SP-345 Evolution of the Solar System





How our solar system was formed is a question that today attracts as much interest as the problem of the Creation did in the past. In many theories advocated today, the basic approach to this problem remains remarkably similar to what it was in ancient times: The author hypothetically assumes some specific primordial configuration of matter and then deduces a process from which some significant features of the present state emerge. When the basic assumption is unrelated to actually observed phenomena, chances are that the result will be the same as over thousands of years: a model which, by definition, is a myth, although it may be adorned with differential equations in accordance with the requirements of modern times.

A realistic attempt to reconstruct the early history of the solar system must necessarily be of a different character. It is essential to choose a procedure which reduces speculation as much as possible and connects the evolutionary models as closely as possible to experiment and observation. Because no one can know a priori what happened four to five billion years ago, we must start from the present state of the solar system and, step by step, reconstruct increasingly older periods. This actualistic principle, which emphasizes reliance on observed phenomena, is the basis for the modern approach to the geological evolution of the Earth; "the present is the key to the past." This principle should also be used in the study of the solar system. The purpose of this monograph is to show how this can be done.

We proceed by establishing which experimentally verified laws are of controlling significance in the space environment. To achieve this, we must rely on the rapidly increasing information on extraterrestrial processes that modern space research is providing, and on laboratory studies of these processes under controlled conditions. If the large body of available empirical knowledge is interpreted strictly in terms of these laws, the speculative ingredient of cosmogonic theories can be significantly reduced.

When analyzing the origin and evolution of the solar system, we should recognize that its present structure is a result of a long series of complicated processes. The final aim is to construct theoretical partial models of all [4] these processes. However, there is often a choice between different partial models which a priori may appear equally acceptable. Before the correct choice can be made, it is necessary to define a framework of boundary conditions which these models must satisfy. We consider this to be a main task of this monograph.



Theories of the formation of the solar system must also account for the satellite systems in a manner consistent with the way in which the planetary system is treated. In certain respects the satellite systems provide even more significant information about evolutionary processes than does the planetary system, partly because of the uncertainty about the state of the early Sun.

Observing that the highly regular systems of Jupiter, Saturn, and Uranus are in essential respects similar to the planetary system, we aim at a general theory of the formation of secondary bodies around a primary body. This approach contrasts with that of the Laplacian-type theories in which the postulated processes for planetary formation fail to explain the structure of the satellite systems. Although it is desirable to avoid excessive terminology, we will frequently make brief reference to this specific aspect of our analytical method by using the term hetegony (from the GreekGreek word, companion, and Greek word
, generate).

The theoretical framework we try to construct should, consequently, be applicable both to the formation of satellite systems around a planet and to the formation of planets around the Sun. Through this requirement, we introduce the postulate that these processes are essentially analogous. Our analysis supports this postulate as reasonable. Indeed, we find evidence that the formation of the regular systems of secondary bodies around a primary body- either the Sun or a planet-depends in a unique way on only two parameters of the primary body, its mass and spin. It is also necessary to assume that the central bodies were magnetized, but the strength of the magnetic field does not appear explicitly; it must only surpass a certain limit.



Applying the actualistic and hetegonic principles, we find that the evolutionary history of the solar system can be understood in terms of five stages, in part overlapping in time:


(1) Most recently-during the last three to four billion years-a slow evolution of the primeval planets, satellites, and asteroids which produced [5] the present state of the bodies in the solar system. By studying this latest phase of the evolution (post-accretional evolution), we prepare a basis for reconstructing the state established by earlier processes.
(2) Preceding this stage, an accretional evolution of condensed grains, moving in Kepler orbits to form planetesimals which, by continuing accretion, grow in size. These planetesimals are the embryonic precursors of the bodies found today in the solar system. By clarifying the accretional processes, we attempt to reconstruct the chemical and dynamic properties of the early population of grains.
(3) To account for grains moving in Kepler orbits around the Sun and the protoplanets, transfer of angular momentum from these primary bodies to the surrounding medium must have occurred in the stage of evolution preceding accretion.
(4) Emplacement of gas and dust to form a medium around the magnetized central bodies in the regions where the planet and satellite groups later accreted.
(5) Formation of the Sun as the first primary body to accrete from the source cloud of the solar system.



