The Solar System and Beyond
Section: Earth and Space | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
The Solar System
The Solar System consists of the Sun and all the objects that orbit around it due to gravity, including eight planets, their moons, dwarf planets, asteroids, and comets.
All eight planets orbit the Sun in the same direction, held in place by its gravity
Components of the Solar System
- The Sun - a star at the centre of our Solar System
- Eight planets that orbit the Sun
- Moons (natural satellites) that orbit planets
- Asteroids - small rocky objects, mostly in the asteroid belt between Mars and Jupiter
- Comets - icy objects that develop tails when near the Sun
- Dwarf planets - including Pluto, Ceres, and Eris
- All objects in the Solar System are held in orbit by the Sun's gravity
- the planets orbit the Sun in elliptical (oval) paths
- the Solar System formed about 4.6 billion years ago
The Sun
The Sun is a star - a massive sphere of extremely hot gas that produces energy through nuclear fusion.
The Sun's energy is produced by nuclear fusion in its core and travels outward through several layers before reaching space
Key Features of the Sun
- Core - the centre where nuclear fusion occurs, producing energy
- Temperature - surface temperature approximately 5,500°C; core temperature about 15 million°C
- Composition - mainly hydrogen (about 73%) and helium (about 25%)
- Size - diameter about 1.4 million km (109 times Earth's diameter)
- Mass - contains 99.8% of all the mass in the Solar System
- The Sun produces energy by nuclear fusion - hydrogen atoms join together to form helium, releasing enormous amounts of energy
- this energy travels through space as light and heat
- the Sun's gravity holds all the planets in their orbits
- never look directly at the Sun - it can cause permanent eye damage
Common Mistakes
- ❌ Thinking the Sun is "on fire" or burning like a bonfire
- ✓ the Sun does not burn the way a fire does - it releases energy through nuclear fusion, where hydrogen nuclei join together to form helium
The Planets
There are eight planets in our Solar System, divided into two groups based on their characteristics.
Inner Planets (Terrestrial Planets)
The four planets closest to the Sun are small, rocky planets:
| Planet | Key Features | Atmosphere |
|---|---|---|
| Mercury | Smallest planet; closest to Sun; extreme temperatures; heavily cratered surface | Very thin (almost none) |
| Venus | Hottest planet (460°C); similar size to Earth; thick atmosphere; spins backwards | Very thick CO₂; sulfuric acid clouds |
| Earth | Only planet with liquid water on its surface; supports life; one moon | Nitrogen and oxygen |
| Mars | Red planet (iron oxide); two small moons; evidence of past water; polar ice caps | Thin CO₂ atmosphere |
Outer Planets (Gas Giants and Ice Giants)
The four planets farthest from the Sun are much larger and made mainly of gas and ice:
| Planet | Key Features | Moons |
|---|---|---|
| Jupiter | Largest planet; gas giant; Great Red Spot (huge storm); strong magnetic field | 79+ moons (including Ganymede, the largest moon in the Solar System) |
| Saturn | Gas giant; spectacular ring system made of ice and rock particles; least dense planet | 82+ moons (including Titan) |
| Uranus | Ice giant; rotates on its side; appears blue-green due to methane; faint rings | 27+ moons |
| Neptune | Ice giant; farthest planet; fastest winds in the Solar System; deep blue colour | 14+ moons (including Triton) |
The rocky inner planets are far smaller than the gas and ice giants of the outer Solar System
- Inner planets: small, rocky, few/no moons, solid surfaces
- Outer planets: large, made of gas/ice, many moons, no solid surfaces
- All planets orbit the Sun in the same direction
- The time for one complete orbit is called a year (different for each planet)
Common Mistakes
- ❌ Thinking Pluto is the ninth planet
- ✓ Pluto was reclassified as a dwarf planet in 2006 - there are only eight official planets
Moons, Asteroids, and Comets
Moons orbit planets, asteroids are rocky and mostly confined to the asteroid belt, while comets are icy and follow long elliptical orbits
Moons (Natural Satellites)
- Moon (natural satellite): an object that orbits a planet, held in orbit by the planet's gravity
- Earth has one moon - our Moon
- Mars has two small moons - Phobos and Deimos
- Gas giants have many moons - Jupiter and Saturn each have over 80
- Mercury and Venus have no moons
- Earth's Moon is thought to have formed about 4.5 billion years ago when a smaller planet collided with the early Earth - this is called the collision theory (giant impact hypothesis). Debris thrown out by the impact came together to form the Moon
- one piece of evidence for this theory is that rocks from the Earth and the Moon have a similar composition
- see the Formation of the Moon topic for the full explanation and evidence
Asteroids
- Asteroid: a small, rocky or metallic object that orbits the Sun. Most are found in the asteroid belt between Mars and Jupiter
- Irregular, rocky shapes - not spherical like planets
- Range in size from a few metres to hundreds of kilometres
- Most asteroids orbit in the asteroid belt between Mars and Jupiter
- Believed to be leftover material from when the Solar System formed
Comets
- Comet: an icy object that orbits the Sun in a very elliptical orbit. When close to the Sun, it heats up and develops a glowing tail
- Made of ice, dust, and rock - often called "dirty snowballs"
- Have highly elliptical orbits that bring them close to the Sun, then far away
- Develop bright tails when near the Sun (ice vaporises due to heat)
- Tails always point away from the Sun (pushed by solar wind)
- Famous example: Halley's Comet (visible from Earth every 76 years)
Common Mistakes
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