 Greetings and welcome to the Introduction to Astronomy. In this video, we are going to be talking about the formation of our own solar system, so how our solar system came to be and the processes that made it the solar system that we see today. So we want to start out looking at what some of the observations are that can help us to understand our solar system. So we can look at a couple of different things. We can look at the motions and we can look at the chemistry of the solar system. These are both important for being able to understand how our solar system formed in the first place. So first of all we see that all of the planets revolve around the sun in the same direction and in approximately the same plane. What does that mean? Well essentially when you've drawn a model of the solar system on a piece of paper, it's a very good approximation of what the solar system actually does. So you take your little piece of paper there and you draw in a sun at the center and you draw planets orbiting around it. It may not be to scale, but in terms of how flat it is that's what we mean by it being in the same plane. That is not you don't have planets coming in and out at all different random directions coming up and above or up and below around the sun there such as the comet's orbit, but everything is essentially in the same plane. We also find that planets will rotate in the same direction for the most part with the big exception being Venus, which rotates in the opposite direction. We see that moons, the major moons of the planets also revolve around the planets in the same direction. So essentially everything in the solar system is moving counterclockwise and what that means is if you look down from above from the north from the north pole of the earth all of the planets revolve around the sun counterclockwise. Almost all of the planets rotate counterclockwise and the vast majority of the major moons of the planets are also moving counterclockwise. So there is this overall motion to the solar system that we have to be able to explain. In terms of chemistry we find that the composition of the planets changes as you get further away from the sun. When we look close to the sun we have the terrestrial planets, which are metal and rocky. And then as we go into the asteroid belt we transition more into carbon compounds and when we get to the outer solar system we get icy materials. So, and the largest planets having lots of hydrogen and helium. So there's a difference in the compositions of the planets when you are close to the sun as to when you are further away. So that's something else we have to be able to explain. We also know about the ages of objects in the solar system. We've been able to determine these earth rocks being about 3.8 billion years old. The oldest moon rocks about 4.4 billion years old and very old primitive meteorites at about four and a half billion years old. So what do these things tell us? Well the ages tell us that things formed at roughly the same time, that the formation of the solar system did not take billions and billions of years, that it formed relatively quickly because we have ancient meteorites and moon rocks and the oldest moon rocks being very close to the same age and very close to our estimated age for the sun. In terms of chemistry we find out that temperature impacted what materials condensed in each area. So the hotter, hotter, hotter, closer to the sun you were able to condense materials that did not vaporize that don't vaporize as easily. So things like metals and rocks could condense close to the sun. When you got further away you could actually form ices and allow that to form planets as well. The motions tell us that there is a common overall motion to the solar system. So we are not just confined to random motions but there is some overall pattern that we see. So when we put this together we look at what we call the solar nebula theory as a way of being able to explain the origin of the solar system. Essentially you had a great cloud of gas and dust that begins to collapse here. So material would then condense down. Why did it condense is a good question. You can condense them through things like collisions. So two clouds striking each other could do that. You could also have a supernova explosion that might happen that would start the compression. Once that does once that happens gravity begins to take over and flattens it into a disc shape. So it begins to flatten and it spins faster and faster. The material gravitates towards the center. So it flattens here and it starts to form a protostar at the center and then we have this more concentration of material and then finally the star begins to form and we have planets and material around it. And we don't form planets right away but we form planetesimals first. Planetesimals would just be the building blocks of the planets that would eventually form. But it's all due to this gas cloud that begins to collapse and gravity takes over pulling most of the material to the center forming what will eventually become the sun. And the rest of the material the debris left behind is what will form the planets. So this little bit of material left behind is what is going to form the planets over time. Now how do we know what types of planets are going to form and let's take a look at what we call the condensation sequence which says essentially that the higher temperatures that were existed closer to the sun kept icy material from condensing. So why do we not have lots of ices on Mercury and Venus and Earth and Mars? Because they were too close to the sun and those materials stayed in a gaseous or vapor state so they were not able to form chunks of material. So when we look at the sequence here at various temperatures it tells you what types of materials are able to condense out. So close to the sun you get metallic materials, further away you get rocky materials and even further away you start to get ices out in the depths of the solar system and that tells us what materials the planets could form from. So metals condensed closest to the sun, rocky materials a little further out and out beyond the frost line somewhere between Mars and Jupiter here then you were able to get ices that were able to able to condense. The key is that the planetesimals and therefore planets could only form from the materials that actually condensed in their region. So in close with Mercury all we had was metallic materials and a little bit of rock so Mercury is primarily metals that's all that could form. When you get further out you still had the metals they could still form but you also add in now rocky materials so planets like Venus, Earth and Mars become made up with some metal but also significant amounts of rocky material. When you get