Planets

    OCR
    GCSE

    Planets are major celestial bodies orbiting a star, distinct from dwarf planets and satellites, maintained in stable orbits by gravitational attraction acting as a centripetal force. The Solar System comprises eight planets: four inner terrestrial planets and four outer gas giants, a structure determined by the accretion processes within the primordial solar nebula. Physics principles dictate that for stable circular orbits, the gravitational force provides the necessary centripetal force, resulting in a specific relationship where orbital speed decreases as the orbital radius increases.

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    Objectives
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    Exam Tips
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    Pitfalls
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    Key Terms
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    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for listing the eight planets in the correct order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
    • Credit responses that identify gravity as the centripetal force acting towards the centre of the orbit
    • Award 1 mark for describing the geocentric model as Earth-centred and the heliocentric model as Sun-centred
    • For Higher Tier, credit the explanation that velocity changes in a circular orbit due to change in direction, even if speed remains constant
    • Award 1 mark for citing Galileo's observations of Jupiter's moons as evidence supporting the heliocentric model

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly listed the planets, but ensure you include the asteroid belt location for a complete description of the solar system structure"
    • "Good definition of the heliocentric model. To improve, explicitly link Galileo's evidence regarding Jupiter's moons to the rejection of the geocentric model"
    • "You stated speed changes in orbit, but remember speed is scalar; velocity changes because the direction of motion changes"
    • "Excellent link between gravity and orbit stability. Now consider what happens to the required orbital speed if the radius of the orbit increases"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for listing the eight planets in the correct order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
    • Credit responses that identify gravity as the centripetal force acting towards the centre of the orbit
    • Award 1 mark for describing the geocentric model as Earth-centred and the heliocentric model as Sun-centred
    • For Higher Tier, credit the explanation that velocity changes in a circular orbit due to change in direction, even if speed remains constant
    • Award 1 mark for citing Galileo's observations of Jupiter's moons as evidence supporting the heliocentric model

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When asked about the history of the solar system, explicitly mention Galileo's observations of Jupiter's moons as evidence against the geocentric model
    • 💡For orbital motion questions, ensure you distinguish between the force direction (perpendicular to motion) and instantaneous velocity direction (tangent)
    • 💡Use the mnemonic 'My Very Easy Method Just Speeds Up Naming' for planet order, but write the full names in the exam
    • 💡In 6-mark extended response questions comparing models, ensure you describe *both* the structure of the model and the evidence that led to the change

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Confusing 'speed' and 'velocity' when describing circular orbits; failing to state that velocity changes because direction changes
    • Incorrectly placing the Sun at the centre of the Universe rather than just the Solar System when describing the modern heliocentric model
    • Omitting the asteroid belt when describing the structure of the solar system between Mars and Jupiter
    • Stating that gravity 'stops' or becomes zero immediately outside the Earth's atmosphere

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Structure of the Solar System and heliocentric model
    Orbital mechanics and gravity as centripetal force
    Formation of the Solar System via nebular accretion
    Natural and artificial satellites

    Likely Command Words

    How questions on this topic are typically asked

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