The Origin and Evolution of the Solar System

IntroductionSince the time of Newton the basic structure of the solar system and the lawsthat govern the motions of the bodies within it have been well understood. Onecentral body, the Sun, containing most of the mass of the system has a family ofattendant planets in more-or-less circular orbits about it. In their turn some ofthe planets have accompanying satellites, including the Earth with its single satellite,the Moon. With improvements in telescope technology, and more recentlythrough space research, knowledge of the solar system has grown apace. Sincethe time of Newton three planets have been discovered and also many additionalsatellites. A myriad of smaller bodies, asteroids and comets, has been discoveredand a vast reservoir of comets, the Oort cloud, stretching out half way towardsthe nearest star has been inferred. Spacecraft reaching out into the solar systemhave revealed in great detail the structures of all the types of bodies it contains—the gas giants, terrestrial planets, comets, asteroids and satellites, both with andwithout atmospheres. At the same time observations of other stars have revealedthe existence of planetary-mass companions for some of them. This suggests thattheories must address the origin of planetary systems in general and not just thesolar system. Observations of young stars have shown that many are accompaniedby a dusty disk and it is tempting to associate these disks with planet formation.In attempting to find a plausible theory the theorist has available not onlyall the observations to which previous reference has been made above but also aknowledge of the basic laws of physics, particularly those relating to conservation.It turns out that finding a theory consistent with both observation of the spins andorbits of solar system bodies and conservation of angular momentum is difficult,and has proved to be an unresolved problem for some current theories. In thisrespect it can be said that for some theories the post-Newtonian knowledge isirrelevant since an explanation of the origin of even the basic simple system, asknown to Newton, has not been found.This book describes the four major theories that have been under developmentduring the last two or three decades: the Proto-planet Theory, the CaptureTheory, the Modern Laplacian Theory and the Solar Nebula theory, and givesthe main theoretical basis for each of them. Also discussed, but not so fully, isthe Accretion Theory, an older model of solar-system formation with some positivefeatures. These theories are examined in detail to determine the extent toxv