 Greetings and welcome to the Introduction to Astronomy. In this video, we are going to talk about the outer planets, Jupiter, Saturn, Uranus, and Neptune, which are the planets that are further away from the Sun. The terrestrial planets are the ones closer to the Sun, and these are the ones that are further away. And because they are further from us, they've also been less well studied, so fewer spacecraft have visited things like Jupiter and Saturn, than have visited Venus and Mars. So let's take a quick look and get an introduction to these planets here. And what we see is that we call these the Jovian planets, meaning Jupiter-like. So these are the Jovian planets. And we note that they are larger than the terrestrial planets, but still very small compared to the Sun. As you can see in the figure here, all four of them would comfortably fit inside by just a small portion of the Sun. However, to compare those sizes, if we were to take the smallest of these Jovian planets, all four of the terrestrial worlds would fit easily inside that. Now the compositions of these are also different. We had rocks and metals in the inner solar system with the terrestrial planets. These are composed instead of ices and gases. And I want to clarify what do we mean by ices? We tend to think of water ice, which is one component and a major component of what we have in the outer solar system, but it also includes ices of ammonia and methane. So they include, when we say ices and are talking about the outer planets, we are not qualifying that to be just water ice. And as I mentioned, these are still really tiny compared to the Sun. Even the largest of the planet's Jupiter is significantly smaller than the Sun and many, many Jupiters could fit with the inside the Sun. None of these have a solid surface at all. We will never land on any of these planets because there is no surface on which to land. However, we can land on their moons and they have a lot of moons and we'll see about that in the future. So let's look at a little bit more of the details of these planets. What are they made up of? Well, essentially they are made up of hydrogen and helium. Their composition is much like that of the Sun. So 75% by mass or 90% by number is hydrogen. So 90% of the atoms within these planets are hydrogen. 10% are helium, if we're just counting the number of atoms. And that leaves less than 1% to be everything else and primarily that is going to be the hydrogen compounds, which are what we call the ices. So that's what we have right here. About 1% or so by mass, less than that if we're just counting the number of atoms or molecules. So the vast majority of these are hydrogen and helium much like the Sun. So their composition is exactly the same and the rest of that material is again these ices and things like iron and silicon that make up a large percentage of the inner terrestrial planets are not a major component of the outer planets. Now let's look at each of these briefly and start off looking at Jupiter. So let's start off here with Jupiter. Jupiter is the largest planet in the solar system. It's diameter 11 times that of the Earth. It's mass over 300 times that of the Earth. However, even though it is so tremendous, it rotates very quickly. Its day is very short, ours is about 24 hours, Jupiter's is less than 10. So you can imagine how fast those outer layers have to be moving because Jupiter is bigger than the Earth but spins more than twice as fast. Its orbital period is nearly 12 years, that's how long it takes to go around the Sun once, and it orbits at an average distance of 5.2 astronomical units or 5 times the Earth's Sun distance. You'll notice that its density is much lower than the other planets that we've looked at previously. We had densities of 3s and 5s generally showing rock and metallic materials. Now we are seeing that there are a lot of ices so when we start to see lower densities that's when we know that ices are making up a significant portion of the composition. Here we see Jupiter, we can see some of its structures. It has some light and dark banding structures across it. It does have the great red spot as well as other storms that are present in its atmosphere. You can see a lot of the turbulence even within these sections. You can see that they're very turbulent and there's a lot to try to understand in the atmosphere of Jupiter. Now let's move on and look at Saturn briefly. Saturn is known as the ringed planet although all of the Jovian planets have rings. This is just the one with the best set of rings visible from the Earth. Its diameter is very similar to that of Jupiter, a little less than 10 times the Earth, but its mass is a lot less, only less than 100 times the Earth's mass. And that means that Jupiter is much more compacted down by its gravity. So with Jovian planets as you make them more and more massive they don't necessarily get much larger and in fact they can get smaller as material crushes down. Its rotational period is similar to that of Jupiter, a little bit longer than 10 hours in this case. But because it is a lot further away from the Sun, nearly 10 astronomical units, it takes a correspondingly longer time to orbit the Sun almost 30 years to make one trip around the Sun. Saturn does have a mean density of less than 1. Now water would have a density of 1 gram per cubic centimeter, so that means that Saturn is actually less dense than water. So that it would, if you had something you could float it on, it would actually float on top of water. And that is because it is very uncompressed. Its mass is, it does not have yet enough mass, it had never gained enough mass to be able to compress