Each of the five main stages in the sequence discussed above was governed by physical and chemical processes which may be characterized in the following way:


1.4.1. Post-Accretional Evolution; Effects of Tides and Resonances

The most striking result of the analysis of this stage, which has lasted for about four billion years, is that there has been very little change. The Earth-Moon system and the Neptune-Triton system have evolved due to tidal effects, but otherwise the primary-secondary systems exhibit a high degree of stability. This high degree of stability is shown not only by the dynamic state of planets and satellites, but also by certain structures in the asteroidal belt and the Saturnian rings. The complicated pattern of resonances between the bodies in the solar system is probably a major cause of this stability.

Comets and meteoroids are exceptions; they are in a state of rapid change. Information on the changes in these populations can be derived from studies of their orbital characteristics and to some extent can be inferred from the structure and irradiation history of meteorites and lunar materials.

[6] An evolution, although much slower, has also taken place in the asteroid belt, resulting in changes in asteroidal jet streams and families.


1.4.2. Accretional Evolution; Viscosity-Perturbed Kepler Motion and the Evolution of Protosatellites and Protoplanets From Jet Streams

Since the planets and, even more so, the satellites are too small to have formed by gravitational collapse, planetesimal accretion is the only feasible theory of formation; that is, the planets and satellites have been formed by accretion of small bodies (planetesimals and, initially, single grains). The conditions in those regions of space where planets or satellites formed must, at a certain stage in development, have borne similarities to the present state in the asteroidal region. Studies of the present asteroidal region consequently provide information on the processes governing planet and satellite accretion. The isochronism of asteroidal and planetary spin periods gives strong support to the planetesimal model which leads to a promising theory of spin.

The phenomenon basic to a study of accretion of planets and satellites is the Kepler motion perturbed by viscosity (mutual collisions between bodies). It is surprising that in all earlier cosmogonic theories the basic properties of such a state of motion have been misunderstood. It has been believed that a population of mutually colliding grains necessarily diffuse out into a larger volume. This is not correct. Because the collisions are essentially inelastic and the collision frequency less than the orbital frequency, the diffusion of a population in Kepler motion is negative, meaning that the orbits become increasingly similar.

This negative diffusion lends to formation of jet streams, self-focusing streams of bodies orbiting around a gravitating central body. Such jet streams are likely to constitute an intermediate stage in the accretion of celestial bodies.

Focused streams observed today in the asteroidal region and meteor streams may be held together by the same effect. If this is confirmed, studies of the essential properties of jet streams under present-day conditions could reduce the speculative element of hetegonic theories.


1.4.3. Processes Relating to the Angular Momentum Transfer and Emplacement

The motion of a dispersed medium under space conditions can obviously not be treated without hydromagnetics and plasma physics as a basis. The criterion for justified neglect of electromagnetic effects in the treatment of [7] a problem in gas dynamics is that the characteristic hydromagnetic parameter L (defined in eq. (15.1.1)) is much less than unity. In cosmic problems involving interplanetary and interstellar phenomena, L is usually of the order 1015-1020. In planetary ionospheres it reaches unity in the E layer. Planetary atmospheres and hydrospheres are the only domains in the universe where a nonhydromagnetic treatment of fluid dynamic problems is justified.

Nonetheless, the misconception is still common that if only a cosmic cloud is "cold" enough, and stellar radiation is absorbed in its outer layer a nonhydromagnetic treatment is legitimate. In the interior of cold, dark clouds, the factor L is certainly much smaller than in most other regions in space, but ionization by cosmic radiation, by natural radioactivity, and especially by currents associated with magnetic fields which are not curl-free is still sufficient to make it much larger than unity. L may possibly reach values as low as 106 in such environments, but this still means that by ignoring hydromagnetic processes one neglects effects which are many orders of magnitude larger than those considered.

