The Origin of the Solar System

In Chapter 1 you met many of the broad features of the Solar System. It is these broad features that any theory of the origin must explain, and this chapter presents the type of theory that is very widely accepted. This is the solar nebular theory, in which the planets form from a disc of gas and dust around the Sun. Such a type of theory also accounts for many of the details of the Solar System, as you will see in subsequent chapters.

You might think that we could deduce the origin of the Solar System by working back from the state in which we observe the Solar System to be today. This cannot be done, for several reasons. First, our knowledge of the present state of the Solar System is incomplete. Second, there are areas of ignorance about the way the Solar System has interacted with its interstellar environment. Third, our understanding of the fundamental physical and chemical processes that operate on all matter, though extensive and deep, is incomplete. Fourth, and most profoundly, even if these three areas of ignorance were eliminated, it would still not be possible to 'reverse time' and deduce the origin. This is because an infinitesimal adjustment in the present state of the Solar System would lead to a very different journey into the past: it is not possible to have sufficiently accurate knowledge to deduce the origin. This is an example of the scientific phenomenon of chaos, and it is a barrier in principle, not just a barrier in practice.

Astronomers must therefore construct theories as best they can, guided by the broad features of the Solar System and by our knowledge of the rapidly growing number of other planetary systems - the exoplanetary systems. Observations of star formation and of young stars are also important, because these increase our understanding of the formation of the Sun, an event that was surely intimately involved in the formation of the rest of the Solar System.

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