The Solar System and Gravity

Our home in space

On a clear night you can see thousands of stars, but our nearest star is the one that lights up the day: the Sun. The Sun and everything held to it by gravity make up the solar system — eight planets, dozens of moons, and countless asteroids and comets, all circling that one giant star.

The Sun is colossal. It holds about 99.8% of all the mass in the solar system. That huge mass is the key to understanding everything else, because mass creates gravity.

What is gravity?

Gravity is a force of attraction between any two objects that have mass. Every object pulls on every other object. You are pulling the Earth toward you right now — but because the Earth is so much more massive, it pulls far harder, holding you on the ground.

Two things decide how strong gravity is:

• Mass — more mass means a stronger pull. The Sun's vast mass gives it a powerful grip.
• Distance — the closer two objects are, the stronger the pull. Move them apart and gravity weakens quickly.

Gravity is what gives objects their weight. Weight is just the force of gravity pulling on an object's mass. Gravity is one of the fundamental forces — you can see another everyday force at work in pushes and pulls.

Why do planets orbit?

Newton realised that the same gravity that makes an apple fall also keeps the Moon circling the Earth. So why don't the planets simply fall into the Sun?

The answer is motion. A planet is moving sideways extremely fast. Left alone it would fly off in a straight line forever. But the Sun's gravity constantly pulls it inward, bending that straight path into a curve. The two effects balance, and the planet loops around the Sun in a near-circle called an orbit. This idea connects directly to Newton's laws of motion.

It is a bit like swinging a ball on a string above your head. The string pulls the ball inward; the ball's motion keeps it circling. Cut the string and it flies off. The Sun's gravity is the invisible string.

The eight planets

In order from the Sun:

Order	Planet	Type
1	Mercury	Rocky
2	Venus	Rocky
3	Earth	Rocky
4	Mars	Rocky
5	Jupiter	Gas giant
6	Saturn	Gas giant
7	Uranus	Ice giant
8	Neptune	Ice giant

The four inner planets are small and rocky. The four outer ones are huge balls of gas and ice. The further from the Sun, the longer a planet takes to complete one orbit — Earth takes 1 year, while distant Neptune takes about 165.

Day, night, and the seasons

Earth does two movements at once.

• It spins on its axis once every 24 hours. The side facing the Sun has day; the side facing away has night.
• It orbits the Sun once every 365.25 days — one year.

Earth's axis is tilted by about 23.5 degrees. As Earth orbits, each hemisphere leans toward the Sun for part of the year (summer, with longer, more direct sunlight) and away for another part (winter). That tilt — not our distance from the Sun — is what creates the seasons.

Worked example: weight on the Moon

Weight depends on local gravity. On Earth, gravity pulls with about 10 newtons per kilogram. On the Moon it is only about 1.6 N/kg.

A student has a mass of 50 kg. What is their weight on each world?

Earth: weight = 50 kg × 10 N/kg = 500 N Moon: weight = 50 kg × 1.6 N/kg = 80 N

Same mass, but they weigh more than six times less on the Moon — which is exactly why astronauts can bounce so high.

Try it yourself! 🧪

Model an orbit with a coin and a bowl.

1. Find a large round bowl and a small coin or marble.
2. Roll the coin around the inside rim of the bowl, fast, in a circle.
3. Watch it loop around and around. The curved wall constantly pushes it inward — just as the Sun's gravity pulls a planet inward.
4. As it slows, it spirals down to the centre, like an orbit decaying.

You have just modelled how inward pull plus sideways motion creates an orbit.