Key Stage 4 science · Physics – Space physics

GCSE space physics: the solar system and orbits

The Key Stage 4 programme of study asks students to know the main features of the solar system, in a single short statement. Schools usually take that to mean the Sun, the planets and dwarf planets, moons, asteroids and comets, and how gravity keeps them in orbit, and many courses also cover how the solar system formed. It builds on gravity and the seasons from Key Stage 3, and it gives students a sense of scale that is often the most memorable part.

The solar system and orbits, explained

The solar system is the Sun and everything that orbits it. The Sun is a star and holds more than 99% of the mass of the whole system, which is why its gravity controls everything else. Around it are eight planets. The four inner planets, Mercury, Venus, Earth and Mars, are small and rocky. Between Mars and Jupiter lies the asteroid belt, a wide ring of rocky fragments, which is much emptier than films suggest. Beyond it come the giant planets: Jupiter and Saturn, made mostly of hydrogen and helium, and Uranus and Neptune, often called ice giants. Further out still is a region of icy objects, including dwarf planets such as Pluto.

Astronomers agreed in 2006 that a planet must orbit the Sun, be massive enough to be nearly round and have cleared other objects from the neighbourhood of its orbit. Pluto meets the first two but not the third, so it is classed as a dwarf planet. Moons are natural satellites that orbit planets: Earth has one, while Jupiter and Saturn each have a great many, and new small ones are still being found. Comets are lumps of ice and dust in elongated, oval orbits. When one swings close to the Sun, some of the ice turns to gas and releases dust, forming a glowing tail that points away from the Sun.

Gravity is the force that holds all this together. A planet in orbit is always being pulled towards the Sun, which keeps changing the direction of its motion so that it follows a curved path. Its speed may stay steady in a circular orbit, but its velocity keeps changing because velocity includes direction. Planets close to the Sun feel stronger gravity, so they move faster and take less time to go round: Mercury’s year is 88 days, Earth’s is about 365 days and Neptune’s is about 165 Earth years. The planets’ orbits are ellipses, but nearly circular. The same ideas apply to moons orbiting planets and to artificial satellites orbiting the Earth.

Many courses also teach that the solar system formed about 4.6 billion years ago, from a huge cloud of gas and dust. Gravity pulled the cloud together, most of the material collected in the middle to form the Sun, and the rest formed a spinning disc in which the planets built up. Scale is the other big idea: the distances are so huge that astronomers use the astronomical unit, the average distance from the Earth to the Sun, about 150 million kilometres. Courses vary in how far they take space beyond this, into the life cycle of stars and the universe, and those topics are not required by this single statement.

Words your child will hear

Orbit: The curved path of an object around a star, planet or moon, held in place by gravity.
Dwarf planet: A round object that orbits the Sun but has not cleared its orbit of other objects, such as Pluto.
Asteroid: A rocky object, much smaller than a planet, orbiting the Sun, mostly in the belt between Mars and Jupiter.
Comet: A body of ice and dust in a long oval orbit that develops a tail near the Sun.
Natural satellite: A moon: a natural object that orbits a planet.
Astronomical unit: The average distance from the Earth to the Sun, about 150 million kilometres.
Gas giant: A very large planet made mostly of hydrogen and helium, such as Jupiter and Saturn.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about the main features of the solar system

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

Try it: building a scale model

Plan a scale model of the solar system in which the Earth is 1 metre from the Sun. Where do Jupiter and Neptune go, and how big are the Sun and the Earth?

Set the scale. One astronomical unit is the Earth–Sun distance, 150 million km, and it becomes 1 metre. Other planets’ distances are given in astronomical units, so they convert directly into metres.
```
150 000 000 km = 1 m
```
Place Jupiter and Neptune. Jupiter is about 5.2 astronomical units from the Sun and Neptune about 30, so on this scale they are 5.2 m and 30 m away. Neptune would be at the far end of a long school corridor.
```
Jupiter: 5.2 × 1 = 5.2 m
Neptune: 30 × 1 = 30 m
```
Scale the Sun. Its diameter is about 1 400 000 km. Divide by 150 million to get the size in metres.
```
1 400 000 ÷ 150 000 000
  = 0.0093 m = about 9 mm
```
Scale the Earth, with a diameter of about 12 700 km.
```
12 700 ÷ 150 000 000
  = 0.000085 m = 0.085 mm
```

Jupiter is 5.2 m and Neptune 30 m from a Sun about 9 mm across, roughly the size of a pea. The Earth is a speck about 0.09 mm wide, about the thickness of a hair. Space is mostly empty, and the nearest other star would be about 270 km away on this scale.

Common mistakes and how to help

There is no gravity in space, so satellites float

Astronauts look weightless on video, which suggests there is nothing pulling on them.

What helps: Gravity is only slightly weaker at the height of a space station. The astronauts appear weightless because they are in free fall, constantly falling around the Earth, not because gravity has gone. Orbiting means falling but always missing.

The asteroid belt is crowded, so a spacecraft has to dodge rocks

Films show dense fields of tumbling rocks that touch or nearly touch.

What helps: The asteroids are spread over an enormous volume, and the typical gap between them is hundreds of thousands of kilometres or more. Spacecraft have crossed the belt without any special manoeuvres.

The solar system is the same thing as a galaxy or the universe

These are all “space” in everyday talk, and big numbers are hard to compare.

What helps: The solar system is the Sun and what orbits it. The Sun is one star among hundreds of billions in the Milky Way galaxy, and the universe holds an enormous number of galaxies. The scale model above shows how far away the nearest star is.

Pluto is a planet, or Pluto was demoted because it is too small

Many people learnt nine planets at school, and “dwarf” sounds like size alone.

What helps: Pluto is a dwarf planet because it has not cleared its orbit of other objects, not simply because of its size. Ask for the three conditions for a planet and test Pluto against each.

How to help at home

Walk a scale model. Using the worked example’s scale of 1 metre per astronomical unit, mark Mercury at 0.4 m up to Neptune at 30 m along a path or garden, with a pea for the Sun. Walking the distances makes the empty space real.
Look at the night sky. Bright steady lights that don’t twinkle are often planets. On a clear evening, spot Venus, Jupiter or Mars with the help of a star chart, and ask what they are and why they move against the background stars over weeks.
Compare year lengths. Look up each planet’s orbital period and plot against distance from the Sun. The trend, that further planets take longer, is a good way to practise describing a pattern in data and linking it to gravity.
Explain an orbit. Swing a soft ball tied to a string in a circle, outdoors in an open space, and ask what provides the force that keeps it moving in a circle, and what plays that part for a planet. Then ask which way the ball would go if the string were let go. Keep well away from people and windows.
Compare two satellites. A satellite used for satellite television stays above the same spot on the Earth: it orbits about 36 000 km up and takes a day to go round, matching the Earth’s spin. A space station orbits only about 400 km up and goes round in about an hour and a half. Ask your teenager why the lower one has to move so much faster.

Builds on

Key Stage 3: Gravity, stars, galaxies and seasons

Everything in Key Stage 4 science

Updated 20 October 2026.