The Origin and Evolution of the Solar System

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xviIntroductionwhich they provide a plausible mechanism for the origin of the solar system andtheir strengths and weaknesses are analysed. The only theory to essay a completepicture of the origin and evolution of the solar system is the Capture Theorydeveloped by the author and colleagues since the early 1960s. This explains thebasic structure of the solar system in terms of well-understood mechanisms thathave a finite probability of having occurred. The way in which planets form, andthe way that their orbits originate and evolve according to the Capture Theory,suggests the occurrence of a major catastrophic event in the early solar system.This event was a direct collision between two early planets, in terms of whichvirtually all other features of the solar system, many apparently disparate, can beexplained. As new knowledge about the solar system has emerged so it has lentfurther support to this hypothesis.There is a tendency in areas of science like cosmogony for a ‘democraticprinciple’ to operate whereby the theory that has the greatest effort devoted to itbecomes accepted, without question and examination, by many people workingin scientific areas peripheral to the subject. These individuals, highly respectedin their own fields, swell the numbers of the apparently-expert adherents and,by a positive feedback mechanism, they enhance the credibility of the currentparadigm—which is the Solar Nebula Theory in this case. Science writers andthose producing radio and television programmes, accepting the verdict of themajority, produce verbal and visual descriptions of an evolving nebula that, ifthey were to illustrate any scientific principle at all, would be illustrating the invalidprinciple of the conservation of angular velocity. In scientific televisionprogrammes material is seen spiralling inwards to join a central condensationhaving jettisoned its angular momentum in some mysterious fashion on the wayin. Computer graphics are not constrained by the petty requirements of science!The ‘democratic principle’ is not necessarily a sound way to determine theplausibility of a scientific theory and there are many examples in the history ofscience that tell us so. The geocentric theory of the solar system, the phlogistontheory of burning and the concept of chemical alchemy were all ideas that persistedfor long periods with the overwhelming support of the scientific communityof the time.The aim of this book has been to present the underlying science as simplyas possible without trivializing or distorting it in any way. None of the importantscience is difficult—indeed most of it should be accessible to a final-year pupilat school. It is hoped that this book will enable those both inside and outside thecommunity of cosmogonists to use their own judgement to assess the plausibility,or otherwise, of the theories described. For those wishing to delve more deeplyinto the subject many references are provided.I must give special thanks to my friend and colleague, Dr John Dormand, forhelp and very useful discussions during the writing of this book. Gratitude is alsodue to Dr Robert Hutchison for providing illustrations of meteorites.
Chapter 1The structure of the Solar System1.1 IntroductionBefore one can sensibly consider the origin of the Solar System it is first necessaryto familiarize oneself with its present condition. Consequently this first chapterwill provide an overview of the main features of the system of planets. The treatmentwill be particularly relevant to the study of solar-system cosmogony. Factorsrelating to the origin of stars and their evolution are left to the next chapter, as isa preliminary discussion of the structure of extra-solar planetary systems.The salient features of the Solar System are split here into five sections,starting with its orbital structure. This exhibits many striking relationships thatare still not fully understood but are now starting to yield to modern celestialmechanics. Secondly, the broad physical characteristics of the planets will beconsidered. The classification of planets into the major and terrestrial categoriesis a key feature here.Most of the planets are themselves accompanied by satellites, thus comprisingmini-systems reminiscent of the Solar System itself. The study of thesesmaller systems has been extremely important in the development of celestial mechanicsand is greatly enhanced by spacecraft data from the outer Solar System.The fourth section will be concerned with the lesser bodies of the system, rangingfrom asteroids with radii up to some hundreds of kilometres down to microscopicparticles that commonly cause meteor trails on entry into the atmosphere. Thevast numbers of smaller bodies ensure frequent collisions with planets and thescars of their impacts are notable features of all solar-system bodies without anatmosphere.The comets, responsible for some of the most spectacular celestial apparitions,will be the topic of the last section of this chapter. Inhabiting the furthestreaches of the Solar System the population of comets is, perhaps, the least wellunderstood feature of the Solar System.The conventional classification of solar-system objects is now challenged byrecent discoveries of remote bodies inhabiting the region beyond Neptune. It is3
Page 1 and 2: The Origin and Evolution of the Sol
Page 5 and 6: ContentsIntroductionxvPART 1The gen
Page 7 and 8: ContentsixPART 2Setting the theoret
Page 9 and 10: Contentsxi7.3 Angular momentum 2257
Page 11: ContentsxiiiPART 4The current state
Page 16 and 17: 4 The structure of the Solar System
Page 23 and 24: Planetary structure 11Figure 1.4. T
Page 25 and 26: Planetary structure 13Figure 1.5. S
Page 27 and 28: Satellite systems, rings and planet
Page 43 and 44: Asteroids 31a precise estimate of t
Page 45 and 46: Asteroids 33Figure 1.17. A schemati
Page 47 and 48: Meteorites 35for in this way. Other
Page 49 and 50: Meteorites 37types of meteorites th
Page 51 and 52: Meteorites 39Figure 1.21. An etched
Page 53 and 54: Comets 41these anomalies contain a
Page 55 and 56: Comets 43Figure 1.23. The structure
Page 57 and 58: Comets 45of the Solar System indica
Page 59 and 60: Stars and stellar evolution 47Figur
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Stars and stellar evolution 53Figur
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The formation of dense interstellar
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The evolution of proto-stars 73Figu
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Observations of star formation 75Be
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Observation of young stars 77Figure
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Theories of star formation 79Figure
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Theories of star formation 81small
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Theories of star formation 83and su
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Planets around other stars 95(a)(b)
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Planets around other stars 97Figure
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Circumstellar discs 99e.g. Vega, ha
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The nature of scientific theories 1
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3.2 Required features of theoriesRe
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Required features of theories 105to
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Required features of theories 107Th
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Chapter 4Theories up to 19604.1 The
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The historical background 113Figure
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The historical background 115Figure
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Buffon’s comet theory 117writing
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The Laplace nebula theory 119Figure
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The Roche model 1214.4 The Roche mo
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The Roche model 123Figure 4.7. A sa
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The Chamberlin and Moulton planetes
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The Jeans tidal theory 127(i) stell
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The Jeans tidal theory 129Figure 4.
