Lesson Video: The Structure of the Solar System Physics

In this lesson, we will learn how to describe the different types of objects that are found in the solar system and the formation of the solar system.


Video Transcript

In this video, our topic is the structure of the solar system. And when we say the solar system, we’re talking about our solar system, the one with our Sun at the center. As we’re going to see, not only does our system include the Sun and the eight full-sized planets we see here on screen, but there are also other masses, such as moons and comets that help make up the overall system.

A good place for us to start in all this is with the question, How did our solar system come together? How did it initially form? As we start to think about outer space, a helpful realization is that even when the space looks empty, that doesn’t necessarily mean it is. Before our star — what we call the Sun — was formed, space wasn’t empty, but rather there were dust and gas clouds all throughout. And thanks to the force of gravity, which tends to make masses attract or pull towards one another, these clouds of gas and dust began to pull towards a center.

As this process went on, over many, many years, all this mass started to coalesce into a roughly spherical shape like a ball. And not only that, but all this mass began to turn around itself. It started to rotate. This rotation meant that the mass started to flatten out from being shaped like a ball and became shaped more like a disc. As all this was taking place, as this mass was becoming more and more disc like as it rotated, at the center of all, this began to form something called a protostar. This is the name for a star in the making. It has the elements of hydrogen and helium, which stars often used to fuel themselves. But the process that stars used to generate energy called fusion is not yet happening. But if we give it some time over many, many more years, this process does eventually begin. And this is what happened with our star, the Sun.

As more and more mass collected at the center of this rotating disk, the temperature and the pressure at that location went higher and higher. Eventually, the conditions were right for fusion to start happening in the core of the Sun. This is a big moment in a star’s life. It’s as though the star’s engine has just turned on. Once fusion starts to happen, a star has reached a fairly stable stage. So long as it still has fuel, it can continue on as it is, in some cases for billions of years.

Now, when we consider all this rotating mass involved, not all of the mass went towards forming our Sun. The material that was left over began to create its own little high-density pockets. These collections of mass gathered in on themselves farther and farther until they eventually became the planets in our solar system. Now, if we count them up, we can see that we’ve drawn in one, two, three, four, five, six, seven, eight planets in a sketch. But these objects are just the eight masses that we’ve classified as full-sized planets. There are also smaller planets, called dwarf planets, that we haven’t sketched in here.

But anyway, there are eight of these full-sized planets, and here are the names of them. Starting closest to the Sun, there’s Mercury, then Venus, then Earth, then Mars, then Jupiter, then Saturn, then Uranus, and then, finally, Neptune. In order to remember all these planets and the order in which they appear from the Sun, there’s a helpful memory device that goes like this: “My Very Educated Mother Just Served Us Nachos.” The first letter of each word in the sentence helps us remember the name of the planet it corresponds to. So when we want to remember the planets and the order in which they appear from the Sun, we can Just recall “My Very Educated Mother Just Served Us Nachos.”

Now, each one of these planets is different, and each one has something interesting about itself. Mercury, for example, is the fastest moving planet. Its speed and orbit is higher than any others. While Venus, thanks to its thick atmosphere, which creates a powerful greenhouse effect, is the hottest of all the planets. Earth, of course, has a special place in our heart because it’s home. And very much related to that, it’s the only one of the planets where liquid water can exist. That is, it has just the right temperature range so that the water on Earth isn’t all frozen, which it would be if the Earth was too cold. And it’s also not all water vapor, which would happen if the Earth was too hot.

Next comes Mars, which, as far as we can tell, shows signs of water being there many, many years ago. And then, there’s Jupiter. Not only is Jupiter the biggest of the planets by far, but also when we see pictures of this planet, we often see a large red dot. That dot is a storm that’s been going on literally for centuries on Jupiter’s surface. And the radius of this red dot, in other words, the size of the storm, is larger than the radius of the Earth. After this comes Saturn, famous for its rings of ice and rock. And then, there is Uranus, which spins on its axis at a very unusual angle. And finally, there’s Neptune, a dark, cold planet which experiences winds moving faster than the speed of sound.

These are the eight full-sized planets in our solar system, and they’re divided up into different categories. The four planets closest to the Sun are called rocky planets. Another name for this is a terrestrial planet. And it’s one which is mostly made of rocks or metal. Then, there’s the gas giants, Jupiter and Saturn. These are larger planets that have a relatively lower density. They’re made mostly of hydrogen and helium. And then, Uranus and Neptune are known as ice giants. These are planets typically made of elements heavier than hydrogen and helium.

Now, if we take a step back and consider this overall picture, the Sun and the eight full-sized planets, we know that this is often how our solar system is represented. And yet, we know that this picture isn’t quite complete. We know that because when we on Earth look up into the night sky, often we’ll see something we call our moon. And it turns out that Earth isn’t the only planet that has a moon. And indeed, some planets have more than one. The current count for moons in our solar system is 193. Some are bigger, some are smaller. But the condition for a moon is that it be a celestial body that orbits or moves around a planet. So in just the same way that planets will orbit a star, in our case, our Sun, so moons will orbit a planet.

