 Greetings and welcome to the Introduction to Astronomy. In this video, we are going to discuss the two types of planets that exist in our solar system. We will see that our solar system is divided into two very distinct types of planets, and while each planet has its own distinct properties, we can easily group them together as there are two sets that easily have a lot of things in common. So let's take a look at those here, and the first set is the terrestrial planets. These are the planets that are a lot like the Earth, and therefore we call them terrestrial, and that includes Mercury, Venus, the Earth itself, and Mars. So these four are what we call the terrestrial planets. They are all very close to the Sun, and these are the first four planets, and they're all very rocky worlds. They're all things that we could actually land on. And in fact, we have had soft landings right now on three of these planets. We have had soft landings on Venus. We've brought astronauts back to the Earth, so we've soft landed there, and we've had soft landings on Mars. The only one that has not yet been landed on, at least as a soft landing, would be Mercury. Mercury has only been visited by a couple of spacecraft so far. Now the other type of planet is what we call a Jovian planet, or Jupiter-like planet. And that includes Jupiter itself as the prototype of these Saturn, Uranus, here, and Neptune. So this includes the Jovian planets. These are all very gaseous, and they are ones that we have never landed on, and will never land on, because there is no solid surface on which to land. Now if we want to look in a little more details, we can look at a little table here showing some of their properties and comparing these two different types of planets. So what we see here is that the terrestrial planets have one set of properties, and the Jovian planets have a completely different set of properties. So we'll note that the terrestrial planets are small in size and mass, whereas the Jovian planets are large. We can note that the terrestrial planets have rocky surfaces, and the Jovian planets are all gaseous. And related to that we have solid surfaces for the terrestrial planets, and no solid surface for any of the Jovian planets. All of the terrestrial planets are high density, all of the Jovian planets are lower density. All of the terrestrial planets have a slow rotation, while all of the Jovian planets have a faster rotation. So even the fastest rotating terrestrial planet, the Earth, rotates slower than the slowest rotating of the Jovian planets. When we look at magnetic fields, the terrestrial planets have weak or no magnetic fields, while all the Jovian planets have strong magnetic fields. When we look at satellites, we see lots of satellites in the outer solar system around the Jovian planets, very few in the inner solar system. And we also note that there are no rings in the inner solar system among the terrestrial planets, while every single one of the Jovian planets has a ring. And then finally, we also note that the terrestrial planets are all close to the Sun, while the Jovian planets are all far away from the Sun. Now, what might cause us to have these two different types of planets? And we believe it has to do with how they have formed and where they formed in the solar system. So if we look at this, when you're close to the Sun, you would have a higher temperature. So it would be much hotter. So as the solar system was forming, you had higher temperatures close to the Sun, colder temperatures further away, and that meant that different types of materials could condense. Close to the Sun, the only things that could actually form solids were rocks and metals. So the terrestrial planets close to the Sun could only form from rock and metal. So we see them with high density materials, rocky materials, metallic materials, very little ices or gases. So while we consider the Earth has an atmosphere and the Earth has a lot of water and ice on it, it's a very small percentage of the overall composition of the Earth. So this would give us terrestrial or Earth-like planets made primarily of rock and metal. When you're far from the Sun, past what we'd call the frost line, where ices could condense, then we'd have ices as well, and ices mean things like water, H2O, but it could also mean things like methane and ammonia. So we can have various different... When we say ice, we're not referring just to water ice, which it might be the first thing you think of, but we can get other compounds as ices as well. These could form as well, and these are much more common in the solar nebula than rocks or metals. So there was a lot more material to build from, and that allowed larger planets to be able to form. So why are large planets in the outer solar system? They had a bigger base of materials from which to build. They certainly could form rocks and metals as well. Those could condense in the outer solar system, but in addition, you could get things like water and methane and ammonia that allowed you to build up much larger planets, and if they got large enough, they would then be able to directly pull in things like hydrogen and helium gas which made up the vast majority of the solar nebula, and that allowed the outer planets, the Jovian planets, to become much larger. Now, how do planets change over time? As we take a look here, planets will change as they form, and we have a little picture of the Earth here. The Earth has a core here. It has a liquid outer core, a solid inner core. This whole section here would be iron, iron and nickel. So primarily iron, also nickel as well, so it would be very metallic, a metallic core that would be present there. The outer layers, the mantle in the outer layers here would then be rocky, primarily made up of silicate rocks, so rocky material, and the outer layers are the least dense materials, things like waters, ices and gases, and that would be the surface, the crust of the Earth, in addition to its atmosphere. So the planets, when they form, if they form large enough, they actually undergo a process called differentiation, and what that is is a separation of materials by density. What that means is that the Earth was at one point molten. The entire Earth would have been molten, and that meant that the denser materials were able to sink down to the core, and the less dense materials rose to the surface. So it gave us Earth with a metallic core, with the densest materials all down at the center, a rocky mantle in crust, with higher density rocks down in the mantle, lower density rocks up here on the crust, and then a lowest density or gaseous atmosphere. So the planets would have undergone this very, very early in their formation, they would have been molten, and they would have separated out by density, just as anything else, anything here, if you take several liquids together, water and oil, they will also separate by density with the less dense material rising to the top and the denser material sinking down below. Now, other things that can change a planet's surface include geological activity. If you have a planet with a hot interior that has internal heat, then you can get geological activity, and we get that here on Earth in terms of plate motions, in terms of volcanic activity, in terms of earthquakes, and various other geological types of activity, and those will reshape the surface of the Earth. So any objects that are there now could disappear, for example, craters that form or other land features could be wiped out by volcanic eruptions, and that's a constant state of change that occurs not only on planets, but also on many of the moons that we will look at. Now, the other thing that can shape the surface of a planet is impacts. Impacts can change the surface appearance of a planet, and they can give us objects that are heavily cratered like the moon, and objects that are less cratered like the Earth. The number of craters doesn't tell us that one object has been hit more than the other. In fact, the Earth and the moon have probably suffered about equal numbers of impacts over the billions of years of the solar system. However, the Earth has this geological activity or weathering effects that wear down the craters. So in reality, the number of craters tells us something about how old the surface is, not how often something has been impacted. And we will see that as we look at other objects elsewhere in the solar system. Impacts can also change the entire planet. Large, massive impacts can essentially shatter a large object, and maybe the reason we actually have a large moon is because of a massive impact billions of years ago. So let's finish up here with our summary, which says that we talk about two distinct types of planets, and those are the terrestrial planets, the Earth-like planets, and the Jovian planets, or Jupiter-like planets. They formed in different regions of the solar system because of different temperatures. So rocky and metallic planets formed close to the sun and gave us the terrestrial planets. The gaseous and icy planets formed further away, and that's where it was a lot colder. And solar system surfaces have been modified over times by various processes, cratering geological activity, just among a couple of them that we have discussed. So that concludes our lecture on the two different types of planets. 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.