This is a course about history. So if we talk about the solar system we should be talking about its history. But can we? History makes sense if something changes. But isn't the solar system just the prime example of something that is not changing? After all, the fact that the planets follow their simple orbits unchanged for long times was what made the start of modern science even possible. There's even a science fiction story by David Gerrard and Larry Niven about this. It's called the Flying Sorcerers and it's based on this problem. On a planet orbiting a system of two sons the motions are so complicated in this book that the inhabitants never managed to start science because there are no regularities. If the planetary motion would not be extremely regular, Johannes Kepler would not have been able to derive his simple laws of planetary motion and either Agnudin would have had a much harder time to find the basic laws of motion and gravity. The solar system is the one model we have for unchanging stability and dynamic simplicity it seems and ideal laboratory to derive the laws of nature. When we want to talk about history we can certainly talk about the formation of the solar system in an historic context. We can also talk about how the solar system will end at some point. And as I will tell you today the solar system is actually not as unchanging as we think. In particular during the first billion years it did change a lot. So there is a history after all. When it comes to the formation of the solar system we look at other very young stars in our vicinity and watch them form a planetary system. Astronomers have now images of proto-planetary disks that are in the process of forming planets. In some of these disks we can even see the proto-planet's SA form. From observations like this we know that our sun form from a collapsing cloud of dust and gas. It collapsed under the influence of gravity and like a dancer pulling in her arms it started to rotate faster and faster as it became smaller. Some of the cloud material did not fall onto the star but it became a flat disk-like structure around the star. In this disk small dust particle started to stick together and to grow all the way to the Earth's like planets and the giant planets that we are seeing now. Most of that happened quickly in a time span of only 10 million years. This is in fact short compared to the age of the solar system which is 4,500 million years. When the sun dies swallowing several of the inner planets it will be more than 10 billion years old. As a comparison we could say that the pregnancy of the star of the solar system took only a month compared to a life of 80 years. In the previous lecture we made an inventory of everything that makes up the solar system. Besides the main eight planets we discussed as that there are also belts of much smaller objects in particular the asteroid belt between Mars and Jupiter and the Kuiper belt outside of the orbit of the outermost planet Neptune. Pluto is part of the Kuiper belt and so are many other bodies large and small. Isn't it curious that there are these two regions where no planet did form while everywhere else in the system planets did form. We now think that this is a reflection of the fact that the solar system in its infancy was not stable and quiet at all. Looking at the amount of Mars present in the belts and studying the distribution of orbits we have concluded that planets moved around quite a bit. They were not formed in the places where they are today. For example one theory says that Jupiter in its early days made a big excursion into the inner solar system almost to the location where the Earth is right now. After that excursion it went back out. But it left behind a stilled up and depleted region which helps to account for the low mass of Mars and for the fact that the asteroid belt has very little mass remaining clearly not enough to make a full-size planet. Looking at the outer belt the Kuiper belt beyond the orbit of Neptune scientists have concluded that Neptune must have formed further in closer to the Sun. Then slowly over a period of tens or even hundreds of millions of years Neptune migrated outward pushing the Kuiper belt and arranging it into the setup that we see today. Some theories even have Neptune and Uranus switch places in a major upheaval when the solar system was maybe 800 million years old acting up like a child in puberty. This type of internal arrangement of a planetary system is really a new idea in astrophysics and it seems to play a crucial role to understand the architecture of other planetary systems around other stars. Some of these look really different from our own. For example a massive planet like Jupiter, Jupiter is at five as Sonoma units from the Sun, can orbit so close to the star only a few percent of the Earth's distance to the Sun that the heat of the star blows it and makes it slowly lose its gas atmosphere. Other systems might have five planets each of them larger than the Earth's more like Neptune maybe, pecked into a region that would completely fit inside the orbit of Mercury, the innermost planet in our own solar system. Studying history always follows the same basic principles. We are looking for artifacts from our own past or we are looking for situations and locations that we believe are good models for our past to study what's going on there and then we spin these facts into the best story that we can make. In the case of the solar system the artifacts are the precise orbit arrangement of everything in the solar system as well as other things like for example the chemical composition of planets. The comparison cases are nearby stars that are young and are producing their planetary systems right now and the story that we make of it is one of a dynamic changing solar system at least during the first billion years. After that first billion the solar system did settle down. Planets were in the positions there in now. Occasionally of course impact might still happen for example an asteroid or comet might impact a planet like the Earth with possibly disastrous consequences. Mars might capture an asteroid and turn it into a moon. Jupiter might rip apart a comet that passes too close and the fragments and can impact on Jupiter or on one of its moons but mostly things have become a lot more quiet. We are lucky to live in this more stable phase because long-term stability is a main requirement when we look at a planet that might develop life.