 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about the formation of our solar system and we'll begin to lead into the ideas of how solar systems form around stars and then later in this set of lectures we will look at the formation of other solar systems around other stars. But let's start off with our own solar system and we have to start off as we do when we try to make some kind of model of things is to look at what we know and what do we know about the motions of the solar system? Well all planets revolve around the sun in the same direction and approximately the same plane. The solar system is very flat. Most planets rotate in the same direction as well and have moons that revolve around them in the same direction. There is definitely a direction to the solar system. When we look at the chemistry of the solar system we see that the composition changes as you get further from the sun. So when you're close to the sun we have one kind of composition which is the metal and rocky terrestrial planets and then we have icy materials in the outer solar system. So there's a big difference between those. We can also look at some more things such as ages. How old are things in the solar system? Well earth rocks go back 3.8 billion years, moon rocks about 4.4 billion years, very primitive meteorites about 4.5 billion years and what does this tell us? Well it tells us about when we look at the ages that pretty much things formed at the same time. We didn't have things forming billions of years apart. The chemistry tells us that the temperature was important and that impacted what materials were able to condense. The motions tell us that there is a common overall motion to the solar system. So there is definitely some order there and we want to look at how we feel that might have formed. Now we look at the solar nebula theory which starts off with a cloud of gas and dust in space which begins to collapse. Now what causes it to collapse is a good question. There are a number of things that can happen. Clouds can collide into each other starting a compression and gravity kicks in. Sometimes a supernova could go off nearby and cause the compression that starts that. But whatever starts it, it begins to flatten. Material condenses in and that's because it had some very slight rotation along an axis. It wasn't very fast, but there was some slight rotation and that becomes magnified as the nebula begins to sink, shrink down. So the material gravitates to the center. That's why the sun is most of the material in our solar system. The left of the planetesimals form from the remaining debris and that eventually condenses in to planets. So it gives us a vague idea of how things started to occur within our solar system. Now this fits of course what we see. How about the temperatures? Well the high temperatures, when you're close to the sun, keep the icy materials from condensing. So when you're this close into the sun where the inner planets are, you don't get much icy material. The metals condense very close to the sun and we see that when we're in by mercury, mostly metal oxides and iron and nickel. We see rocky material a little further out where we get silicates and other types of rocky materials. And we see ices when we get out beyond the frost line and past the asteroid belt, we start to get ices, water, ammonia and methane. The planetesimals and therefore the planets could only form from the materials that condensed in that region. So when you're forming a planet out here, you can build from water, methane and ammonia plus everything to the left of it. All of that can condense. If you're trying to form a planet around earth, you only have a limited amount. You can only form the material that was within that so earth did not get to form with lots of water, ammonia or methane. It has rocky materials and metals that form up the majority of earth. So it gives you more to build with and gives you bigger planets in the outer solar system and smaller planets in the inner solar system. So what happens to these planets? Planetesimals form through accretion and some become large enough to attract their neighbors gravitationally and start to grow. We start to form what we call protoplanets, which are getting up to the mass of Mercury and these start to become dominant. They accrete more planetesimals and throw some material out of the solar system altogether. And the energy causes the planetesimals to become molten. The energy of this accretion caused them to become molten and start to differentiate as we see our planets today. For the Jovian planets, it's similar, but the one difference is that we have more to build from. We have icy material as well. More material, larger planetesimals. So we've got more to build from. We're going to get larger objects and they then become massive enough to collect gases through gravity. So earth was not massive enough to be able to collect things like hydrogen and helium through gravity. The planetesimals that formed Jupiter had more to build with and were able to do this. So much larger protoplanets. This is a relatively short process, taking only a few million years as the solar system is forming. So it does not take billions of years. It actually seems to go relatively fast. Then we have to clean out the solar system. How do we clean the solar system? Well, the protoplanets collide, giving rise to some major exceptions. Why does the earth have a large moon? That could be because of a massive collision. The backward rotation of Venus could possibly have been caused through a collision and the loss of Mercury's rocky crust. The reason that it does not have as much rock as we might expect. Comets are material that was ejected out by gravitational interactions and this could be where we form things like the Kuiper Belt and the Oort Cloud, which now give us our comets. Other icy objects could have been moved into the inner solar system. So it's all interactions with Uranus, Neptune, Jupiter, and Saturn, and they could get flung into the inner solar system as well as getting flung out. And that could account for the water and ice that we see on the terrestrial planets. Not much water and ice should have been able to condense, but since there is some, it could have come from the outer part of the solar system. So what happened? Most of the planetesimals then disappear. They're gone, they either are ejected from the solar system, or accreted by a planet. They're gone, they're no longer part, except for the stable areas found in the asteroid belt, the Kuiper Belt, and the Oort Cloud, those remain. The rest of the gas was removed by solar wind and radiation pressure that cleared those out. However, that wasn't the end. We also had planetary migration that occurred, and we're gonna see this more when we look at other solar systems, but planets likely did not originally form in their current locations. So that can explain an era of heavy bombardment that appears to have occurred early on in the history of the solar system. There was a lot of debris being sent in, and explains why that was when the planets were moving and disrupting the orbits of smaller objects. And we will look at that more when we look at planets outside our solar system. So let's go ahead and finish up with our summary. And what we've looked at here, the solar nebula theory is what we use to explain the formation of our solar system from a cloud of gas and dust starting about five billion years ago. The material formed planetesimals, to proto became protoplanets, and the protoplanets finally became the planets that we see today. Compositions determined by location. Where were they in the solar system? And the planets in the sun worked together to clear out that remaining material and remove it from the solar system, leaving much less debris today than there was early on. So that concludes this lecture on formation of the solar system. We'll be back again next time for another topic in astronomy. So until then, have a great day everyone, and I will see you in class.