10.1 The solar system and orbits — Learn

Prerequisites: from earlier science, that gravity is a non-contact force that pulls masses together; from Chapter 3, that velocity is speed in a given direction; from Chapter 9, that nuclear fusion joins light nuclei and releases energy; and writing very large numbers in standard form.

By the end of this section you can: describe the main features of the solar system; compare the planets, their moons and artificial satellites; explain how gravity keeps a planet or a satellite in a circular orbit; explain how the Sun formed from a cloud of dust and gas; and explain why fusion reactions keep the Sun stable.

1. What is in the solar system

The solar system is the Sun and everything that orbits it. It has one star, the Sun. Eight planets orbit the Sun, and dwarf planets such as Pluto orbit it too. Smaller bodies called asteroids and comets also orbit the Sun.

A moon is a natural satellite. It orbits a planet. Some planets have many moons, and some have none. An artificial satellite is built by people and put into orbit. Hundreds of artificial satellites orbit the Earth.

The Sun is one of billions of stars in the Milky Way galaxy. The solar system is a very small part of the Milky Way.

The planets are spread out over a huge range of distances. This table gives the distance of each planet from the Sun, and the time it takes to orbit once.

Two patterns stand out. The planets further from the Sun are much further apart than the inner planets. They also take much longer to go round once. Neptune is 30 times further from the Sun than the Earth, and its orbit takes 165 times longer.

Worked example 1. Describe the main features of our solar system.

• There is one star, the Sun.
• Eight planets orbit the Sun, and dwarf planets orbit it too.
• Natural satellites, called moons, orbit the planets.
• Artificial satellites built by people also orbit the Earth and other bodies.
• Asteroids and comets orbit the Sun.
• The solar system is a small part of the Milky Way galaxy.

2. Planets, moons and artificial satellites

Planets, moons and artificial satellites are all held in orbit by gravity. That is what they have in common. They differ in where they orbit and in how they were made.

A planet is large and roughly spherical. A moon is smaller than the planet it orbits. An artificial satellite can be any size and shape, because engineers build it for a job. A communications satellite carries aerials. A weather satellite carries cameras.

Worked example 2. Give one similarity and one distinction between a moon and an artificial satellite.

One similarity is that both orbit a larger body, and gravity keeps both of them in orbit. One distinction is that a moon is natural and always orbits a planet. An artificial satellite is built by people and can orbit a planet or another body.

3. Gravity and circular orbits

Gravity is a force of attraction between any two masses. It acts across empty space. Every object with mass pulls on every other object with mass.

Gravity provides the force that keeps a planet or a satellite in a circular orbit. The force on the orbiting body is directed towards the centre of the body it orbits. This force bends the path of the object. Without the force, the object would travel in a straight line.

The telescope in the chapter problem is 540 km above the Earth. The Earth still pulls on it. The pull bends its path into a circle, so it falls around the Earth instead of into it.

Watch out. A pupil may say that the telescope floats because there is no gravity in space, or because it is above the air. That is a trap. Gravity acts between all masses, so the Earth still pulls on the telescope. The force is weaker far from the Earth, but it is not zero. Re-check by asking what force keeps the Moon in orbit 384 000 km away.

Worked example 3. Explain what keeps a planet in a circular orbit around the Sun.

Gravity provides a force of attraction between the planet and the Sun. The force on the planet is directed towards the Sun, the centre of the orbit. The force changes the direction of the planet, so its path curves into a circle instead of going in a straight line.

Worked example 4. The telescope orbits 540 km above the Earth. A radio signal from the telescope travels at $3.0 \times 10^{8}\,\text{m/s}$. Calculate the time for the signal to reach the ground.

The signal travels at a steady speed, so use $\text{time} = \dfrac{\text{distance}}{\text{speed}}$. First change the height to metres, so $540\,\text{km} = 5.4 \times 10^{5}\,\text{m}$. The answer is $1.8 \times 10^{-3}\,\text{s}$, which is 1.8 ms.

4. Velocity and a stable orbit

Velocity is speed in a given direction. A body in a circular orbit moves at a constant speed, but its direction changes all the time. Its speed stays the same, yet its velocity changes, because the direction is part of the velocity.

A stable circular orbit needs a speed that matches its radius. If the speed of the body changes, it can no longer stay in a stable orbit at the same radius. The radius must change until the speed and the radius match again. The exact rule for this is studied later; at GCSE you explain it in words.

Worked example 5. The telescope moves at a constant speed in its orbit, but its velocity changes. Explain how this is possible.

Velocity is speed in a given direction. As the telescope goes round its circular orbit, the direction of its motion changes all the time. Its speed stays the same, but the changing direction means the velocity changes.

5. How the Sun formed and why it is stable

The Sun was formed about 4600 million years ago. It began as a cloud of dust and gas, called a nebula. Gravity pulled the dust and gas together. As the cloud shrank, it became hotter and denser.

When the centre became hot enough, hydrogen nuclei began to join together to form helium. This process is nuclear fusion. Fusion releases a large amount of energy. A star is born when fusion starts in its core.

Fusion pushes outwards and tends to make the star expand. Gravity pulls the material inwards and tends to make the star collapse. In a stable star, these two are equal. The star is in equilibrium, so it keeps the same size and shines at a steady rate.

Worked example 6. Explain why the Sun stays the same size.

Fusion reactions in the core release energy. This pushes outwards and tends to make the Sun expand. Gravity pulls the material of the Sun inwards and tends to make it collapse. The outward push and the inward pull are equal, so the Sun is in equilibrium and stays the same size.

6. Try it: the balance inside a star

The figure shows a star with two opposing tendencies. Gravity pulls the material inwards. Fusion pushes it outwards. The sliders change the strength of each one.

Predict first. Do not move the sliders yet. Write down your answers.

1. The Sun has been stable for billions of years. What must be true about the two strengths?
2. If the outward push from fusion became larger than the inward pull of gravity, what would happen to the star?

Test. Set both sliders to the same value and watch the readout. Now move the fusion slider up, then down. Watch the size of the star and the word in the readout.

Explain. When the two strengths are equal, the star keeps the same size. Explain why. Use the words gravity, fusion and equilibrium in your answer.

7. The life cycle of a star

A star goes through a life cycle. The path it takes depends on its mass.

A star about the size of the Sun follows this path.

1. A cloud of dust and gas is pulled together by gravity.
2. Fusion starts, and the star becomes a main-sequence star like the Sun.
3. The star expands into a red giant.
4. It then cools and collapses into a white dwarf.

A star much more massive than the Sun follows a different path.

1. It forms from a nebula and becomes a main-sequence star.
2. It expands into a red supergiant.
3. It explodes as a supernova.
4. The core is left behind as a neutron star, or as a black hole if the star was massive enough.

Fusion in stars makes the naturally occurring elements up to iron. Elements heavier than iron are made in a supernova. The explosion spreads those elements through space, and they become part of new stars and planets.

Progress check

Return to the chapter problem. The telescope orbits the Earth at a height of 540 km. State the force that keeps it in orbit, and describe the direction of that force.

You should now be able to

• describe the main features of the solar system, including the planets, the dwarf planets, the moons and the artificial satellites;
• compare a planet, a moon and an artificial satellite, and state one similarity and one distinction;
• explain how gravity keeps a planet or a satellite in a circular orbit;
• calculate the time a signal takes to travel a distance at a steady speed;
• explain how the Sun formed from a cloud of dust and gas;
• explain why fusion reactions keep the Sun stable;
• describe the life cycle of a star the size of the Sun and of a much more massive star;
• explain where the naturally occurring elements were made.