Welcome back everyone. So now that we're further out in the solar system, it's time for us to unpack a bit more the difference between the different kinds of giant planets. So there are gas giants and ice giants, and we want to sort of distinguish between them in our understanding of both our own solar system and other solar systems. So in our solar system, the gas giants are Jupiter and Saturn. And if we could peel them like an onion, what we'd see is that they are composed mainly of hydrogen and helium. And as you dive inward, what you'd find is that there is essentially no place to stand and the atmosphere smoothly merges into a hydrogen ocean essentially. And it's a metallic hydrogen ocean, the forces, the pressures that hydrogen experiences as you dive down into the gas giants is really quite extreme, unlike anything that we experience here on Earth. And those forces, the inter-atomic forces, can change the phase or the behavior of the gas such that it begins to act like not only a liquid, but also a liquid metal. So that's really one of the interesting things about the gas giants as you go deep into them. If you go very deep, you'll eventually reach the core. And the core is essentially, sometimes people think about the core being a rocky core, but really what it is, is it's a ball of metals, perhaps we think, at very, very high temperatures and pressures. Now because of the metallic hydrogen and also perhaps the core of metals in the gas giants, both Jupiter and Saturn have very strong magnetic fields, extremely strong magnetic fields, on the order of 20,000 to 500 times the magnetic field of Earth. That's for Jupiter and Saturn, respectively. Now these magnetic fields are so large, particularly for Jupiter, that it actually, the magnetic field extends all the way out through the moons that orbit Jupiter. And so many of those moons, it can actually be under the umbrella, the protective umbrella of Jupiter's magnetic field. Of course, that magnetic field also captures charged particles. So for example, the closest moon to Jupiter is Io and Io is in a constant state of volcanic activity. And so a lot of ions get burped out from Io and then get caught in the magnetic field. So there are very large radiation belts, just like the Earth's radiation belts, of trapped, charged particles moving around the magnetic fields of Jupiter. Now these planets are so large that they are still slowly gravitationally contracting. They're being squeezed by their own gravity, so much so that they give off more radiation as they sort of slowly, very slowly, contract, than they actually get from the sun. So Jupiter certainly gives off more energy than it receives from the sun, and Saturn approximately does as well. So let's move on now to the ice giants, so that's Uranus and Neptune. Now, the ice giants are still very large planets, but they are certainly far smaller than Jupiter. And the interesting thing about these worlds is that they are, mainly what we have are there's a large composition that's made up of things like CH3 or NH3. So ammonia and methane and lots of water as well. And things are cold enough that these are sort of slushes or slurries essentially. And if you could peel these like an onion and dive deep enough down, what you'd find is that there were these slurry of ices with also dissolved gases in them. And salts as well, and those salts can carry a charge essentially. And that's going to be an interesting thing when we look to the magnetic fields. Now once again, we do have quote unquote rocky cores of these planets. But again, they're under extreme pressures and temperatures. And so the cores are in some sense, there's approximately about the size of the earth, but they are in fact ten times the mass of the Earth. So as we explore extrasolar planets, we're going to find lots of what we would call super-Earths, these are planets that are about ten times the size of the Earth. And we think that these are sort of what is living at the center of Uranus and Neptune, except the pressures and temperatures are such that these things have been squeezed down to sizes about the size of the Earth. So you're getting ten times the mass of the Earth in about the same volume. Now these planets also have magnetic fields, which was a little bit surprising to people. We didn't really expect them to have magnetic fields, and in some cases the magnetic fields are in very strange orientations. We know that on Earth, for example, the magnetic field is pretty nicely aligned with the rotation axis and we certainly find this is true also with Jupiter and with Saturn. But with the ice giants, we can see quite a bit of misalignment between the rotation axis and the magnetic axis and in fact for Uranus, the alignment is so strong that it looks, it's very difficult to understand. Now the origin of these fields is quite interesting, and it may be from the salts and maybe because of rotational motion or swirling motions because of the ionized salts that actually could create the magnetic fields in this case. Now, these planets are also slowly contracting, and it turns out that Neptune gives off more energy than it receives from the Sun, but Uranus, for reasons that we don't quite understand, does not. So again, these planets are large enough that there is gravitational contraction still ongoing. But there does seem to be some difference between the two planets for reasons that we don't yet understand.
