# Explain why planets closer to the Sun orbit at greater orbital speeds than planets further away.

> AQA GCSE Physics (8463) — 4.8.1 Solar system; stability of orbital motions; satellites (Phys only) · Explain · 4 marks

> The planets in our Solar System all orbit the Sun in stable, roughly circular orbits. However, Mercury completes an orbit in about 88 days whilst Neptune takes approximately 165 years.

## Mark scheme (4 marks)

1. Gravitational force / gravitational attraction between the Sun and a planet acts as the centripetal force
2. Gravitational force is greater for planets closer to the Sun (because the distance is smaller)
3. A greater centripetal force requires a greater orbital speed (for a stable circular orbit)
4. Closer planets also travel a shorter orbital path / circumference, so the combination of higher speed and shorter path results in a much shorter orbital period

## Key terms

- [orbital speed](https://www.gradenine.co.uk/glossary/orbital-speed)

## Related

- [Revision notes for AQA GCSE Physics (8463)](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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-planets-closer-to-the-05a4ae4d) · Published by Druglandscape Ltd.