If we assume that the formation of the solar system took place in a cloud of the same general character as the dark clouds observed today, we can get observational indications of the minimum possible effects of hydromagnetic and plasma processes in the hetegonic nebulae. Recent observations of strong magnetic fields and of radio emission from complex molecules in certain dark clouds give clues to the state of matter in these clouds.

In an early nebula where, according to all theories, dispersed matter was dissipating large amounts of energy, the inevitable hydromagnetic effects must have been still more pronounced. A theory of the formation of the solar system is obviously meaningless unless it is based on modern plasma physics and magnetohydrodynamics.

Our analysis shows the controlling phenomena during the emplacement of matter and transfer of angular momentum in the circumsolar region to be as follows:


(1) Critical velocity, a plasma phenomenon which has been studied extensively in the laboratory and also analyzed theoretically. It defines the conditions under which neutral gas falling toward a magnetized central body becomes ionized and stopped. The phenomenon is sufficiently well understood for its importance in cosmic processes to be recognized.

The application of the critical velocity phenomenon suggests an explanation of the mass distribution in the planetary system as well as in the satellite systems. It further accounts for some of the processes of chemical] differentiation indicated by bulk properties of planets and satellites and by the interstellar medium.

(2) Partial corotation, a state of revolution of a plasma surrounding a rotating magnetized body. Evidence for the basic role of partial corotation [8] in transfer of angular momentum in the primordial planetary and solar nebular processes is found in the detailed structure of the Saturnian rings and the asteroidal belt.

Transfer of angular momentum from a central body to a surrounding medium is a process which is fundamental in the formation of secondary bodies revolving around their primary. This process can be studied by extrapolation of present-day conditions in the magnetosphere and in the solar atmosphere. In fact, the electric current system in the auroral region is now known to be of the same type as is necessary for the transfer of angular momentum from a central magnetized body to a surrounding plasma. Hence, this primordial process can be derived from processes now studied by space probes.

(3) Formation and plasma capture of grains. The solid grains in the solar system may have formed by condensation from the nebular plasma emplaced in the circumsolar region. But it is also possible that much of the solid material is interstellar dust condensed in other regions.


Infall of such preexisting grains may have been an important process contributing material to the early solar system. This is suggested by the present-day distribution of dust in dark clouds in interstellar space and possibly by some of the chemical features of the material preserved in meteorites. Since grains in space are necessarily electrostatically charged, small dust particles of transplanetary origin now found in bodies in nearcircular Kepler orbits are likely to have been brought into corotation with the revolving magnetized plasma in the circumsolar and circumplanetary regions.


1.4.4. Origin of the Sun

Theories concerning the origin of the Sun and other stars clearly also must have a foundation in hydromagnetics and plasma physics. Even if appropriately constructed, such theories are by necessity speculative and uncertain. For this reason, in the present study of the formation of the solar system, we do not rely on any initial assumptions concerning the primeval Sun or its history except that during the hetegonic era it existed and was surrounded by a plasma. From our analysis of the solar system, however, we can draw specific conclusions about the primeval Sun: Its mass was approximately the same as today, but its spin and magnetization were much larger.



The completion of a set of quantitative, mutually consistent, and experimentally supported models for the evolution of the solar system is [9] obviously still in the remote future. We need much more information from space data and from laboratory investigation to be able to reduce to a manageable level the speculative element which such models necessarily still contain.

As a first step, we have tried to identify those physical and chemical laws that, at our present state of knowledge, emerge as most important in controlling the processes in the solar system now and in early times. Within the constraints obtained in this way, we have attempted to develop a series of partial models which both satisfy the general principles outlined above and also, when taken together, define what we regard as an acceptable framework for theories of the evolution of the solar system (ch. 23). We construct a matrix (table 23.8.1) complete in the sense that it comprises all the groups of planets and satellites (with the exception of the tiny Martian satellites). The general framework also includes asteroids, comets, and meteoroids.