What we're going to start is going to be Comparative Planetology because this section of the course is about planets and life. So well the idea of comparative planetology is to look at all the different kinds of planets that we see, and we really see a really remarkable variation even in our own solar system. and to try and use that difference to compare and contrast, to understand the forces which drive the evolution, the formation and the evolution of planets. Because it's interesting in its own right, but also because it's going to effect things like habitability. How habitable is a planet? Is a planet remain habitable for a long time? If it has moons, are those moons habitable? So those are the kinds of questions we want to ask. and so let's just remind ourselves of our guiding principles in thinking about this. First, we have different planets and those different planets are going to tell us different stories, and we want to use those different stories. And in each one of these, we want to understand how those planets might be in terms of a home for life, either for long-term or even for short-term. Okay. So, let's focus in on one planetary system. Our own. To see what it tells us about its own evolution and perhaps about more general principles for what we've learned about other planets as well. Okay. So, there's a couple of different things that we immediately get by doing a survey of the bodies in our own solar system. The first is there are essentially, in our solar system, three flavors of planets. The first are what we would call terrestrial planets. Rocky planets like the Earth or Mars. The second kind of planet are what we call gas giants. And these are enormous planets where there may or may not be a rocky core. But essentially they're built of fairly simple material, although in different states. The third kind of planet are what we call an ice giant, and these are planets that also are quite large compared to the Earth, but, a great deal of their material is locked up in ices. So it's a mix of gases and ices in different states. Okay so that's the three different kinds of planets, when we look at individual planets we also find some similarities. For example many of the objects in our solar system have moons, not all, but many of them do have satellites orbiting around them and in some cases there are many moons orbiting around a single planet. Now, along with the planets, these three kinds of planets, and their satellites, we also find a lot of debris. material left over from the construction of the solar system, which didn't form planets. But was instrumental in forming the planets. For example we have what, what are called Dwarf planets. These are bodies that, some of them are you know spherical or quite spherical, but are not as large as a planet. in particular they may not have done much in terms of having their gravity vacuum up material around them. That's one of the things we use to define a planet, the ability for the gravity of the body to sort of clear out a gap around it. so there's dwarf planets, and actually these days that's what we would sort of think of Pluto as being. Though there are others in our solar system. Then there are things like asteroids, which are planetesimals. They are small enough that they could not form they did not become spherical. There was not enough gravity to sort of take the, you know, the rocks and crush them down under their own weight. To be able to form a sphere. So these are basically, in many cases, floating mountains essentially. and then along with the asteroids are also things called comets, which are, again, debris left over from the solar system, but there tends to be a lot more frozen water in objects like that. Comets, as we'll see, are a mix of both frozen water and rocks, slushy mud balls as some people call them. Okay, so those that tells us that's a good overview of the objects in the solar system. What about the structure of the solar system? Even just by finding out where the objects are and how they're moving already tells us something very important. So one thing is when we look at all the objects in the solar system, we find that they are all roughly orbiting in the same plane. So, if this is the sun, I don't necessarily have some planets orbiting like this and other planets orbiting like that. Pretty much all the planets and most of the even the construction debris, is orbiting in a disk. Certainly all the planets are pretty much orbiting in that disk. Some of them have slight inclinations but pretty much it defines a single disk of rotation. now there's also the, and most of the spinning motion. So we just have the orbiting motions, and most of the objects in the solar system are also spinning. The planets pretty much, not all, but are all spinning in the same direction as well, and they're spinning in the same direction as the sun is spinning, and that tells us something very fundamental about the origin of the planets and the solar system as a whole. along with the planets there are also belts of debris. We talked about before that there is construction debris left over from the origin of the solar system, and much of that debris is in the specialized belts. There's the asteroid belt that we all know about. There's also something called the Kuiper Belt beyond the orbit of Neptune, that much like the the asteroid belt, is made up of smaller bodies that were left over from the construction of the solar system. And then even larger distances there are the, is what's called the Oort cloud. Now the Oort cloud is a, sort of, storage area, in some sense, for comets. as is the Kuiper belt as well. Most of the comets we see come from either the Kuiper belt, or the the Oort cloud.
