You are describing the solar nebula theory to a friend. You point out that it provides an explanation for the regular motion in our solar system, the division of the planets into terrestrial and jovian types, and an explanation for the origin and nature of comets and asteroids. Your friend agrees that this is all very nice, but that it seems that the solar nebula theory fails the basic criteria for a scientific theory, to make predictions about the natural world that can be observationally tested. In her opinion, all the solar nebula theory does is explain known facts, albeit in a compelling and economical way. How would you answer your friend and make the case that the solar nebula theory is indeed a scientific theory that has passed many observational tests? Can you think of any predictions that the solar nebula theory has made that were confirmed only within the last decade or so?

We now come to the point where we think "How did this solar system come about?". In a way,we are returning to the thoughts that began the course about what is our cosmology, our view of the universe, based on the observations we can make and our ideas of the physical processes that control the organization and behavior of matter. In this course we have only considered a very small part of the universe, the solar system. On the other hand, it is in our exploration of the solar system that our observations and understanding have come such a long way since the Greek astronomers started us on this road of scientific exploration. So, will consider how the basic properties of the solar system relate to the formation process. As we shall discuss, there is strong evidence that the solar system formed about 4.5 billion years ago and it took about 500 million years for the planets to form in more or less their current location and orbits. In subsequent sessions we shall consider how the planets and other solar system material evolved for the next 4 billion years. The discussion of solar system formation follows the topics.

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