 In how old are stars, we found that the sun will burn hydrogen for 10 billion years. But because the sun is a field star, we could not determine how long ago the hydrogen burning started. In how old is the Earth-Moon system, we found that the oldest solid Earth and Moon material was 4.3 billion years old. But we could not determine the age of the Earth from its original start because the giant impact turned the mantle to magma. In this chapter, we'll cover the age of the solar system itself, which will give us both the age of the sun and the age of the Earth. To get a handle on the age of the solar system, we'll need to review planetary formation theory. Any such theory would need to explain our solar system as we see it today. We'll start with a look at some of the key characteristics of our system. We have the sun at the center and four relatively small rocky planets in the inner solar system and four much larger gaseous planets in the outer solar system. They are all in nearly circular orbits and they are all in the same orbital plane. None of the planets are orbiting outside the plane like some comet's asteroids and dwarf planets do. Our theory of planetary formation will need to explain these facts.