The solar system contains eight planets orbiting the Sun, along with dwarf planets, moons, asteroids and comets.
The eight planets (in order from the Sun)
| Planet | Type | Relative distance | Key feature |
|---|---|---|---|
| Mercury | Rocky | 0.39 AU | Smallest planet, no atmosphere |
| Venus | Rocky | 0.72 AU | Hottest surface (greenhouse effect) |
| Earth | Rocky | 1.00 AU | Liquid water, supports life |
| Mars | Rocky | 1.52 AU | Red surface (iron oxide), thin atmosphere |
| Jupiter | Gas giant | 5.20 AU | Largest planet, Great Red Spot |
| Saturn | Gas giant | 9.54 AU | Prominent ring system |
| Uranus | Ice giant | 19.2 AU | Rotates on its side |
| Neptune | Ice giant | 30.1 AU | Strongest winds in the solar system |
Rocky vs gas/ice giants
The inner four planets (Mercury to Mars) are rocky (terrestrial): small, dense, solid surfaces.
The outer four planets (Jupiter to Neptune) are gas or ice giants: large, low density, no solid surface, deep atmospheres of hydrogen and helium.
Dwarf planets
A dwarf planet orbits the Sun and has enough mass for gravity to make it roughly spherical, but has NOT cleared the neighbourhood around its orbit of other debris.
Examples: Pluto, Ceres, Eris.
Pluto was reclassified from planet to dwarf planet in 2006 because it has not cleared its orbital neighbourhood.
Common errors and how to correct them
- Listing Pluto as a planet. Pluto is a dwarf planet since 2006.
- Saying Jupiter is the hottest planet. Venus has the hottest surface temperature due to its thick greenhouse atmosphere.
How to apply this in an exam
List the planets in order (use a mnemonic if needed). Know the distinction between rocky and gas/ice planets. Explain the difference between a planet and a dwarf planet.
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