6.1.2 The Solar System
The Solar System is the collection of bodies that are bound together by the gravity of a single star – the Sun. It is organised hierarchically into the Sun, eight major planets (listed in order from the Sun), a variety of smaller bodies (minor planets, dwarf planets and the asteroid belt), natural satellites (moons) and even smaller objects such as comets and meteoroids.
1. The Sun – a single star
The Sun is a G‑type main‑sequence star. It contains about 99.9 % of the total mass of the Solar System and supplies the heat and light that make life on Earth possible.
2. The eight named planets (in order from the Sun)
Planets are large bodies that orbit the Sun, are massive enough for their own gravity to make them nearly round, and have cleared their orbital neighbourhood of other debris.
| Order | Planet | Type | Mean distance from Sun (AU) | Mean radius (km) | Orbital period (yr) | Surface gravity (g) | Key feature |
|---|
| 1 | Mercury | Terrestrial | 0.39 | 2 440 | 0.24 | 0.38 | Smallest planet; virtually no atmosphere |
| 2 | Venus | Terrestrial | 0.72 | 6 052 | 0.62 | 0.90 | Thick CO₂ atmosphere; runaway greenhouse effect |
| 3 | Earth | Terrestrial | 1.00 | 6 371 | 1.00 | 1.00 | Only known planet with liquid water on the surface |
| 4 | Mars | Terrestrial | 1.52 | 3 390 | 1.88 | 0.38 | Red surface; thin CO₂ atmosphere |
| 5 | Jupiter | Gas giant | 5.20 | 69 911 | 11.86 | 2.53 | Largest planet; strong magnetic field |
| 6 | Saturn | Gas giant | 9.58 | 58 232 | 29.46 | 1.07 | Prominent system of rings |
| 7 | Uranus | Ice giant | 19.2 | 25 362 | 84.01 | 0.89 | Rotates on its side (≈98° tilt) |
| 8 | Neptune | Ice giant | 30.1 | 24 622 | 164.8 | 1.14 | Very strong winds; vivid blue colour |
How to calculate orbital speed (optional)
Formula: \(v = \dfrac{2\pi r}{T}\) where
r = average orbital radius (≈ semi‑major axis) and T = orbital period.
Worked example – Earth
- Mean distance \(r = 1.00\;{\rm AU}=1.496\times10^{11}\;{\rm m}\)
- Orbital period \(T = 1.00\;{\rm yr}=3.156\times10^{7}\;{\rm s}\)
- \(v = \dfrac{2\pi(1.496\times10^{11})}{3.156\times10^{7}} \approx 2.98\times10^{4}\;{\rm m\,s^{-1}} = 29.8\;{\rm km\,s^{-1}}\)
Note: this box is optional material for the syllabus.
3. Minor planets, dwarf planets and the asteroid belt
- Minor planets – bodies that orbit the Sun but are not massive enough to be classified as planets.
- Dwarf planets – a subset of minor planets that are massive enough for their own gravity to make them roughly spherical, yet have not cleared their orbital zone.
- Pluto – Kuiper‑belt dwarf planet; orbital period ≈ 248 yr.
- Eris, Haumea, Makemake – recognised dwarf planets in the trans‑Neptunian region.
- Ceres – resides in the asteroid belt and is also classified as a dwarf planet.
- Asteroid belt – a torus of rocky bodies between the orbits of Mars (1.52 AU) and Jupiter (5.20 AU). The largest member is Ceres (≈ 940 km diameter), which is itself a dwarf planet.
4. Moons (natural satellites)
Moons are bodies that orbit planets. They illustrate the concept of natural satellites and provide clear examples of tidal forces, orbital resonances and geological activity.
- Earth – one moon, simply called “the Moon”.
- Jupiter – four large Galilean moons:
- Io – most volcanically active body in the Solar System.
- Europa – icy surface; possible subsurface ocean.
- Ganymede – largest moon in the Solar System (larger than Mercury).
- Callisto – heavily cratered, ancient surface.
- Saturn – notable moon Titan, which possesses a dense nitrogen‑rich atmosphere.
- Uranus – major moons include Titania, Oberon, Miranda, Ariel and Umbriel.
- Neptune – Triton, a large retrograde moon with geyser‑like plumes.
5. Smaller Solar‑System bodies
- Comets – icy nuclei that develop a coma and a tail when they approach the Sun. Their highly elliptical orbits take them from the distant Oort cloud or Kuiper belt into the inner Solar System.
[Diagram of a comet: nucleus, coma, dust tail, ion tail]
- Meteoroids – tiny fragments of rock or metal. When they enter Earth’s atmosphere they become meteors; if they survive to reach the surface they are called meteorites.
6. Comparative sizes and masses (quick reference)
- Jupiter’s mass ≈ 300 × Earth’s mass; its diameter ≈ 11 × Earth’s.
- Saturn’s mass ≈ 95 × Earth’s mass; its diameter ≈ 9 × Earth’s.
- Uranus and Neptune each have a mass ≈ 15 × Earth’s.
- The Moon’s mass ≈ 1 % of Earth’s; its diameter ≈ 27 % of Earth’s.
7. Summary
- One star – the Sun.
- Eight planets, listed in order from the Sun, with type, distance, and quantitative data for AO2 work.
- Minor planets, including dwarf planets (e.g., Pluto, Eris, Ceres) and the asteroid belt.
- Moons that orbit the planets – key examples illustrate natural satellites and tidal effects.
- Smaller bodies such as comets (with diagram) and meteoroids.