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The Origin of the Solar System: The Capture Theory
Pinner. Spuce Sk., Vol. 38, No. 4, p. 587, 1990 Pergamon Press plc. Printed in Great Britain.
BOOK REVIEW
The Origin of the Solar System: The Capture Theory by R. D~RMAND and MICHAEL M. WOOLFSON. Ellis Horwood Ltd, Chichester (1989). JOHN
This is a notable book about the reason for this journal’s existence. The years since World War II have seen much progress in understanding the evolution of astronomical systems--even though certain things such as the solar neutrino problem or the behaviour of the supernova SN1987a may somewhat shake the confidence of astrophysicists. At least they may claim to have discovered much about when, where and under what physical conditions most components of a galaxy were formed. How these were formed is a totally different case. There is still no consensus as to how any single item was formed out of matter in some essentially different state. Astronomers may know much cosmology, they know no cosmogony. The aim of all basic science is surely to solve problems of origins, The historically oldest and the most famous such problem is the topic of this excellent book. Twenty-five years ago Prof. Woolfson began to present his own proposed solution, now know as the “capture theory” (the book’s subtitle). With the collaboration of students and colleagues, especially his present co-author, Dr Dormand, together with the everexpanding capabilities of computational resources, this has become the most comprehensive and thoroughly explored of all existing theories in this field. The model proposed, in crudest outline, is : about 5 billion years (5 x 109) before the present, a train of events, triggered perhaps by a supernova outburst, produced a certain gravitationally contracting cloud of interstellar matter. Turbulence caused the condensation to be taken over by fragments of the cloud. These became the protostars of a galactic stellar cluster with the Sun as a member. The young Sun then had a close encounter with a still diffuse, less massive, protostar. The tidal action of the Sun pulled a filament of material out of the diffuse star. This broke into six protoplanets captured into highly eccentric orbits about the Sun. In this initial diffuse state these would be tidally distorted in passing through perihelion, resulting in the shedding of material that would condense to become satellites of the forming planets. Their orbits becoming “rounded” by the action of debris in the system, the middle two planets would become Jupiter and Saturn with their “regular” satellites. The two A, B with perihelia nearest the Sun are inferred to have collided. This would have resulted in the break up of A or B to form the terrestrial planets, Mercury, Venus, Earth,
the planet Mars having been an original satellite of A or B, Moon having been another. Triton could have been yet another that was captured by Neptune in a collision resulting in the ejection of an original satellite of Neptune which then became the Pluto-Charon system. Other fragments from the A-B collision could have formed asteroids, comets, meteorites ; the more nearly intact of A, B could have been ejected from the Solar System. Uranus and Neptune are the outermost two, again with “rounded” orbits. Woolfson and his colleagues have established the dynamical feasibility of all this by comprehensive computer modelling. The fashion in which they were led from one development to the next as the work progressed makes it all more impressive to someone who has witnessed its progress than it may seem to a reader presented all at once with the theory in its present state. The book gives an admirably lucid descriptive account of the theory, and of the physics and dynamics behind it, and summarizes the main quantitative conclusions. The account can be comprehended without the mathematics. For this along with some description of the computations, the reader is referred to the original papers. The great value of the book is in showing in a comprehensive way the whole range of phenomena that have to be taken into account in any theory that claims attention. There have been any number of theories that seem to account almost perfectly for some subset of properties of the Solar System, but that offer no hope at all of accounting for other properties. It is very hard at present to say how much of the capture theory should be even provisionally accepted; the authors themselves admit “we can never know”, but they are fully justified in claiming that this theory “is worth taking seriously”. One way of taking it seriously would be to use it as a guide to formulating programmes for future planetary exploration to show what observations may be most crucial in this context. Another way that one is bound to hope may be taken very seriously indeed would be to establish in some U.K. university a centre of Solar System science with special reference to the problem of its origin. As a postscript to a hearty recommendation to read this book, I should wish to recommend also the reading, with the same motivation, of the theory developed by Dr Andrew J. R. Prentice of Monash University in Australia. It also provides a comprehensive outlook upon the range of phenomena to be studied, but from a somewhat different viewpoint. WILLIAM
