The Solar System
The Solar System
Our Solar System formed about 4.6 billion years ago from a collapsing cloud of gas and dust. It consists of the Sun at the centre, surrounded by everything that orbits it: 8 planets, their moons, dwarf planets, asteroids, comets, and interplanetary dust.
The Solar System is located in the Milky Way galaxy, about 26,000 light-years from the galactic centre. The entire Solar System takes approximately 225 million years to orbit the centre of the Milky Way.
The Sun
The Sun is a medium-sized, middle-aged star — a G-type main sequence star. It accounts for about 99.8% of all the mass in the Solar System.
Key facts:
- Diameter: approximately 1.4 million km (109 times Earth's diameter)
- Composition: approximately 75% hydrogen, 25% helium
- Energy source: nuclear fusion — hydrogen nuclei fuse to form helium in the core, releasing enormous amounts of energy as light and heat
- Surface temperature: approximately 5,500°C; core temperature: approximately 15 million °C
- Distance from Earth: approximately 150 million km (1 AU — astronomical unit)
Light from the Sun takes approximately 8 minutes to reach Earth.
The Eight Planets
Mnemonic: My Very Energetic Mother Just Served Us Nachos (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune)
Terrestrial Planets (rocky, inner planets)
| Planet | Key features |
|---|---|
| Mercury | Smallest planet; closest to Sun; no atmosphere; extreme temperature swings (-180°C to 430°C) |
| Venus | Hottest planet (465°C average) due to greenhouse effect; thick CO2 atmosphere; rotates backwards; brightest object in sky after the Moon |
| Earth | Only known planet with liquid water on the surface and life; one large moon; protective magnetic field |
| Mars | Red colour from iron oxide; thin CO2 atmosphere; largest volcano in the solar system (Olympus Mons, 22 km high); two small moons (Phobos and Deimos) |
Gas and Ice Giants (outer planets)
| Planet | Key features |
|---|---|
| Jupiter | Largest planet (1,300 Earths fit inside); mainly hydrogen and helium; Great Red Spot is a storm larger than Earth; 95 known moons including Europa (possible subsurface ocean) |
| Saturn | Spectacular ring system (ice and rock particles); least dense planet — would float on water; 146 known moons; moon Titan has a thick atmosphere |
| Uranus | Ice giant; rotates on its side (axial tilt ~98°); faint rings; extremely cold atmosphere (-224°C) |
| Neptune | Furthest from Sun; strongest winds (up to 2,100 km/h); large moon Triton orbits backwards; deep blue colour from methane |
Orbital Motion
Planets orbit the Sun in elliptical paths (slightly squashed circles) with the Sun at one focus. This was described by Kepler's First Law.
Kepler's Laws (simplified):
- Planets orbit in ellipses with the Sun at one focus
- Planets move faster when closer to the Sun (sweep equal areas in equal times)
- The further a planet is from the Sun, the longer its orbital period
What keeps planets in orbit? Gravity between the Sun and each planet provides the centripetal force needed to keep planets moving in curved paths. Without gravity, planets would travel in straight lines (Newton's First Law).
Mercury orbits the Sun in just 88 Earth days. Neptune takes 165 Earth years for one orbit.
The Asteroid Belt
Located between Mars and Jupiter, the asteroid belt contains millions of rocky bodies ranging from tiny pebbles to Ceres (diameter ~940 km, classified as a dwarf planet). The total mass is less than 4% of the Moon's mass — these are the remnants of material that never coalesced into a planet due to Jupiter's gravitational influence.
Comets
Comets are icy bodies with highly elliptical orbits that bring them close to the Sun and then far out into the outer Solar System.
Structure:
- Nucleus — the solid core of ice and dust (typically 1–10 km across)
- Coma — a halo of gas and dust surrounding the nucleus as ice vaporises near the Sun
- Tail(s) — always points away from the Sun (pushed by solar wind and radiation pressure); can be millions of km long
Famous comets: Halley's Comet (visible from Earth every ~75–76 years; last seen 1986, expected 2061).
Moons and Dwarf Planets
Moons: natural satellites orbiting planets. Earth has one (the Moon); Jupiter and Saturn each have over 100. Our Moon stabilises Earth's axial tilt and drives tides.
Dwarf planets: bodies that orbit the Sun, have enough mass to be roughly spherical, but have not cleared their orbital neighbourhood. Examples: Pluto, Eris, Ceres, Makemake.
Key Terms
- Solar System — the Sun and everything gravitationally bound to it
- Terrestrial planet — a small, rocky inner planet (Mercury, Venus, Earth, Mars)
- Gas giant — a large planet made mostly of gas (Jupiter, Saturn)
- Ice giant — a large planet with icy composition (Uranus, Neptune)
- Asteroid — a rocky body orbiting the Sun, mainly in the asteroid belt
- Comet — an icy body with a highly elliptical orbit; develops a tail near the Sun
- Nuclear fusion — process in stars where hydrogen nuclei fuse to form helium, releasing energy
- Ellipse — the oval shape of planetary orbits
Common Mistakes
- Saying Pluto is a planet — it was reclassified as a dwarf planet in 2006
- Thinking the asteroid belt is dense and dangerous to fly through — it is mostly empty space
- Saying Venus is the closest planet to Earth always — sometimes Mercury is closer due to orbital positions
- Confusing gas giants (Jupiter, Saturn) with ice giants (Uranus, Neptune)
Tips and Tricks
- Mnemonic for planet order: My Very Energetic Mother Just Served Us Nachos
- Inner planets = small and rocky; outer planets = large (gas or ice)
- Further from Sun = longer year (orbital period); also moves more slowly in orbit
- Venus is hotter than Mercury despite being further from the Sun because of its dense CO2 greenhouse atmosphere