Neptune orbits the Sun at a much greater distance than Mercury. Explain why Neptune's orbital speed is much lower than Mercury's orbital speed.

Pearson Edexcel International GCSE Physics (4PH1) — 1.4 Astronomy · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Mercury orbits the Sun at an average distance of approximately 58 million km, whilst Neptune orbits the Sun at an average distance of approximately 4 500 million km.

Model answer (4 marks)

Gravity from the Sun acts on both planets, keeping them in orbit.
The gravitational pull on Neptune is weaker than on Mercury because Neptune is much further from the Sun.
The weaker gravitational force provides a smaller centripetal force, so a lower orbital speed is sufficient to keep Neptune in a stable orbit.
Thus Neptune’s orbital speed is much lower than Mercury’s.

Examiner tips

Use the word "gravity" and "centripetal force" to show understanding of the force keeping the planets in orbit.
Explain the distance–force relationship (inverse square law) to justify why Neptune’s pull is weaker.
Show that a smaller force allows a lower speed to maintain the orbit.
Keep the answer concise and use correct UK spelling.

Common mistakes

Confusing orbital speed with orbital period; students may say Neptune moves slower because it takes longer to complete an orbit.
Using vague terms like "less gravity" without linking it to distance or force magnitude.
Failing to mention that the weaker force means a lower speed is sufficient for a stable orbit.

Mark scheme (4 marks)

Gravity (a non-contact force) from the Sun acts on both planets, keeping them in orbit
The gravitational force (pull) on Neptune from the Sun is weaker/smaller than on Mercury because Neptune is further away
The gravitational force provides the resultant force that causes the planet to accelerate (change direction) / keeps the planet in its circular orbit
A smaller resultant/gravitational force means a lower orbital speed is sufficient to maintain Neptune's orbit at its greater distance

Key terms in this question

orbital speed