About 200 years ago the French astronomer and mathematician Laplace proposed a model for the formation of the Solar System in which the Sun and the planets condensed from a rotating disk of gas1. This hypothesis, while undergoing many modifications, has survived and forms the basis of our present understanding of the origin of stars and planets. In his wide-ranging 2001 Chesley Bonestell lecture, Dr. Frank Shu of the University of California at Berkeley showed us how the nebular hypothesis of Laplace, originally very unsatisfactory, has grown into a modern theory which not only explains much of what we observe in the Solar System but also provides a link to recent observations of what we believe to be the formation process of stars. The wealth of information presented by Dr. Shu made it quite impossible for this reviewer to take complete notes, much less to reproduce all that he had to say. What follows is thus only a sample from the lecture, containing mainly those items which particularly struck the reviewer.
It appeared, as was shown by Sir George Darwin2, that the hypothesis of Laplace in which the Sun and the planets condensed from the same gas cloud could not be correct, since the distribution of angular momentum throughout the system was quite unrealistic. This difficulty led to the development of the collision hypothesis in which the material of the planets was torn from the Sun by the close passage of another star. Although this alternative solves the angular momentum problem, it leaves so many properties of the planetary system unexplained that, after many years in favor, it has now been largely abandoned. We can rescue the nebular hypothesis if we assume that the Sun and planets did not form from the same original cloud, but that the Sun condensed first and that the planets condensed largely from material that was accreted from interstellar space after the formation of the Sun was well underway. This will also help us to understand the composition of the outer planets. Much of their material may have been brought in by comets, and the dramatic collision of Comet Shoemaker-Levy with Jupiter is an example of what must have been a frequent occurrence during the early history of the Solar System.
Dr. Shu discusses his research with Tom Lougheed of the Friends of MIRAAs we have heard at other recent MIRA talks, there are an increasing number of examples of planets and planetary systems belonging to solar type stars other than the Sun. Many of these are Jupiter-sized planets at Earth-like distances from the stars. Although selection favors discovery of systems like this, their very existence raises questions which must be resolved. The way in which the outer planets may be created is fairly well understood. The creation mechanism for the inner planets is less well understood. But there seems to be no possibility whatsoever for creating a Jupiter-like planet as close to the Sun as the Earth is. Thus the Jupiter-like planets in these close-in orbits must have been formed elsewhere and migrated inward. Dr. Shu himself had already discovered the means by which this could happen when, in the early sixties, he developed, in cooperation with others, the mathematical theory of spiral density waves. The theory was originally developed in order to explain the structure of spiral galaxies, but Dr. Shu himself has applied it to several other situations, such as the structure of the rings of Saturn. One of its consequences is that objects inside a certain critical radius will migrate inward and those outside will migrate outward. An obvious requirement is that there be enough mass in the diffuse matter of the density waves that it will have a significant effect upon other objects in the system. In the early solar system the density was probably not sufficient to move Jupiter far from its original position. But there is no reason to suppose that it was not much greater around some other stars, and it is around these stars, whose Jupiter-sized companions have been moved inward, that we are now discovering planetary systems.
Amongst the many types of meteorite that we recognize, the carbonaceous chondrites have been the most mysterious and are now perhaps the most revealing. These strange objects are made up of nodules of rock or metal imbedded in a matrix of carbon and other relatively volatile elements (see cover illustration). The nodules are clearly the result of a melting process and are all of about the same size. They must have been produced under high temperature, much higher than the carbonaceous matrix could have withstood, so the matrix must have been accreted under cold conditions. Dr. Shu suggests a process in which the Sun, in an early stage of evolution, is located in a disk of dust and various sized particles. The Sun has a magnetic field which transfers angular momentum to the inner part of the disk and sweeps it clear of this material. When a solid particle approaches the inner edge of the disk it emerges from the cover of the dust layer and is suddenly exposed to an intense burst of sunlight. If it is large, nothing much will happen, and it will fall inward toward the Sun. If it is small, it will be evaporated (at least in part), caught up in the magnetic field and ejected from the system. But if it is just the right size, it will be melted but not evaporated and flung outward by the angular force of the magnetic field. The whole process happens very rapidly, in a matter of hours. The field configuration is such that the particles of the right size will often be returned to the outer part of the disk where they will act as accretion nuclei for the volatile material that is plentiful there. When such objects encounter one another they may stick together and form the clumps which we see as the carbonaceous chondrites. It is not surprising that they are scarce.
The above may seem a bit contrived, and it is surely remarkable that four or five billion years later we are able to resolve the timing of events that lasted for one or two hours, but it is supported by such a wealth of observational data that it must be considered the best picture currently available of the early Solar System. The distance between the Sun and the inner edge of the disk, which determines the local temperature and thus the size of the nodules, is strongly dependent upon the magnetic field strength. When a magnetic field is found frozen into one of the nodules, it generally agrees with the field predicted by this model. Observations of solar mass stars in the process of formation show that a magnetic field determines the configuration and motion of the surrounding material, and the required magnetic field is very like the one proposed for the early Solar System. Thus the whole picture has a coherence which, despite its complexity, makes it very attractive as an explanation for the origin of stars, planets, and much more that we observe both inside and outside the Solar System.
Notes
1. Immanuel Kant had made a somewhat similar suggestion some 40 years earlier, but the hypothesis of Laplace appears to have been developed independently.
2. Yes, George Darwin was the son of the biologist Charles Darwin.
| Spring 2002 Contents | Newsletter Index | Mira Home Page |
Last updated 12/14/02 DMC