State and explain how the orbital speed and surface temperature of a planet are each related to its distance from the Sun.

Cambridge International IGCSE Physics (0625) — 6.1 Earth and the Solar System · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Orbital speed decreases as distance from the Sun increases because the Sun’s gravitational field strength falls off with distance, so the force pulling the planet along its orbit is weaker.
Surface temperature decreases as distance from the Sun increases because planets further from the Sun receive less solar radiation/energy per unit area.

Examiner tips

Use the word ‘decreases’ for both speed and temperature. Explain the cause: weaker gravity for speed, less radiation for temperature. Keep sentences short and use exact terminology from the mark scheme.

Common mistakes

Confusing ‘increases’ with ‘decreases’ for speed or temperature. Failing to mention the Sun’s gravitational field or solar radiation as the cause. Using vague phrases like ‘farther away’ without linking to the physical reason.

Mark scheme (4 marks)

Orbital speed decreases as distance from the Sun increases
Because the Sun's gravitational field strength decreases with distance, so there is less force pulling the planet along its orbit
Surface temperature decreases as distance from the Sun increases
Because planets further from the Sun receive less radiation / energy per unit area from the Sun

Key terms in this question

orbital speed · surface temperature