Lesson video

Hello, my name's Dr.

George, and this lesson is called "The solar system." It's part of the unit "Gravity in space." The outcome for this lesson is I can describe the main features of the solar system.

I'll be introducing these keywords during the lesson and this slide's here in case you want to come back any time and remind yourself of the meanings.

The lesson has three parts which are called "The Sun, planets, and dwarf planets," "Moons" and "Universal force of gravity." Our solar system contains a single star at its centre, the Sun.

And the solar system also contains eight planets, several dwarf planets, many moons, and millions of comets and asteroids.

Our sun is a fairly typical star and it only looks different from the other stars because they're so much nearer than they are.

There are four rocky planets, inner planets, including Earth.

In order of distance from the Sun, they're Mercury, the smallest planet, Venus, a planet of very similar size to Earth, Earth, our own planet, with liquid water, and Mars, which is smaller and cooler than Earth.

So a question for you.

Which of the following is a rocky inner planet? And when I ask a question, I'll wait five seconds.

But if you need longer, press pause, and press play when you have your answer ready.

And the correct answer is Earth.

It's the third closest planet to the Sun and it has a rocky surface which is partly covered in water.

The four outer planets in the solar system are much larger than the inner planets.

They're composed of gases and ices instead of rocks.

Jupiter is a gas giant and it's the largest of all the planets in our solar system.

Saturn is a gas giant with an enormous set of rings.

Since Jupiter and Saturn are both mostly made of hydrogen and helium gases, there would be no solid surface that a spacecraft could land on.

Deeper down, the pressure is so high that the hydrogen is liquid.

Jupiter could fit 1,000 Earths inside it.

It has a feature called the Great Red Spot, which can be seen with only a fairly small telescope from Earth.

It changes over time but is currently around 16,000 kilometres across, wider than the diameter of our planet.

It appears to be a huge and very long-lasting storm.

And the two outer planets are often called gas giants but they can also be called ice giants.

They're composed of gases and ices instead of rocks.

There's Uranus, an ice giant, and Neptune and other ice giant and the furthest planet from the Sun.

Like Jupiter and Saturn, Uranus and Neptune have a hydrogen and helium atmosphere, but what makes them different is that beneath this atmosphere is compressed slushy water mixed with methane and ammonia.

Only one spacecraft has travelled close to Uranus and Neptune briefly making observations as it passed each of these distant planets.

Which two of the four giant planets are sometimes called ice giants? And the answer is Uranus and Neptune.

These two are colder and further from the Sun, so they contain large amounts of frozen material.

There are many ways of remembering the order of the planets.

Most of these are acronyms, so something where you make up a sentence using the first letters of the planets.

Here's an example.

My very easy method just speeds up naming.

Using the first letters of Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Now on this page the list shows the planets in alphabetical order.

Can you place them into the order of distance from the Sun, starting with the closest to the Sun? And the correct order is this.

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

There are other very large objects which orbit the Sun called dwarf planets.

There are five named dwarf planets in the solar system.

Ceres, Pluto, Haumea, Makemake, and Eris.

Pluto is the most well-known example of a dwarf planet and it used to be considered to be a planet.

The reason why dwarf planets aren't counted as proper planets is because they're not completely spherical and they haven't cleared their orbital path of other objects, so they haven't made an empty path all the way around the Sun.

The solar system contains billions of smaller rocks orbiting the Sun, and these are called asteroids.

There is a ring of millions of asteroids spread between the orbits of Mars and Jupiter in a region called the asteroid belt.

Asteroids can also be found in a region beyond the orbit of Neptune, a region called the Kuiper Belt, and also at the edge of the solar system in what's called the Oort Cloud.

Asteroids can range in diameter from just a few metres to nearly 100 kilometres.

And in addition, there are also billions of icy rocks in the solar system called comets.

The Oort Cloud contains most of these, so we can't usually see them as they're too far away from the Earth.

But sometimes comets get knocked out of the Oort Cloud and they begin to orbit the Sun in very long, elliptical orbits.

