What is in the Solar System?
The Solar System is a star (the Sun) and all the objects that go round it because of its gravity:
- 8 planets and more than 290 known moons;
- dwarf planets such as Pluto, Ceres and Eris;
- millions of asteroids (rocky pieces) and comets (icy lumps);
- dust and the solar wind (charged particles from the Sun).
Our Solar System is one of billions in the Milky Way galaxy, and the Milky Way is one of billions of galaxies in the universe. The Sun is about 26,000 light-years from the centre of the galaxy.
The Sun and the planets
The Sun is a medium-sized star, about 109 times wider than Earth, with a surface near 5,500 °C. In its core, hydrogen joins to make helium (nuclear fusion), giving out light and heat.
| Planet | Type | Distance (AU) | Year (Earth years) |
|---|---|---|---|
| Mercury | rocky | 0.39 | 0.24 |
| Venus | rocky, hottest | 0.72 | 0.62 |
| Earth | rocky, liquid water | 1 | 1 |
| Mars | rocky, red | 1.52 | 1.88 |
| Jupiter | gas giant, largest | 5.2 | 11.9 |
| Saturn | gas giant, bright rings | 9.5 | 29.5 |
| Uranus | ice giant, tilted on its side | 19.2 | 84 |
| Neptune | ice giant, windiest | 30.1 | 165 |
1 AU (astronomical unit) = average Sun–Earth distance ≈ 150 million km. Light takes about 8.3 minutes to cover it.
Why does Earth have life? It sits in the habitable zone where water can stay liquid, it has an atmosphere that traps just enough heat and a magnetic field that shields it from the solar wind.
Gravity and orbits
Gravity is the pull between any two masses. The Sun's huge mass pulls every planet towards it. At the same time each planet is moving sideways very fast. The pull keeps bending its path, so it goes round and round: an orbit. Orbits are ellipses (slightly squashed circles) with the Sun near the centre.
- Closer to the Sun: stronger pull, faster speed, shorter year.
- Moons and artificial satellites orbit planets in the same way. A satellite in a stable orbit keeps the right speed for its height.
From Earth, planets seem to wander against the stars. Sometimes Mars appears to move backwards for a few weeks (retrograde motion): Earth, on its faster inside track, is overtaking it. The time between two similar line-ups of a planet with the Sun and Earth is its synodic period (about 780 days for Mars).
Small bodies: dwarf planets, asteroids, comets, meteors
- Dwarf planet: round and orbits the Sun, but has not cleared other objects from its path (Pluto since 2006, Ceres, Eris).
- Asteroids: rocky or metal bodies, mostly in the belt between Mars and Jupiter.
- Kuiper belt beyond Neptune: icy bodies including Pluto. The Oort cloud, far beyond, is thought to hold trillions of comets.
- Comets: dirty snowballs. Near the Sun, ice turns to gas and forms a glowing head and tails that point away from the Sun, pushed by sunlight and the solar wind.
- Meteoroid (small rock in space) → meteor (the streak of light as it burns in our air, a "shooting star") → meteorite (the piece that reaches the ground).
How did the Solar System form?
About 4.6 billion years ago a huge cloud of gas and dust (a nebula) began to collapse under its own gravity, perhaps after a nearby star exploded.
- The cloud spun faster and flattened into a disc.
- Most material fell to the centre, got very hot and dense, and fusion began: the Sun was born.
- In the disc, dust stuck together into bigger and bigger lumps (planetesimals), which collided to form planets.
- Near the hot Sun only rock and metal could stay solid, so rocky planets formed. Farther out, ice could also form, so the cores grew big enough to pull in lots of gas: the giants.
That is why all planets orbit in the same direction and almost in one flat plane.
Key formulas and definitions
- 1 AU ≈ 1.5 × 10⁸ km (Sun–Earth distance)
- Light-time = distance ÷ speed of light (3 × 10⁸ m/s); Sun to Earth ≈ 8.3 min
- 1 light-year ≈ 9.46 × 10¹² km
- Closer orbit → stronger gravity → faster speed → shorter year
- Order: Mercury, Venus, Earth, Mars | belt | Jupiter, Saturn, Uranus, Neptune
Worked examples
1. Which planets are rocky and which are giants?
Rocky: Mercury, Venus, Earth, Mars. Giants: Jupiter, Saturn (gas) and Uranus, Neptune (ice).
2. Jupiter is 5.2 AU from the Sun. About how far is that in km?
5.2 × 150 million km ≈ 780 million km.
3. How long does sunlight take to reach Neptune (30 AU)?
30 × 8.3 min ≈ 250 min, a little over 4 hours.
4. Why was Pluto reclassified in 2006?
It is round and orbits the Sun, but it has not cleared its path of other Kuiper-belt objects, so it is a dwarf planet.
Common mistakes
- Thinking Mercury is the hottest planet. Venus is hotter because its thick CO₂ atmosphere traps heat.
- Thinking a comet's tail trails behind it. The tail always points away from the Sun.
- Believing pictures show true scale. Real distances are huge compared with planet sizes.
- Calling the Moon a planet or the Sun a planet. The Moon is a natural satellite; the Sun is a star.