it down. Were it to gain more Earths essentially to it, more Earth masses to it, it would not grow much in size, but it would in density, it would essentially compact down more. Now the next planet to look at would be Uranus. And Uranus is the first planet that we mentioned that actually has a discovery date. It was discovered in 1781. Prior to that it was not known, known to exist. Now this is, this was discovered quite by accident when William Herschel was actually looking for other objects that perhaps were nearby looking for what we call stellar parallax. And he happened to find this object that looked a little bit bigger, had it actually some size to it in his telescope. Now this is the planet that is lying on its side. And that means that its pole at times points towards the sun and its other pole points away. So it's tilt of its axis is closer to 90 degrees. Remember that the Earth's axial tilt was 23 and a half degrees. So this is really tilted significantly and therefore would have really extreme seasons. Its diameter is four times the Earth's. You can imagine four Earths fitting across the diameter of this planet here. And its mass is about 14 times that of the Earth. So significantly smaller than Jupiter and Saturn that we looked at previously. Its rotation period is a little bit longer, 17 hours. But it's so much further away, almost 20 astronomical units that it takes 84 years to orbit the sun once. Its density is very comparable to that of Jupiter, 1.3 grams per cubic centimeter. Again showing a concentration of ices. Now if you look at Uranus here it has a very bland blueish-green color. And that's because it does not have a lot of surface clouds that are seen. We saw a lot in Jupiter. They were a little more washed out on Saturn. And now on Uranus they're essentially gone. And that simply means that any structures are too far down below the surface of the clouds and the hazes for us to be able to see. Now as we get out to the last of the major planets and that would be Neptune. Neptune was discovered in 1846 and that was actually predicted to exist based on deviations in the orbit of Uranus. So it was predicted that it would exist and then happen to be found at that proper location. Previously it was actually observed by Galileo and recorded in his star charts but just recorded as another star. So while he saw it he did not actually recognize it as the planet. This is the most distant planet and its diameter and mass you should recognize. Those are pretty close to Uranus as well as its rotational period. So Uranus and Neptune have a lot of similarities between them just as some of the inner planets did. However it is a lot further away. 30 astronomical units meaning that it takes 165 years to orbit around the Sun once. Its density is actually a little bit larger so in a higher concentration of maybe some rock and icy materials and a little bit less of just the gases. Now all of the outer planets have been visited to some extent by a spacecraft. So each one has been visited at least once. Jupiter and Saturn we have had orbiters so we have had detailed studies of Jupiter and Saturn by having objects that orbit around them. The only craft that visited all four of these planets was the Voyager 2 craft. It actually visited Jupiter, Saturn, Uranus and Neptune and is the only one to have ever visited Uranus and Neptune. It was launched back in the 1970s when there was an alignment of the planets and they were all lined up so it was able to go from one to another rather conveniently. You could not go travel back and forth through the Solar System because of the immense distances so you can't go get to one planet on one side of the Sun and then cut back across the Sun to go to another planet. It simply is not time efficient to be able to do that. It will take far too long of a time. However when Voyager 2 was launched it was actually able to visit all four of these and gave us our only close-up looks at Uranus and Neptune back in 1986 and 1989 respectively. Some of the other craft Jupiter has been visited by a number of craft. Some of these just kind of passed by. Ulysses is one example and Cassini on its way to Saturn and New Horizons on its way to Pluto did not actually go into orbit around Jupiter or study it in any great detail. However they did take images as they passed by. Juno was one orbiter encountering in 2016 and the Galileo craft that went there in 1995 with the two orbiters that have studied the planet Jupiter. Saturn visited fewer times being further away. Pioneer 11 which also visited Jupiter in 1979. The two Voyager craft and then the Cassini mission that orbited around Saturn as well for over a decade. And again Uranus and Neptune only visited the one time by that same craft and have not been visited again since. So let's finish up here with our summary and what we find is that the outer or Jovian planets have really very different properties than the inner terrestrial planets. They are two very different types of planets. One being solid and rocky and have surfaces on which we can land and the other being large and icy and gaseous and not having any surfaces that we could ever land on. However each of the Jovian planet has its own unique characteristics as well that we can look at in more detail. All of them have been visited by a spacecraft but to a much smaller extent than Mars and Venus which have been very explored in great detail by a number of craft. Only a few craft have gone out and the further out you get in the solar system the fewer craft have visited. So that concludes our lecture on the overview of the giant 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.