The solar system · Grade 7

Our solar system is the Sun – a star – and everything held in orbit by its gravity: eight planets, their moons, dwarf planets such as Pluto, asteroids and comets. The four inner planets are small and rocky; the four outer planets are giants made mostly of gas and ice. Distances are so huge that sunlight takes over four hours to reach Neptune.

Key facts

  • Order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune (“My Very Easy Method Just Speeds Up Naming”).
  • Inner planets are rocky; outer planets are gas and ice giants. The asteroid belt lies between Mars and Jupiter.
  • Jupiter is the largest planet; Venus is the hottest (its thick atmosphere traps heat).
  • Further planets take longer to orbit: a year on Neptune is about 165 Earth years.
  • 1 astronomical unit (AU) = Earth–Sun distance ≈ 150 million km.
  • Light travels 300,000 km/s, so sunlight reaches Earth in about 8.3 minutes.

Worked examples

Example 1

Mars is 228 million km from the Sun. How long does sunlight take to reach it?

Answer: about 12.7 minutes

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