempt was made by Roche (1854) to overcome the angular momentum difficulty. He assumed that the nebula had a high central condensation and was rotating as a rigid body. Such a distribution of material could result in a large proportion of the angular momentum of the system being posTectonophysics, 2 (4) (1965) 333-340
THE ORIGIN OF THE SOLAR SYSTEM
335
sessed by a small fraction of the total mass but Jeans (1916) showed that the density of material at the periphery of such a nebula would be so small that planetary condensations could not have formed. There seemed to be no escape from the angular momentum quandary and the idea that the.solar system originated in a nebula form was abandoned for about eighty years. An interesting new idea was introduced in a theory suggested by Birkeland (1912). He proposed that ions emitted by the sun and being under the influence of the solar magnetic field would pursue spiral paths to limiting circles. The radii of these circles would depend on the ratio of ionic charge to mass and thus rings of different ions would be set up. There are many objections to this idea; the accepted compositions of the planets do not agree with the pattern suggested by this mechanism and, in any case, it can be shown that even the ~nermost rings would lie outside the known limits of the solar system . Alfven (1942, 1943) also invoked the solar magnetic field and Berlage (192’7, 1930a) the solar electric field to describe how ionic particles could have produced the planets but it is generally true to say that most astronomers expect the process to have been mechanical and not to have involved fields other than those due to gravitation. The failure of the nebula hypothesis led to the in~o~ction of a number of so-called “tidal theories”. Such a theory, somewhat elemental in form, had been advanced by Buffon (1745) who thought in terms of the collision of a “comet” with the sun. However, the first tidal theory to be seriously considered was that of Chamberlin (1901) and Moulton (1905) who proposed that the sun was disrupted by the tidal action of a passing star. The material so removed acquired angular motion round the sun due to the sideways pull of the star and later small condensed portions of this material (called planetisimals by Chamberlin and Moulton) coalesced to form the planets. Another variety of tidal theory was advanced separately by Jeffreys (1918) and Jeans (1919). Both these workers proposed that the planets were formed more-or-less directly from the removed solar material. Jeans pictured the material as removed in the form of a long cigar-shaped filament which later condensed at various points along its length to form the planets. The shape of the filament has the merit of providing a rough explanation of the relative sizes of the planets. However where Jeans envisaged that disruption of the sun took place when the sun extended to beyond the orbit of Neptune, Jeffreys considered the sun to have been much more highly condensed and approximately in its present state. With Jeans’s model it is difficult to understand how the inner planets could have been produced; either the initial orbits had to intersect the body of.the sun or the orbits had to reduce in dimensions by large and almost inexplicable amounts. On the other hand, it was shown by Russell (1935) and, with the aid of an electronic computer, by Lyttleton (1960) that if the sun had been disrupted while in its present state (as assumed by Jeffreys) then it would have been impossible to produce planets at distances from the sun of more than a few, say four, solar radii. Another very serious objection to tidal theories was advanced by Spitzer (1939) who showed that if sufficient material had been ejected from the sun to form the planets it would have been at such a high average temperature (ca. 10s OK ) that it would have dissipated violently into space rather than condensed into planets. This objection was so discouraging that Tectonophysics,
2 (4) (1965) 333-340
336
M.M.
WOOLFSON
tidal theories were no longer seriously considered. More recently an even Stronger reason has been advanced for discounting any theory which takes hot solar material as the origin of planets. Lithium, beryllium and boron are consumed by nuclear reactions at high temperatures and these elements are fairly abundant in the earth’s crust. There followed a number of what can be termed “catastrophic theories” which were advanced to overcome Russell’s objections. Lyttleton (1936) suggested that the sun was originally one member of a binary pair, the other member of which was struck by an invading star. The colliding stars could both have left the vincity of the sun leaving behind enough material to form the planets. Later Lyttleton (1941) considered the possibility that the sun could have had two companions which collided and left the sun’s vicinity. Hoyle (1946) suggested a super-nova explosion as a means of removing a companion of the sun and leaving behind potential planetary material. These catastrophic theories would all seem to be open to the objections which are due to the high temperature of the original planetary material. Other difficulties, regarding the masses of the planets so formed, have been raised by Jeffreys (1952).’ In any case such theories seem, at best, to explain only the presence of material to form the planets and very little else. There has been a recent consolidation of opinions in favour of the idea that planetary systems are common and a natural consequence of the evolution of some types of stars. This is supported by some observational evidence it has been found for example that the star 61 Cygni moves in a way which shows that it has a dark companion with mass about fifteen times that of Jupiter. Many astronomers have therefore concluded that, on the basis of the density of stars in our galaxy, stellar endounters would be so rare that to attribute the formation of planetary systems to such-events would be unreasonable There have been several recent attempts to revive nebula theories. These mostly consider the nebula in the form of a rotating disk and all have as their main feature some explanation of the observed distribution of angular momentum. One of the most widely known of these theories is attributable to Weizsacker (1944). In this it is assumed that the rotation of the disk nebula would give rise to vortices which would act as centres of planetary formation. Jefffeys (1952) doubts whether turbulent motion could arise in the way described. In any case the angular momentum difficulty is only overcome if the original planets were very large compared with their present size and if the excess material was lost from the solar system altogether. Jeffreys (1952) has pointed out that this material could not possibly have left the system completely and that if it had been reabsorbed into the sun then the angular momentum problem is brought back all over again. An ingenious explanation of how the angular momentum difficulty can be overcome has been advanced by Hoyle (1955). He suggested that as the central part of the nebula condensed in a spherical form and as the outer part, in the shape of an annular disk, expanded, so a gap was formed between the two parts. The action of magnetic field across this gap is presumed to have permitted the transfer of angular momentum and Hoyle describes how this might have occurred in such a way that the ring gained angular momentum at the expense of the central nebula. Hoyle admits that the theory leaves a number of difficulties to be resolved; Lyttleton (1961) asserts that it is very doubtful whether magnetic fields of the type required actually occur. The next and final class of theories that we shall consider may be Tectonophysics, 2 (4) (1965) 333-340
THE ORIGIN OF THE SOLAR SYSTEM
337
called “accretion theories”. Schmidt (1958) described a process whereby if a star has passed near the sun whilst they were both moving through a cloud of dust and gas then the sun could have captured material from the cloud, Lyttleton (1961) has suggested that the presence of the star was not necessary for the capture process if the relative speed of the sun and cloud had been sufficiently small. These theories might be open to the objection that there seems to be no reason for the captured material to coagulate into planets; however, Lyttleton considers that non-elastic collisions between “sticky” particles could bring about this process., This type of theory does have the advantage of postulating a cold origin of the planetary material. The above survey of theories is by no means comprehensive but it does indicate the main lines of approach to the problem. It is probably fair to say that at present there is no completely acceptable and accepted theory of the origin of the solar system. Theories which attempt to answer one problem are usually overcome by another and the consideration of increasingly more complex and less understood physical processes does not seem to have ameliorated the position. A theory which has been advanced by the author (Woolfson, 1964) appears to solve the most pressing of the problems while not invoking complicated physical processes to do so. Explanations of many .of the finer details of the solar system also seem possible in terms of the proposed model. This theory is now briefly described.
THE CAPTURE
THEORY
This theory describes the process by which a light diffuse star may be tidally disrupted by the sun to provide the material for the planets. The manner of the distortion and break-up of the light star is considered both theoretically and by computation with a model star. It is concluded that the star would have ejected material in the form of a filament, that condensations within the filament could have produced planets and that these planets would have been captured by the sun. A schematic representation of the process, based on the computed results, is shotin in Fig.1. Many of the snags which beset tidal or indeed other types of theory do not apply to this one. We may note that: (1) Russell’s and Lyttleton’s objections concerning the maximum orbital radii no longer apply. Planets are protluced naturally within the required range of distances from the sun; in the case of the detailed computation given in the author’s paper (Woolfson, 1964) planets are produced in orbits with perihelia between 1.4 and 38.4 astronomical units. (2) There is no difficulty concerning the distribution of angular momentum. That of the planets is derived directly from the orbital motion of the star while that of the sun is derived mainly from a comparatively small quantity of stallar material which it captures. (3) The star was in a cool state. This obviates Spitzer’s objection and also satisfies the chemical requirements that the earth has a cpld origin. ’ (4) The approach and recession of the star would have given an increasing and then decreasing rate of loss of stellar material thus giving the largest planets in the middle of the system. The difficulty which appears to remain is that of probability and the Tectonophysics, 2 (4) (1965) 333-340
338
M.M. WOOLFSON
Protoplanets r\
C
