Our solar system — AQA GCSE Physics
Test yourself on Our solar system with AQA GCSE practice questions.
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Our solar system explained
Our solar system contains exactly one star, the Sun, and everything else in it orbits that star either directly or indirectly.
Read the full explanation
Eight planets orbit the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Dwarf planets, including Pluto and Ceres, orbit the Sun as well, but they have not cleared other objects out of their orbital path, which is why they are counted separately. Size alone does not decide the label, and exam questions exploit that: the largest dwarf planet is smaller than the smallest planet, yet some moons, such as Ganymede and Titan, are larger than the planet Mercury. The Sun holds the whole system together by gravitational attraction, and it is the only object in it that gives out its own light.
Natural satellites, the moons that orbit planets, are also part of the solar system.
A satellite is anything in orbit around something else, so the word splits two ways. Natural satellites were not put there by people: the moons that orbit planets are natural satellites, and strictly so is a planet orbiting the Sun. Artificial satellites are the ones we launched, such as the International Space Station or a weather satellite, and they orbit the Earth. If a one mark question asks what a moon is, the answer the mark scheme wants is a natural satellite that orbits a planet, and a rock in space or something that goes round the Earth will not do. The number of moons varies enormously: the Earth has one, Mercury and Venus have none, and Jupiter and Saturn have dozens each.
Our solar system is a small part of the Milky Way galaxy.
Scale is the point of this statement. A galaxy is a collection of billions of stars held together by gravity, and the Milky Way is ours. The Sun is one ordinary star among perhaps a hundred billion of them, sitting far out from the centre, so the whole solar system is a speck on that map. Beyond the Milky Way there are billions of other galaxies, separated by distances far larger than the gaps between stars inside a galaxy. The ladder runs in this order: moons orbit planets, planets orbit a star, stars sit within a galaxy, and galaxies together make up the universe. The faint band of light called the Milky Way in a dark sky is the disc of our own galaxy seen edge on from the inside.
The Sun was formed from a cloud of dust and gas (nebula) pulled together by gravitational attraction.
Stars begin as a nebula, a huge cloud of dust and gas that is mostly hydrogen. Gravitational attraction between the particles pulls the cloud inwards, and as it collapses it becomes denser and hotter, because the particles lose gravitational potential energy and gain kinetic energy as they fall together. The hot dense lump at the centre is a protostar. It is not yet a star: only when the core is hot and dense enough for hydrogen nuclei to fuse do fusion reactions begin and the object become a main sequence star. Our own Sun formed in this way about five billion years ago, out of the remains of earlier stars, which is why the solar system contains heavy elements at all.
Students should be able to explain: how, at the start of a star's life cycle, the dust and gas drawn together by gravity causes fusion reactions that fusion reactions lead to an equilibrium between the gravitational collapse of a star and the expansion of a star due to fusion energy.
A star begins as a cloud of dust and gas. Gravity draws this matter together, causing the cloud to collapse. As it compresses, gravitational potential energy is transferred to thermal energy, making the core incredibly hot and dense. Once the temperature and pressure are high enough, hydrogen nuclei fuse into helium, releasing vast amounts of energy. This fusion marks the start of the main sequence. During this long, stable period, the star is in equilibrium. Gravity continuously pulls the star's mass inwards, attempting to collapse it. However, the energy released by fusion creates an outward pressure that perfectly balances this inward gravitational pull. The star remains at a constant size until the hydrogen fuel in the core begins to run out, disrupting the balance.
Your focus
- Compare the current model of the solar system with an older model in which the Sun and the planets orbit the Earth, giving a similarity and a difference.
- Distinguish a dwarf planet from a planet.
- Use a table of radii to disprove a claim such as planets are always bigger than moons.
Show all 15 objectives
- Define a moon as a natural satellite that orbits a planet.
- Classify given objects as natural or artificial satellites.
- Name one natural and one artificial satellite of the Earth.
- Order the terms moon, planet, star, solar system, galaxy and universe by size.
- Describe what a galaxy is and name the one that contains our solar system.
- Explain what is being seen when the Milky Way appears as a band of light in the night sky.
- Describe the formation of a star from a nebula through to a protostar.
