# Explain why a planet remains in a stable orbit around a star rather than flying off into space or spiralling inward.

> WJEC A-Level Physics (Wales) — 4.3 Orbits and the wider universe · Explain · 5 marks

> Planets in our solar system have been orbiting the Sun for billions of years without crashing into it or drifting away.

## Mark scheme (5 marks)

1. Gravity/gravitational force acts between the star and the planet
2. The planet is moving/has velocity (tangentially/sideways)
3. Gravity continuously changes the direction of the planet's motion (acts as centripetal force / pulls it inward)
4. The planet does not spiral inward because its speed/velocity prevents it from falling into the star (the two effects balance)
5. A stable orbit means the planet travels in a (roughly circular/elliptical) path at a (roughly) constant speed/distance from the star

## Key terms

- [orbit](https://www.gradenine.co.uk/glossary/orbit)
- [stable orbit](https://www.gradenine.co.uk/glossary/stable-orbit)

## Related

- [Revision notes for WJEC A-Level Physics (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-a-planet-remains-in-7fd545aa) · Published by Druglandscape Ltd.