 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to discuss the formation of the solar system and trying to figure out how the solar system formed in the first place. So we will look at some of the patterns that we see and then some of the evidence that we see for our idea of solar system formation that we can see perhaps elsewhere in the galaxy. So let's get started with some of the patterns that we see here and we see a number of different patterns that occur in the solar system. So these patterns are quite important because we can learn about the origin of the solar system by looking at what remains today, what we still have around. And what we notice is that all of the planets orbit in approximately the same plane. That means the solar system is flat. So if you draw the solar system on a piece of paper that is really not a bad approximation for what the solar system looks like. We also notice that all of the planets orbit around the Sun in the same direction. So when we see the Sun here we see planets going around and around and every single one goes in the same direction. They all go around counterclockwise if we look down from the Earth's North Pole. Most but not all of the planets also rotate on their axes in the same direction, in that counterclockwise direction. So not only are they going around the Sun in the same direction but they rotate in the same direction. Meaning that there is some overall motion to the solar system that is probably a remnant of the original material that formed the solar system billions of years ago. We also note that the planets are separated by type. You have the terrestrial planets, the Earth-like planets, and the Jovian or Jupiter-like planets. And we also note that there are a lot of small bodies in the solar system, asteroids and comets. And we want to look at what that can tell us about the history of the solar system. So let's see what does that tell us. And what we see here is that we believe that the solar system formed from a large cloud of gas and dust that was primarily hydrogen that started collapsing about five billion years ago. So it collapsed down. And what we see, looking at some of the pieces of evidence that we saw, is that the rotation of that cloud is reflected in the motions of the planets today. So the fact that everything orbits counter-clockwise now means that that giant gas cloud at one point was spinning counter-clockwise, much, much slower than the planets are orbiting or the planets are rotating. But it had that very slight motion that then got magnified as it began to collapse. Now if we look at that, we would have had hotter temperatures near the Sun that would have caused different types of materials to condense. So hotter temperatures near the Sun means that we would have rock and metal materials forming planets close to the Sun, and icy materials further away. And that gives rise to the two types of planets that we see. The small bodies that we see, asteroids and comets, are actually remnants of the planetary formation. So comets could be bits of the material that might have gone into forming the outer or Jovian planets. The asteroids, the rocky material, might be leftover material that would have been the type that would have formed the terrestrial or the inner planets. So we may be seeing some of those bits and pieces and may have a few bits of the early solar system still around with us today. So let's look at building a planet. What would have happened? And we would have started, first of all, the nebula would begin to cool. The hotter it is, the fewer objects are able to start to form solid particles. So the first things would have been formed would have been metallic and rocky materials close to the star. So the particles begin to start coalescing into larger objects, starting off with simple grains, pebbles, smaller rocks, larger rocks, eventually becoming what we call planetesimals. And those would have been the bits and pieces that would have then formed the terrestrial planets. Now outer in the outer part of the solar system you would have had more icy material forming, and you would find more icy chunks again coalescing into grains and pebbles and smaller rocks and larger rocks, not being rocks in the traditional sense, but larger chunks of ice that would have then coalesced to help form the outer planets. Over billions of years these would slowly grow larger, eventually becoming the planets that we see today. So these planetesimals would slowly coalesce and one would become dominant in each area, and it would then clean out that part of the nebula. It would collect a lot of material into it, and it could use its gravitation to throw material out of the solar system altogether. Then we have to clean up the solar system because there's not a lot of these planetesimals left today. So how do we clean it out? Well as the planets are forming, some of that material, some of that debris left over actually becomes part of the planet. The planet will gravitationally pull it in and it will become a part of the planet impacting in it and causing the planet to grow. Some of it is ejected out into space so you might have a planet here, and you might have an object that hits it and becomes part of the planet, but you might also have something that just comes close to it and then gets ejected completely out of the solar system. It doesn't quite hit it, it just gets ejected, gives it a lot of velocity, and gets kicked out. And this is how the planets cleaned out the debris that remained after the initial formation. And that's one of the things that is required for an object to be a planet is that it was able to clear its region of debris. Now also once the Sun fully forms, the solar wind and the radiation pressure will clear out the remaining gas and dust. So anything that had not yet condensed would then be removed from the solar system by the solar wind, which is an outflow of particles from the Sun, and the radiation pressure with a light from the Sun provides a pressure. And this can remove any small objects, it would not affect things like planets or even smaller things like asteroids and comets, but it would be able to get rid of very small things, grains of dust or gas atoms that were still remaining in the solar system would have been cleared out by this. Now what is the evidence that this may have occurred? And we can't go back five billion years to see what our solar system looked like. We can only use the evidence that we see today, but we can also look at other stars. So we can look at other stars and see if this process does occur or is occurring today. And these are some Hubble Space Telescope images of the star Beta Pictoris, which does seem to have actually a couple of disks around it, a primary and a secondary disk, which could be regions where this is undergoing right now, where stars or planets are actually forming around this star. So we could be seeing in Beta Pictoris what our solar system looked like five billion years ago. And some of the keys are that now, that we didn't know several decades ago, is that we know of many planetary systems outside of our own and that they are quite common. We now know of thousands of planets outside our solar system, and back as early as the mid-1980s, we didn't know for sure of any. We might have seen bits of evidence of things like dust here, but we didn't know of any planets that were around for sure. And now we know of thousands of planets that are around. And we detect, as I showed here in Beta Pictoris, we do detect these dusty disks around stars. So are we observing the process of planet formation? And it looks like that is a pretty good bet, because we do know that planetary systems are common, so that when we see things like this, we have a pretty good idea that planetary systems are forming here. So let's finish up here with our summary, as we do. And in our summary, we would see that we can look at, we can find out what's going on by looking for those patterns. So what patterns exist in the solar system now, today, that understand how the solar system formed billions of years ago? What we believe is what we call the nebular theory, and that is that a large cloud of gas and dust collapsed to form our solar system. And we do see evidence that this occurs around, is occurring today, around other stars. So we see evidence of dusty disks, and we know of many planets that have formed outside of our solar system. So we know that the planetary formation seems to be a natural part of star formation, so we expect that it will be very likely that planets, that stars, many stars, would form planets, and that they would be very common in the universe. So we do see some evidence of this today, looking at stars like Beta Pictorus, to be able to see an example of star formation. So that completes our lecture on the formation of the solar system, and 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.