An ellipse, sometimes called an oval, is a squashed circle.

Sometimes from Earth we can see the gassy tail of a comet as it passes near the Sun.

Which of the following lists of objects in the solar system is most accurate? And the correct answer is A.

A star, the Sun, planets, dwarf planets, moons, asteroids, and comets summarises the objects found in our solar system.

Our solar system doesn't contain galaxies.

Our solar system itself is within a galaxy.

And now here's a longer task for you.

There's a table showing the distances of the eight planets from the Sun compared to Earth's distance.

So Earth will be counted as distance 1 and, for example, Mars distance 1.

5 because it's 1.

5 times as far from the Sun as the Earth is.

The table also shows the mean surface temperature on each planet and the density of the planet, the average density.

There are three questions for you here.

Describe and explain the relationship between distance from the Sun and surface temperature.

Then pick out which planet is the exception to this pattern.

And finally, use the data to discuss why the planets are often categorised into rocky planets and gas giant planets.

Press pause while you write your answers and press play when you're ready to check them.

And here are the answers to the questions.

The further from the Sun, the lower the temperature of the planet.

This is because the planet will receive less energy from the Sun to heat it.

The exception to the pattern is Venus, which is far hotter than the pattern would suggest.

You can see this in the table.

The temperature is generally going down except for Venus, which is much hotter than Mercury.

This is because it has a thick atmosphere that traps energy from the Sun.

The four inner planets have a much higher density than the four outer planets.

This is because the inner planets are mostly dense rock, while the outer planets are made from less dense gases and ices.

Well done if you got most or all of those right.

And now let's take a look at moons.

Earth has a single natural satellite in orbit around it, which is the Moon.

The Moon takes about one month to completely orbit Earth, to travel around the Earth once.

As it orbits, it also rotates on its axis, it spins, taking the same amount of time to rotate once as it does to orbit once.

This means that the same side of the Moon faces the Earth at all times.

It turns out that that's not an amazing coincidence.

When a moon has been orbiting its planet for long enough, the forces acting between them over time can cause it to end up rotating in the same time as it takes to orbit.

We sometimes call the other side of the Moon that we can't see from Earth the dark side.

So let's check you were paying attention.

Why do we always see the same side of the Moon when it is observed from Earth? And it's because the Moon's period of rotation, time taken to spin, is the same as the period of its orbit, the time taken to travel around the Earth.

Most of the other planets in the solar system also have objects in orbit around them, and these are also moons.

A moon is any natural satellite of a planet.

Moons are in orbit around planets, while planets are in orbit around a star.

That's what makes a planet, a planet, and a moon, a moon.

Most of the moons in the solar system are much smaller than the planet they orbit.

Earth's moon actually is unusually large in comparison to the size of the Earth.

It was probably formed when another small planet collided with Earth during the formation of the solar system.

Mercury and Venus don't have any moons.

Mars has two moons, Phobos and Deimos.

These moons are very small compared with the size of Mars or compared with our moon.

They're thought to have been asteroids that at some point in the past were captured by Mars' gravity.

Phobos is only 25 kilometres across and you can see from this photo that unlike much larger objects in the solar system it's not very close to spherical.

The gas and ice giants each have many moons with a wide range of sizes, from a few kilometres to thousands of kilometres in diameter.

Ganymede, one of Jupiter's moons, is the largest moon in the solar system and it has a larger diameter than the planet Mercury.

But remember, it's a moon because it's orbiting a planet.

Jupiter has over 90 moons, including four large ones.

Ganymede, Callisto, Io, and Europa were all discovered by Galileo Galilei in 1610 using an early telescope.

But many of the smaller moons were discovered much later by space probes visiting the planet in the 20th and 21st centuries.

Each of the four largest moons of Jupiter is interesting in its own way.

Ganymede, the largest moon in the image shown here, seems to have an underground ocean of salt water and it's also the only moon in the solar system that has its own magnetic field.

Callisto, the one on the far right in the image, is more covered in craters than any other object in the solar system.

