Explain why a planet remains in a stable orbit around a star rather than flying off into space or spiralling inward.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Planets in our solar system have been orbiting the Sun for billions of years without crashing into it or drifting away.
Model answer (5 marks)
1. Gravity between the star and planet provides the attractive force that keeps the planet bound.
2. The planet has a tangential velocity that makes it move sideways relative to the star.
3. Gravity continuously changes the direction of the planet’s motion, acting as a centripetal force that pulls it inward.
4. The planet’s speed is such that the inward pull does not cause it to fall into the star; the two effects are in balance, so it does not spiral inward.
5. As a result the planet follows a stable, roughly circular or elliptical orbit at a nearly constant distance and speed from the star.
2. The planet has a tangential velocity that makes it move sideways relative to the star.
3. Gravity continuously changes the direction of the planet’s motion, acting as a centripetal force that pulls it inward.
4. The planet’s speed is such that the inward pull does not cause it to fall into the star; the two effects are in balance, so it does not spiral inward.
5. As a result the planet follows a stable, roughly circular or elliptical orbit at a nearly constant distance and speed from the star.
Examiner tips
- Use the exact terms: gravity, velocity, centripetal force, stable orbit, circular/elliptical path.
- Show the cause–effect chain: gravity → centripetal force → balance with velocity → stable orbit.
- Keep the answer concise – one sentence per point.
- Mention the balance of forces to justify no spiralling.
Mark scheme (5 marks)
- Gravity/gravitational force acts between the star and the planet
- The planet is moving/has velocity (tangentially/sideways)
- Gravity continuously changes the direction of the planet's motion (acts as centripetal force / pulls it inward)
- The planet does not spiral inward because its speed/velocity prevents it from falling into the star (the two effects balance)
- A stable orbit means the planet travels in a (roughly circular/elliptical) path at a (roughly) constant speed/distance from the star
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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