Earth and Space · Grade 6

What holds the planets in their orbits?

One force only — gravity. The Sun pulls every planet towards it, and the planet moves forward fast enough to keep falling around the Sun instead of into it. The same force holds the Moon around the Earth. An orbit is nothing more than a fall that never reaches the ground.

Learning objectives

The order, and two families

Out from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Eight, since Pluto was reclassified.

The first four are small and rocky, with a surface you could stand on. The last four are enormous and made mostly of gas — there is no ground there to land on.

The split is not an accident. Close to the Sun it was too hot for light gases to stay, so only heavy rocky material was left. Further out, the cold let gases gather.

Why the distant ones are slow

A distant planet has a far longer journey — a larger circumference around the same Sun.

On top of that, the Sun's gravity weakens with distance, so it also travels more slowly along that path.

The two combine. The Earth completes an orbit in a year; Neptune, about thirty times further out, needs more than a hundred and sixty. Nobody has yet watched a full Neptunian year since it was discovered.

Mass, weight and gravity

Your mass is how much material you are made of, and it is the same everywhere in the universe. Your weight is the force a world exerts on you, and it changes with where you are.

On the Moon gravity is about a sixth of the Earth's, so you would weigh about a sixth there. Your mass has not changed at all — you lost nothing on the way.

And it is the same force holding the whole system together. It pulls you to the ground, holds the Moon around us, and holds the Earth around the Sun. One force, at every scale.

Worked examples

  1. Which planets are rocky and which are gas giants?

    1. The four inner ones are close to the Sun
    2. The four outer ones are distant and cold

    Answer: Rocky: Mercury, Venus, Earth, Mars. Gas giants: Jupiter, Saturn, Uranus, Neptune

  2. Why does Neptune orbit the Sun more slowly than the Earth?

    1. Its path is far longer
    2. The Sun's gravity is weaker out there
    3. So its speed is smaller too

    Answer: A longer path and a lower speed — more than a hundred and sixty years per orbit

  3. A 40 kg child travels to the Moon. What changes?

    1. Mass is the amount of material and does not depend on place
    2. Weight is a force and depends on local gravity
    3. On the Moon gravity is about a sixth

    Answer: The mass stays 40 kg; the weight falls to about a sixth

Common mistakes

Thinking there is no gravity in space
There is, and it is exactly what holds the Moon and the planets in orbit. An astronaut floats because they are falling around the Earth along with the spacecraft, not because the force has gone.
Saying someone 'weighs less' on the Moon and so contains less material
The mass has not changed. Only the force pulling on it is smaller, so the scale shows a smaller number.
Assuming all the planets are alike
Four are rocky and four are gas giants with no surface to stand on. The distance from the Sun when the system formed is what made the difference.

What to remember

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