It's made of roughly equal amounts of rock and ice and may have liquid water more than 100 kilometres below its surface.

Io, the moon on the far left in the image, has very powerful volcanoes and there are lakes of lava on its surface.

And Europa, the second from the left in the image, has the smoothest surface observed of any solid object in the solar system.

Below its surface crust of ice, there seems to be a salt water ocean containing about twice as much water as all of the water in the Earth's oceans.

It may also have all of the chemical ingredients needed for life as we know it.

Ganymede has a larger diameter and greater mass than the planet Mercury.

Why isn't Ganymede classed as a planet? It's because it's orbiting the planet Jupiter and that makes it a moon.

Saturn has over 100 moons, including one that's larger than Earth's moon and another four that are over 100 kilometres in diameter.

Saturn also has many orbiting rings of small rocks and dust.

It may be that Saturn's ring system was formed in the past from moon fragments.

The giant planets Uranus and Neptune also have fairly large numbers of moons, many of which are small.

Uranus has five large moons and over 20 smaller ones, while Neptune has one large moon, Triton, and 15 smaller ones.

There are not as many known details or images of these moons as the Uranus and Neptune systems are very distant and haven't been explored as much as closer planets.

Now a question.

Saturn's moon Titan has a greater diameter than the planet Mercury, but it has a smaller mass.

Why do you think this is? The correct answer is B, Titan is less dense than the planet Mercury, so it's possible for Titan to be smaller and yet have more mass.

Well done if you picked that one.

The reason Titan is less dense is because it contains a large amount of water, whereas Mercury is entirely rock, which is denser than water.

And now a written task for you.

I'd like you to describe some differences between a planet, a dwarf planet, and a moon, giving an example of each.

And then there are a couple of questions about the moons of Jupiter.

Press pause while you write your answers and press play when you're ready to check them.

And here are some example answers.

"A planet orbits around a star and is large and spherical and has also cleared other objects from its orbit.

An example is Jupiter." Of course, you could pick any of the eight planets here.

"A dwarf planet is large and orbits a star, but it may not be spherical or have cleared away other objects.

An example is Pluto." And there are other examples you could choose, such as Makemake or Haumea.

"Moons orbit around planets.

They can be as large as some planets, but may also only be a few kilometres across.

An example is Europa, one of Jupiter's moons." And there are many, many possible examples you could use there, including our own moon, which is simply called the Moon.

The next question in part A asked, why the first four moons of Jupiter were not discovered until 1610? To answer that, you could say this.

"The moons of Jupiter are too faint to see with the naked eye because they're so far away.

They were discovered shortly after the telescope was invented as this could magnify the images and make the moons visible." And then Part B asked, why moons were still being discovered in the 20th century? "Some of the moons are too small to see, even with powerful telescopes.

They were discovered when space probes were sent to Jupiter in the late part of the 20th century and after." Well done if you got most or all of those right.

And now let's think a bit more about gravity.

Isaac Newton developed his theory of gravity based on observations of objects falling on Earth and observations of the movements of planets.

He realised that the same force could be acting on both.

So the same force that attracts all objects with mass towards the Earth could be the force that makes the planets move the way they do.

He called this force gravity and developed an equation to describe the size of the attraction.

We're not going to use that equation here, we'll just be using descriptions of how gravity behaves.

Newton's universal law of gravity can correctly predict the movement of the planets.

It says that the size of the gravitational force between any two objects is directly proportional to the masses of the objects, so the force is larger between objects with more mass, and inversely proportional to the square of the separation of the objects, the distance between them, so the force is smaller when objects are further apart.

Remember, when two variables are directly proportional, then one variable is multiplied by a number, the other variable gets multiplied by the same number.

For example, when one doubles, the other doubles.

With inverse proportion, if one variable is multiplied by a number, the other variable gets divided by that same number.

The gravitational force is inversely proportional to the square of the distance between the objects, so it falls quite rapidly as the separation increases.