587
MCCREA
BOOK REVIEW
The Origin of the Solar System: The Capture Theory by R. D~RMAND and MICHAEL M. WOOLFSON. Ellis Horwood Ltd, Chichester (1989). JOHN
This is a notable book about the reason for this journal’s existence. The years since World War II have seen much progress in understanding the evolution of astronomical systems--even though certain things such as the solar neutrino problem or the behaviour of the supernova SN1987a may somewhat shake the confidence of astrophysicists. At least they may claim to have discovered much about when, where and under what physical conditions most components of a galaxy were formed. How these were formed is a totally different case. There is still no consensus as to how any single item was formed out of matter in some essentially different state. Astronomers may know much cosmology, they know no cosmogony. The aim of all basic science is surely to solve problems of origins, The historically oldest and the most famous such problem is the topic of this excellent book. Twenty-five years ago Prof. Woolfson began to present his own proposed solution, now know as the “capture theory” (the book’s subtitle). With the collaboration of students and colleagues, especially his present co-author, Dr Dormand, together with the everexpanding capabilities of computational resources, this has become the most comprehensive and thoroughly explored of all existing theories in this field. The model proposed, in crudest outline, is : about 5 billion years (5 x 109) before the present, a train of events, triggered perhaps by a supernova outburst, produced a certain gravitationally contracting cloud of interstellar matter. Turbulence caused the condensation to be taken over by fragments of the cloud. These became the protostars of a galactic stellar cluster with the Sun as a member. The young Sun then had a close encounter with a still diffuse, less massive, protostar. The tidal action of the Sun pulled a filament of material out of the diffuse star. This broke into six protoplanets captured into highly eccentric orbits about the Sun. In this initial diffuse state these would be tidally distorted in passing through perihelion, resulting in the shedding of material that would condense to become satellites of the forming planets. Their orbits becoming “rounded” by the action of debris in the system, the middle two planets would become Jupiter and Saturn with their “regular” satellites. The two A, B with perihelia nearest the Sun are inferred to have collided. This would have resulted in the break up of A or B to form the terrestrial planets, Mercury, Venus, Earth,
the planet Mars having been an original satellite of A or B, Moon having been another. Triton could have been yet another that was captured by Neptune in a collision resulting in the ejection of an original satellite of Neptune which then became the Pluto-Charon system. Other fragments from the A-B collision could have formed asteroids, comets, meteorites ; the more nearly intact of A, B could have been ejected from the Solar System. Uranus and Neptune are the outermost two, again with “rounded” orbits. Woolfson and his colleagues have established the dynamical feasibility of all this by comprehensive computer modelling. The fashion in which they were led from one development to the next as the work progressed makes it all more impressive to someone who has witnessed its progress than it may seem to a reader presented all at once with the theory in its present state. The book gives an admirably lucid descriptive account of the theory, and of the physics and dynamics behind it, and summarizes the main quantitative conclusions. The account can be comprehended without the mathematics. For this along with some description of the computations, the reader is referred to the original papers. The great value of the book is in showing in a comprehensive way the whole range of phenomena that have to be taken into account in any theory that claims attention. There have been any number of theories that seem to account almost perfectly for some subset of properties of the Solar System, but that offer no hope at all of accounting for other properties. It is very hard at present to say how much of the capture theory should be even provisionally accepted; the authors themselves admit “we can never know”, but they are fully justified in claiming that this theory “is worth taking seriously”. One way of taking it seriously would be to use it as a guide to formulating programmes for future planetary exploration to show what observations may be most crucial in this context. Another way that one is bound to hope may be taken very seriously indeed would be to establish in some U.K. university a centre of Solar System science with special reference to the problem of its origin. As a postscript to a hearty recommendation to read this book, I should wish to recommend also the reading, with the same motivation, of the theory developed by Dr Andrew J. R. Prentice of Monash University in Australia. It also provides a comprehensive outlook upon the range of phenomena to be studied, but from a somewhat different viewpoint. WILLIAM
587
MCCREA