Knowledge Organiser: Our Solar System
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 Facts
- 1 light year ≈ 9.46 × 10¹⁵ m (distance not time)
- Gravitational force: gets weaker as the distance from the planet or star increases
- 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