Fig.1. A representation of the stages in the process of planetar formation due to the interaction of two stars. A. Time = 0. B. Time = 2.1 d sec. C. Time = 4*10gsec. D. Time = 6.108 sec. idea that interaction theories must lead to a small occurrence of planetary systems. This argument, based on the observed density of stars in our own environment, is not really valid. It is accepted that stars are born in clusters through the fragmentation of large dust-gas clouds, typically of some 1,000 solar-mass units. Some of the larger stars, condensing quickly, will heat up the intersellar medium and generate random motions which will gradually cause the cluster to disperse. However,while the stars are comparatively close together there should be a great deal of interaction between them. This could give rise to planetary systems or, in the many-bodied environment, to binary systems. The binary and planetary systems could then be broken up by a further interaction but nevertheless there will probably be some rough dynamic equilibrium established and when the cluster has finally dispersed a number of binary and planetary systems would persist. The computations referred to have been carried out with many sets of trial parameters and, under a wide variety of conditions, with various masses and densities of star and different interaction orbits, the results have indicated that a planetary system would result. The author proposes to show in future publications how the development of the system leads to many other features such as satellites, asteroids, Tectonophysics,
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THE ORIGIN
OF THE SOLAR
SYSTEM
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and some of the anomalies. However this theory, like all others before it is vulnerable to criticism, as any respectable theory ought to be, and the fact that it is unchallenged at this time may be no more than a sign of its recent origin. CONCLUSIONS
It is doubtful whether there will ever be a theory of the origin of the solar system which is universally accepted. A great difficulty is that the system has probably developed so much from its original form, due to perturbation effects and perhaps to the influence of a once-dense interplanetary medium, that one cannot be certain what the original state would have been. If only this was known it would certainly help astronomers to deduce the mechanism of formation of the system. Another difficulty is that there is only one planetary system on which we can make detailed observations. If there were several systems which could be examined than it would be possible to decide which features were the most characteristic and this would be a vital clue as to the mechanis, of their production. The situation with regard to postulating new theories or examining old ones has changed radically in the last two decades. The advent of the electronic computer makes feasible the examination of the properties of proposed mechanisms with a detail hitherto out of question. While it may not be possible to define a set of conditions which would unmistakably indentify a correct theory it is not difficult to define the contrainfs which the correct theory must satisfy. Better observation of the members of the solar system, perhaps made with satellite telescopes or space probes may improve our knowledge of the system to the point where the contraints are so severe that it is improbable that any theory other than the correct one could satisfy them all. REFERENCES Alfven, H., 1942. On the cosmology of the solar system. Ann. Stockholm Obs., 14 (2): l-33. AlfvCn, H., 1943. On the cosmology of the solar system.11. Ann. Stockholm Gbs., 14 (3): 1-3: Berlage, H.P., 1927. Versuch einer Entwicklungsgeschichte der Planeten. Beitr. Geophys., 17, l-68. Berlage, H.P., 1930. On the structure and internal motion of the gaseous disc constituting the original state of the planetary system. Proc. Koninkl. Ned. Akad. Wetenschap., Proc. Ser. B., 35: 553-562. Birkeland, K., 1912. Sur i’origine de planetes et de leurs satellites. Compt. Rend., 155: 89%895. Buffon, G.L.L., 1745. De la formation des plan&es. In: Oeuvres Complets. 1. Adolf Deros. Bruxelles. 1852. rm.126-140. Chamberlin; T.C., 19OI. On a possible function of disruptive approach in the formation of meteorites, comets, and nebulae. Astrophvs. J. 14: 17-40. Hoyle, F., 1946. The synthesis of the elements from-hydrogen. Monthly Notices Roy. Astron. Sot., 106: 34*383. Hoyle, F., 1955. Frontiers of Astronomy. Harper, New York, 360~~. Jeans, J.H., 1916. The part played by rotation in cosmic evolution. Monthly Notices Roy. Astron. Sot., 77: 186199. Jeans, J.H., 1919. Problems of Cosmology. Cambridge Univ. Press, London, 293 pp.
Tectonophysica,
2 (4) (1965) 333-340
M.M. WOOLFSON
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Jeffreys, H., 1918. Early history of the solar system. Monthly Notices Roy. Astron. Sot., 78: 424-442. Jeffreys, H., 1952. Bakerian lecture. The origin of the solar system. Proc. Roy. Sot. London, Ser. A, 214: 281-291. Laplace, P.S., 1796. Exposition du Systeme clu Monde. Imprimerie Cercle-Social, Paris. Lyttleton, R.A., 1936. The origin of the solar system. Monthly Notices Roy. Astron. Sot. 96: 55’9-568. Lyttleton,‘R.A., 1941. The origin of the solar system. Monthly Notices Roy. Astron. Sot., 101: 216-226. Lyttleton, R.A., 1960. Dynamical calculations relating to the origin of the solar system. Monthly Notices Roy Astron. Sot., 121: 551-569. Lyttleton, R.A., 1961. An accretion hypothesis for the origin of the solar system. Monthly Notices Roy Astron. Sot., 122: 399-407. Maxwell, J.C., 1859. On the stability of the motion of Saturn’s rings. Cambridge Univ Press. London. Moulton. F.R.. 190.5. On the evolution of the solar system. Astrophys. J., 22: 16.ilXl. Roche, E., 1854. La figure dcs atmospheres. Mem. Acad. Montpellier. 2: 39’5-439. Russell, H.N., 193.7. The Solar System and its Origin. Macmillan. Nc\\ York. N.Y., 144 pp. Schmidt, A., 1958. A Theory of the Earth’s Origin.-Foreign Languages Publishing House, Moscow, 133 pp. Spitzer, L., 1939. The dis*Fipation of planetary filaments. Astrophys. J., 90: 675-688. Weizsacker, C.F., 1944. Uber die Entstehung des Planeten systems. Z. Astrophys., 22: 319-355. Woolfson, M.M., 1964. A capture theory of the origin of the solar system. Proc. Roy. Sot. London, Ser. A, 282, 485-507.
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