P8.1 Our solar system
Structure of the Solar System
The Solar System consists of one star (the Sun), eight planets and everything bound by the Sun's gravity:
| Component | Examples |
|---|---|
| The Sun | A middle-aged main-sequence star — our nearest star |
| Eight planets | Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune (in order) |
| Dwarf planets | Pluto, Eris, Ceres |
| Moons (natural satellites) | Earth's Moon; Ganymede (Jupiter); Titan (Saturn) |
| Asteroids | Rocky bodies mainly in the asteroid belt between Mars and Jupiter |
| Comets | Icy bodies with highly elliptical orbits; develop tails as they approach the Sun |
What holds it together?
Gravity — the attractive force between any two objects with mass. The Sun has 99.86 % of the Solar System's mass, so its gravitational pull dominates. Planets travel in roughly circular (actually elliptical) orbits at speeds that balance the inward pull of gravity.
Distances
Distances in the Solar System are enormous. We often use the Astronomical Unit (AU): 1 AU = distance from Earth to Sun ≈ 150 million km. Neptune is ~30 AU from the Sun. For interstellar distances we use light-years (the distance light travels in one year ≈ 9.46 × 10¹⁵ m).
Comets
Comets have highly elliptical orbits. When a comet approaches the Sun, solar radiation and wind blow material off its surface, forming a glowing tail that always points away from the Sun. The comet moves fastest at perihelion (closest point) and slowest at aphelion (furthest point).
Exam technique
- You must know the order of the eight planets (mnemonic: My Very Eager Mother Just Served Us Nachos).
- Distinguish asteroid (rocky, main belt) from comet (icy, elliptical orbit).
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