So which of the following will increase the size of the gravitational forces acting between two objects? And the two correct answers are moving them closer together or increasing the mass of the objects.

Newton's law can explain the orbital motion of the planets and moons.

The force of the Sun's gravity is always pulling the planets towards it, but the planets never get any closer because they're in motion.

How gravity causes orbits can be pictured using what's called a thought experiment.

It's an experiment we can imagine but can't really carry out, but it's still useful for helping us understand something.

Imagine standing on a very tall hill and throwing a ball horizontally, and let's say there's no air resistance, to keep things simple.

So we throw in this direction.

But then the gravitational force on the ball will pull it downwards towards the ground as it travels.

So the ball follows a curved path and soon hits the ground, and we know this is how objects behave when they're thrown.

Now imagine throwing the ball a little harder.

It'll travel further before it reaches the ground, as shown by the dash line.

And if we throw it even harder, we follow the path shown by the dotted line.

The harder the ball is thrown, the further it travels before it hits the ground.

But the ground isn't really flat because Earth is a sphere.

Before we go onto that, though, can you answer this question? Which of the balls whose paths are shown below was thrown with the greatest horizontal speed? And the answer is A, the one shown by the dotted line, which travels the furthest before hitting the ground.

Now in reality, Earth is almost perfectly spherical, it's not flat.

And so the ground curves away in the direction the ball is thrown.

The ball will fall towards the ground as it travels, but it will travel a greater distance before hitting the ground than it would if the Earth had horizontal ground.

If we now show the whole of Earth in this picture, the ball could be thrown in this thought experiment at such a great speed that it falls to the ground at the same rate as the surface of the planet curves away from it.

So what's happening to the ball here is it's constantly falling towards the ground but never gets any closer to it.

It's now in orbit.

Planets orbit the Sun because its gravitational force is always pulling them towards it.

They do accelerate towards the Sun without getting any closer to it, as they're travelling at speed which makes them move in circles.

By the way, real orbits aren't perfectly circular, they're slightly squashed, they're slightly elliptical.

Which of the balls, whose paths are shown below, is in orbit around the planet? And the answer is the one with the path shown by the dash line, the one that goes in the circle all around the Earth.

It was Isaac Newton who came up with this thought experiment, which links the idea of falling objects with the idea of orbiting objects.

Orbiting is a kind of falling.

In modern times, we actually do launch objects from Earth into orbit.

These are artificial satellites.

We don't actually fire them sideways from the tops of mountains, but by firing them upwards from the ground and then turning them so that they settle into an orbit.

And a final task for you to show what you've learned.

Which factors affect the size of the gravitational forces acting between two objects? And then can you explain why the Moon travels around the Earth in an almost circular path? Include a diagram to show how the Moon is moving and then the forces acting on it.

You don't need to explain why it's not quite circular, just assume it is circular and explain that.

Press pause while you write your answers and press play when you're ready.

And here are the correct answers.

"The size of a gravitational force depends on the masses of the two objects; greater masses experience larger forces.

And the separation, the distance that the two objects are apart.

The further apart they are, the smaller the gravitational force between them." And then explaining the Moon's orbit.

"The Moon is attracted towards Earth by a gravitational force.

This force pulls it towards Earth but it doesn't get any closer because it's moving at the right speed to follow a circular path around the Earth." You may have worded your answer slightly differently, so you'll need to check whether it's equivalent to what's written here.

And that's the end of this lesson, so I'll finish with a summary.

The solar system contains the Sun, a star, and the objects in orbit around it.

There are eight planets in almost circular orbits, of which four are inner rocky planets, Mercury, Venus, Earth, Mars, and four are outer gas and ice giants, Jupiter, Saturn, Uranus, and Neptune.

There are several dwarf planets, including Pluto, an asteroid belt, clouds of comets, and over 100 moons orbiting the planets.

The solar system is held together by gravitational forces which cause the moving planets to orbit around the Sun and the moons to orbit the planets.

Well done for working through this lesson.

I hope you found it interesting and I hope to see you again in a future lesson.

Bye for now.