Key Terms
- Solar system: The Sun and all objects that orbit it (planets, moons, asteroids, comets)
- Main sequence star: A star that fuses hydrogen into helium in its core — the Sun is one
- Natural satellite (moon): An object in orbit around a planet
- Asteroid: A rocky body orbiting the Sun, mostly found in the asteroid belt between Mars and Jupiter
- Comet: An icy body with a highly elliptical orbit; develops a tail when near the Sun
- Light year: The distance light travels in one year (~9.46 × 10¹² km) — a unit of DISTANCE not time
- Galaxy: A vast collection of stars, gas, and dust held together by gravity
Must-Know Facts
- Planet order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
- Rocky (inner): Mercury, Venus, Earth, Mars — small, dense, solid surfaces
- Gas/ice giants (outer): Jupiter, Saturn, Uranus, Neptune — large, low density
- The Sun contains 99.8% of the solar system's mass
- The Sun's surface temperature is ~5,500°C; its core is ~15 million°C
- The Milky Way contains ~200 billion stars; the universe contains ~2 trillion galaxies
- Nearest star (Proxima Centauri) = 4.2 light years away
Planet Order Mnemonic
- My Very Educated Mother Just Served Us Nachos
- Mercury · Venus · Earth · Mars · Jupiter · Saturn · Uranus · Neptune
- Closer to Sun → shorter orbital period (Mercury = 88 days)
- Further from Sun → longer orbital period (Neptune = 165 years)
Exam Tips
- A light year is a DISTANCE — examiners often ask students to correct this common mistake
- Classify inner planets as rocky, outer planets as gas/ice giants
- Comets have highly elliptical orbits; planets have nearly circular orbits
- The asteroid belt lies between Mars and Jupiter — not beyond all planets
Key Equations
- v = 2πr ÷ T (orbital speed from radius and period)
- 1 light year ≈ 9.46 × 10¹⁵ m (distance not time)
- Gravitational force: F ∝ 1/d² (inverse square law)
- Orbital period increases with orbital radius
Common Mistakes
- Saying a light year measures time: A light year is a unit of distance — the distance light travels in one year; it is not a unit of time
- Getting planet classifications wrong: Inner planets (Mercury to Mars) are rocky/terrestrial; outer planets (Jupiter to Neptune) are gas or ice giants — classify by composition not just position
- Confusing asteroids and comets: Asteroids are rocky and orbit in the asteroid belt; comets are icy with highly elliptical orbits and develop bright tails when near the Sun
- Saying all orbits are circular: All planetary orbits are ellipses (slightly elongated circles); comet orbits are highly elliptical
- Confusing the Sun's energy source: The Sun generates energy through nuclear fusion (hydrogen fusing to form helium) — not chemical reactions or burning
This topic summary covers Knowledge Organiser: Our Solar System within Our Solar System for GCSE Physics. Revise Our Solar System in Space Physics for GCSE Physics with 13 exam-style questions and 12 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 8 of 8 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Our Solar System
Which of the following is NOT an inner rocky planet in our solar system?
Explain how gravity keeps a planet in a circular orbit around the Sun.
Quick recall flashcards
What is a dwarf planet?
A planet-like object that is too small to clear its orbital path. Examples: Pluto, Ceres, Eris
What is a light year?
The distance light travels in one year = 9.46 × 10¹² km. It's a DISTANCE, not a time!