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y ×The Jeans tidal theory 131(4.
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The Schmidt-Lyttleton accretion the
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The Schmidt-Lyttleton accretion the
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The major problems revealed 137were
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The major problems revealed 139suma
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Chapter 5A brief survey of modern t
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The Proto-planet Theory 145their ra
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The Capture Theory 147Figure 5.3. T
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The Solar Nebula Theory 1495.4 The
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The Modern Laplacian Theory 151(iii
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The Modern Laplacian Theory 153Figu
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5.6 Analysing the modern theoriesAn
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Formation of the Sun: dualistic the
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Formation of the Sun: monistic theo
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Formation of the Sun: monistic theo
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andFormation of the Sun: monistic t
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Formation of planets 169but, of cou
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Formation of planets 171been brough
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Formation of planets 173Figure 6.5.
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Formation of planets 175Figure 6.8.
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Formation of planets 177Figure 6.10
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Formation of planets 179Table 6.3.
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Formation of planets 185within whic
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Formation of planets 187Table 6.4.
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and the time-scale for the formatio
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Formation of planets 193the main ne
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Formation of satellites 1956.5 Form
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Formation of satellites 197Figure 6
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Formation of satellites 203field pe
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Successes and remaining problems of
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Successes and remaining problems of
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Chapter 7Planetary orbits and angul
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The evolution of planetary orbits 2
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Initial planetary orbits 221Figure
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Initial planetary orbits 223Table 7
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Angular momentum 225Figure 7.9. Var
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Angular momentum 227it is not possi
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The spin axes of the Sun and the pl
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Chapter 8A planetary collision8.1 I
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Interactions between proto-planets
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The Earth and Venus 245being subjec
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The Earth and Venus 247Figure 8.6.
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The Earth and Venus 249Table 8.3. P
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Chapter 9The Moon9.1 The origin of
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The origin of the Earth-Moon system
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The chemistry of the Earth and the
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The chemistry of the Earth and the
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The physical structure of the Moon
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Lunar magnetism 283Figure 9.19. The
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Lunar magnetism 285isted. Another i
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Lunar magnetism 289much smaller tha
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Summary 293The maximum magnetizing
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Mars 295of regular satellites since
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Mars 297Figure 10.1. The densities
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Mars 299Figure 10.2. The structure
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Mars 301Figure 10.4. A model for Ma
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A general description of Mercury 30
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A general description of Mercury 30
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Neptune, Pluto and Triton 307Table
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Neptune, Pluto and Triton 309Table
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Neptune, Pluto and Triton 311where
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Irregular satellites 313It could be
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Summary 315sumed to be close in to
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Meteorites 317and Dodd 1973). This
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Meteorites 319Table 11.1. Percentag
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Meteorites 32111.2.1.2 AchondritesA
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Stony irons 323(a)(b)Figure 11.1. (
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Information from meteorites 325Figu
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Isotopic anomalies in meteorites 32
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Explanations of isotopic anomalies
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Comets—a general survey 355Figure
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Comets—a general survey 357popula
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Comets—a general survey 361the ra
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The inner-cloud scenario 365to have
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Comets from the planetary collision
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Ideas about the origin and features
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374 Comparisons of the main theorie
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Appendix IThe Chandrasekhar limit,
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388 The Chandrasekhar limit, neutro
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390 The Chandrasekhar limit, neutro
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392 The Virial TheoremCombining the
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394 Smoothed particle hydrodynamics
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396 Smoothed particle hydrodynamics
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Appendix IVThe Bondi and Hoyle accr
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400 The Bondi and Hoyle accretion m
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ReferencesAarseth S J 1968 Bull. As
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404 ReferencesFreeman J W 1978 The
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406 ReferencesMullis A M 1991 Geoph
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IndexAannestad P A, 61Aarseth S J,
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410 Indexcirculation, 132circumstel
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412 Indexgrain formation, 347-349Gr
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414 Indexlunarbasalt, , 286highland
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416 Indexdiscovery, 7Great Dark Spo
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418 Indexformation, 145, 148, 195-2
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420 IndexVan Flandern T C, 308, 310
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The Origin and Evolution of the Solar System

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