to the outer planets you get the metals and the rocks and in addition you add in ices so more ices that are able to form and condense and they're able to help build the planets as well. Now the key is that metals were a very small part of the solar nebula so you didn't have a lot of material to build from. Rocky materials were a little bit larger portions you gives you more material to build planets but the vast majority of the solar nebula would have been hydrogen and helium and therefore hydrogen compounds things like water and methane and ammonia so you had lots and lots of these so you were able to form very large planets because when you got to the outer solar system a lot more material was able to condense. So how did we form these planets? Well it's through a process we call accretion and what that means is that some of these planetesimals begin to form larger and larger and become large enough to gravitationally attract their neighbors. The planetesimals then that originally formed begin to become what we call protoplanets so these are smaller maybe the mass of mercury or so and they begin to accrete more planetesimals so their gravity is stronger and they start to pull in more of these objects that are beginning to form so they will gather more and that will cause the planet to come larger and larger and then eventually the heat of all those materials impacting in will cause it to be molten and will differentiate the planet so that metals will sink down to the core and lighter materials will rise to the surface so they build up through a slow process that could take millions or tens of millions of years to occur to actually build the planets but it's very small compared to the entire life of our solar system. Now when we look at the differences when we get to the outer solar system what's the difference with the Jovian planets the difference is that ices were able to condense in this region we had more material and that means larger planetesimals so larger objects were able to form here eventually they became massive enough to collect gases through gravitation they had enough material in them enough gravity that they formed that they collected the gases the hydrogen and the helium the inner planets never became massive enough to do this there was not enough material in the inner solar system to form a large enough planet so they were never able to attract gases directly through gravitation only the outer planets could do this and it meant we formed much larger protoplanets over the course of a few million years so relatively short time but as we begin to build up these planets over time. Alright let's look at how we can clean up the solar system because we don't see a lot of this material around anymore so how do we clean clean this up well first of all the protoplanets likely collided together over time so they they built up together and they may account for some of the exceptions that we see to the general properties of the solar system the earth has a large moon it's the only of the smaller planets with a very large moon venus rotates backwards mercury probably had some kind of rocky crust that seems to be lost over time large impacts could explain these how about the comets we talked about comets in the kuiper belt and the oort cloud they could have been ejected from the outer part of the solar system by interactions with the planets so planets would have collided with each other making them larger and clearing out all of the little debris and they would have ejected some of the material out into the outer solar system. Other icy objects not only were they ejected out of the solar system but they could have been moved into the inner solar system and that accounts for water and ice that we find on things like earth and mars that some of these planetesimals that that condensed in the outer solar system could have come towards the inner collided with the earth and could account for a proportion of our water that we have here on earth today and of course other types of ices now over time the planetesimals disappear and they are either ejected from the solar system by gravitational interactions with the planet so a planetesimal may come and collide into the planet and become a part of it but it might also pass very close to it and not collide and then get kicked out of the solar system altogether picking up a little bit of energy from the planet and getting kicked out so they're either ejected or accreted by a planet and that cleaned up the solar system so we don't have lots of debris left around. The couple of stable areas in things like the asteroid belt the kuiper belt in the oort cloud still remain but most of the rest of the areas the planets cleaned out all of the larger objects that used to orbit around them. The solar wind and the radiation pressure of the sun also cleared out the remaining gases sorry remember that gases were not able to be accreted by some of these planets they were not massive enough so the solar wind would then push those away and clear out the solar system that way. One other important factor that we haven't talked about yet is planetary migration and what we believe is that the planets did not originally form in their current locations based on the condensation sequence we can expect that the planets have actually moved around over time before reaching the stable orbits that they have today and this can possibly explain other solar systems where we see very large planets close to their stars that would not be able to exist based on the condensation sequence that we've covered here so it can explain that it can explain how the planets exist it can also explain the era of heavy bombardment with large planets being unstable and moving around a little bit that would then fling more material into the inner solar system perhaps causing that era of heavy bombardment that occurred early in the history of the solar system so let's finish up as we do with our summary which what we talked about this time first of all the solar nebula theory is a way of explaining the formation of the solar system about five billion years ago from a cloud of gas and dust the materials condensed to form first planet tessimals which accreted together to become proto-planets which finally became the planets that we see today the compositions were determined by their location in the solar system rocky and metallic worlds close to the sun where it was very hot icy worlds further away from the sun where it was much colder and then the planets and the sun were what cleared out much of the remaining material whether it be planet tessimals whether it be gas and dust were then cleared out of the solar system so that concludes our lecture on the 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