- Explain why a collapsing cloud of dust and gas becomes hotter.
- State the condition that must be met before a protostar becomes a main sequence star.
- Explain how gravity drawing dust and gas together initiates nuclear fusion.
- Describe the opposing forces that create equilibrium in a main sequence star.
- Explain why a main sequence star maintains a constant size over a long period.
Our solar system exam tips
Marking Points
- one mark for one star, the Sun, at the centre of the solar system
- one mark for eight planets orbiting the Sun
- one mark for dwarf planets also orbiting the Sun and being counted separately from the planets
- one mark for using data to show that a moon can be larger than the smallest planet
- one mark for defining a moon as a natural satellite
- one mark for stating that it orbits a planet
- one mark for distinguishing a natural satellite from an artificial one by how it came to be in orbit
- one mark for naming an example of each, such as the Moon and the International Space Station
- one mark for defining a galaxy as a very large collection of stars held together by gravity
- one mark for the Sun being one star among billions in the Milky Way
- one mark for placing the solar system within the galaxy and the galaxy within the universe
- one mark for stating that there are many other galaxies beyond our own
- one mark for a cloud of dust and gas, or nebula, as the starting point
- one mark for gravitational attraction pulling the cloud together
- one mark for the temperature rising as the cloud collapses
- one mark for naming the protostar stage that comes before fusion begins
- Gravity draws dust and gas together, causing the initial cloud to collapse and heat up.
- The core becomes hot and dense enough for hydrogen nuclei to undergo nuclear fusion.
- Gravity produces an inward force that attempts to collapse the star.
- Energy released by nuclear fusion creates an outward pressure.
- The star is in equilibrium because the inward gravitational force equals the outward pressure from fusion.
Examiner Tips
- 💡If a table of radii is given, quote a specific pair of objects that breaks the claim you are asked about.
- 💡Keep the words star, planet, dwarf planet and moon distinct, because each has its own job in the mark scheme.
- 💡Learn the planets in order, since naming the fifth planet from the Sun is a free mark.
- 💡One mark definitions still need both halves: what the object is, and what it orbits.
- 💡Use the term natural satellite rather than moon when the question is testing vocabulary.
- 💡Name a real example if there is room; it costs one clause and often secures the mark.
- 💡Write the order of scale in the margin before answering; it stops the classic mix-up.
- 💡Say billions rather than lots when you describe how many stars a galaxy holds.
- 💡If asked where the solar system is, name the galaxy rather than describing a position.
- 💡Use the word nebula and then explain it, because the definition is often the second mark.
- 💡Name the stages in order, nebula, protostar, main sequence star, when a sequence is asked for.
- 💡Say that gravity does the pulling and that the temperature rises, not just that the cloud gets smaller.
- 💡Always mention both gravity and the outward pressure from fusion when explaining a star's equilibrium.
- 💡Explicitly state that the initial collapse under gravity increases the temperature to initiate fusion.
Common Mistakes
- counting nine planets by including Pluto, which is a dwarf planet
- concluding from a table that planets are always bigger than moons, when Ganymede and Titan are bigger than Mercury
- saying dwarf planets orbit planets, when they orbit the Sun
- describing the Sun as a planet, or as the largest planet
- defining a moon as something that orbits the Earth, which leaves out the moons of every other planet
- calling the International Space Station a moon because it orbits the Earth
- saying that moons orbit the Sun
- writing natural satellite with no mention of what it orbits, which leaves the definition incomplete
- using solar system, galaxy and universe as though they meant the same thing
- saying the Milky Way orbits the Sun, or that the solar system contains galaxies
- believing the Milky Way is the only galaxy
- describing the Sun as the largest or the brightest star in the galaxy
- saying the Sun formed in an explosion rather than by gravitational collapse
- calling a protostar a star, when fusion has not yet started
- writing that the nebula was pulled together by magnetism, or by the Big Bang
- leaving out the heating, so there is no reason given for fusion to start
- Stating that forces balance without identifying gravity and outward pressure from fusion as the specific causes.
- Forgetting to explain that the initial collapse of dust and gas causes the temperature to rise enough for fusion.
- Confusing nuclear fusion with fission, or describing the star as burning fuel chemically.