And along with moons, there are bodies in our solar system known as comets. Comets are objects that have a core that’s made of ice and dust and then have a distinctive tail of gas and dust behind them. Just like moons and just like planets, comets too move in orbits. And when we think about orbits, there are generally two types that we see. One type is a circular orbit where there’s, say, a planet at the center of the orbit and a moon moving circularly around it. And the other type of orbit is called an elliptical path. This looks like a circular orbit that’s been squished in one direction. Comets are known for traveling in orbits that follow an elliptical shape like this.

Now, as we consider our solar system and space overall, one of the most intriguing questions is “is there life on other planets?” Though we haven’t yet found any such life, there are clues that help us know where to look. Perhaps, the biggest clue is “is it possible for liquid water to exist on this planet?” When we’re looking for this, we’re looking for what’s called a habitable zone. This is a place where water can exist as a liquid, in other words, of a location is not too hot, and it’s not too cold.

Now that we know a bit about our solar system, how it began and what it consists of, let’s get a bit of practice through an example.

In our solar system, what type of planets are found closest to the Sun?

Okay, so to answer this question, we’re going to need to know about the different types of planets in our solar system. Now, in general, a planet is a body that orbits around a star, in our case, our Sun. But not all planets are the same. We’ve classified them according to their types. In our solar system, there are three types of full-sized planets. First, there are rocky planets, these are planets made mostly of rock or metal. They’re also called terrestrial planets. Then, there are planets known as gas giants. This term describes large planets that have a low density. And typically, these planets are made of hydrogen and helium. And then, finally, some planets in our solar system are ice giants. These masses are made of elements that are heavier than helium and hydrogen.

So these are the types of planets that we find in our solar system. But we want to know which type of planet is found a closest to our Sun. Now, at this point, we’d like to be able to recall the different planets in our solar system and the order that they appear from the Sun. To do that, we can recall this memory aid. It goes like this: “My Very Educated Mother Just Served Us Nachos.” The first letter in each one of these words stands for the name of a planet in its order from the Sun. Since there are eight words in that sentence, we know that there are eight planets we want to write down.

And the first one — the one closest to the Sun — starts with an M and its name is Mercury. So let’s say that’s Mercury. Then comes the planet starting with V. That’s Venus. There we go with Venus. Next comes our favorite planet of all, planet Earth. And after that, another planet starting with M. This is Mars. Then, the one starting with J. That’s Jupiter, the biggest of the planets. Then S stands for Saturn, the planet with rings. U stands for Uranus. And lastly, N — in nachos — stands for Neptune.

Now that we have the planets in our solar system in order, we can divide them up according to their type. Starting from the ones furthest from the Sun, Uranus and Neptune, these are both ice giant planets. Then, the next two in Jupiter and Saturn are gas giants. And lastly, the next four planets, the ones that are closest to the Sun, are known as rocky planets. So that includes Mercury, Venus, Earth, and Mars. But our question just wants to know what type of planets are found closest to the Sun. And we can now answer that question. Rocky planets are the planet type found closest to our Sun.

Let’s now look at a second example about our solar system.

What type of planet is Uranus?

Okay, starting off, we know that Uranus is a planet. And this question is asking, what type of planet is it. And it’s true there are different types of planets. Our solar system has three different types. First, there are rocky planets. These are planets made mostly of metal or rock. Then, there are planets known as gas giants. These are larger planets that have low density. And they’re often made of the two elements helium and hydrogen. And then, in our solar system, there’s a third planet type called an ice giant. These planets are typically colder and made of heavier elements than gas giants. So in our solar system, there are rocky planets, there are gas giants, and there are ice giants. And so, we know that Uranus must be one of these three types.

To start figuring out which one it is, let’s recall the order in which the planets appear from the Sun. So here we have our Sun. And it’s at this point that we can recall a memory help for recalling the order of the planets as they appear from the Sun. It goes like this: “My Very Educated Mother Just Served Us Nachos.” In this sentence, the first letter of each word stands for the name of a planet. And all these words are in careful order. The planet closest to the sun starts with M. Its name is Mercury. There is mercury right there. Then, the next planet starts with V. It’s Venus. Then, after that, the planet’s name starts with an E. That’s Earth. Next is another planet whose name starts with an M. That’s Mars. Then, we go on to J. That’s Jupiter. Then S, that stands for Saturn. Then, U stands for Uranus. And finally, N stands for Neptune.

Now that we have all the full-sized planets in our solar system written out in order, we can start to classify them. These first four planets, the ones closest to the Sun, are called rocky planets. And then, the next two Jupiter and Saturn are gas giants. And lastly, Uranus and Neptune are the same type of planet. These are ice giants. And since our question asked, what type of planet is Uranus, that answers our question. Uranus is an ice giant planet, one made up of elements heavier than helium and hydrogen.

Let’s summarize what we’ve learned about the structure of the solar system. First off, we saw that the star of our solar system, our Sun, was formed from dust and gas in space that was pulled together thanks to the force of gravity. And then, the planets of our solar system formed from matter left over from the Sun’s formation. These planets, in order from being nearest to the Sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. And then, we learned a sentence that helps us remember the order of those planets, where the first letter of each word of the sentence stands for the name of a planet. And lastly, we saw that planets, moons, and comets all have orbital paths that are either circles or ellipses. And its movement along these paths that gives the whole system a dynamic aspect.

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