The Solar System
Section: Space Physics | Syllabus: Cambridge IGCSE Physics (0625)
Components of the Solar System
The Solar System consists of the Sun and all the objects that orbit it.
- One star: the Sun
- Eight planets, in order of increasing distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
- Memory aid: My Very Educated Mother Just Served Us Noodles
- Minor planets: smaller bodies that orbit the Sun directly, including dwarf planets such as Pluto and asteroids - most asteroids are found in the asteroid belt between Mars and Jupiter
- Moons: natural satellites that orbit the planets, e.g. Earth's Moon
- Smaller Solar System bodies, including comets - frozen balls of gas, rock and dust with very elongated orbits; as a comet approaches the Sun it heats up and develops a bright tail that always points away from the Sun
The Sun, the eight planets, and the asteroid belt between Mars and Jupiter
Inner vs Outer Planets
Compared with each other, the four planets nearest the Sun are rocky and small, while the four planets furthest from the Sun are gaseous and large.
| Property | Inner Planets (Mercury–Mars) | Outer Planets (Jupiter–Neptune) |
|---|---|---|
| Composition | Rocky and solid | Gaseous (gas giants) |
| Size | Small | Large |
| Density | High (~4000–5500 kg/m³) | Low (~700–1700 kg/m³) |
| Examples | Earth, Mars | Jupiter, Saturn |
Formation: The Accretion Model
This difference between the rocky inner planets and the gaseous outer planets can be explained by an accretion model for how the Solar System formed.
- The Solar System formed from a large, slowly spinning interstellar cloud of gas and dust, containing hydrogen and other elements produced by earlier generations of stars
- Gravity pulled the cloud material together, causing it to collapse inward
- The spinning motion caused the collapsing cloud to flatten into a rotating disc - an accretion disc
- The central region of the disc became the Sun; the remaining material clumped together under gravity to form the planets
- Close to the Sun, intense heat drove away lighter gases, leaving only heavy, rocky material → small, rocky inner planets
- Further from the Sun, temperatures were much lower, so lighter gases such as hydrogen and helium could be retained → large, gaseous outer planets
The accretion model: gravity collapses a spinning cloud of gas and dust into a disc, forming the Sun and planets
Why Planets Orbit the Sun
- The Sun contains approximately 99.8% of the total mass of the Solar System, which is why its gravitational field dominates the Solar System
- The gravitational attraction of the Sun is the force that keeps planets, minor planets, comets and other bodies in orbit around it
- Gravitational field strength at a planet's surface depends on the mass of the planet - more massive planets have stronger surface gravity
- Gravitational field strength decreases as distance from the planet increases
Analysing Planetary Data
Extended Only
Data about the planets can be analysed and interpreted to spot patterns and relationships.
| Planet | Orbital Distance (million km) | Orbital Period (years) | Density (kg/m³) | Surface Temperature (°C) | Surface Gravity (N/kg) |
|---|---|---|---|---|---|
| Mercury | 58 | 0.2 | 5500 | 167 | 4 |
| Venus | 108 | 0.6 | 5200 | 464 | 9 |
| Earth | 150 | 1.0 | 5500 | 15 | 10 |
| Mars | 228 | 1.9 | 4000 | −65 | 4 |
| Jupiter | 778 | 12 | 1300 | −110 | 26 |
| Saturn | 1427 | 30 | 700 | −140 | 11 |
| Uranus | 2870 | 84 | 1300 | −195 | 11 |
| Neptune | 4497 | 165 | 1700 | −200 | 12 |
- As orbital distance from the Sun increases, orbital period increases - this is because the Sun's gravitational field strength decreases with distance, so the orbital speeds of the planets decrease as distance from the Sun increases
- The inner four planets are denser (rocky); the outer four are less dense (gaseous)
- Surface gravity depends on the planet's mass, not on its distance from the Sun
- Surface temperature generally falls with distance from the Sun - except Venus, which is hotter than Mercury despite being further away, because its thick carbon dioxide atmosphere traps heat through a strong greenhouse effect
Light Travel Time in the Solar System
The distances in the Solar System are so vast that it takes a measurable amount of time for light to travel between objects.
- The speed of light in a vacuum is c = 3 × 10^8 m/s (300 000 km/s)
- Travel time can be found by rearranging the wave speed equation:
- t = (d/c)
Worked Example: Light from the Sun to Earth
Muchindu wants to know how long light takes to travel from the Sun to the Earth, a distance of 1.5 × 10^11 m.
- t = (d/c) = 1.5 × 10^113 × 10^8 = 500 s ≈ 8.3 minutes
Worked Example: Light from the Sun to Mars
Muumbe repeats the calculation for Mars, a distance of 2.28 × 10^11 m from the Sun.
- t = (d/c) = 2.28 × 10^113 × 10^8 = 760 s ≈ 12.7 minutes
Elliptical Orbits and Conservation of Energy
Extended Only
Planetary orbits are not perfect circles - they are ellipses (slightly squashed circles).
- Planets, minor planets and comets all have elliptical orbits
- The Sun is not at the centre of the ellipse, but at a special point called the focus - except for orbits that are approximately circular (such as Earth's), where the two foci lie so close together that the Sun appears to be at the centre
- Comets have the most eccentric (elongated) elliptical orbits of all Solar System bodies
- An object in an elliptical orbit travels faster when closer to the Sun and slower when further away
- This is explained by conservation of energy: total mechanical energy (kinetic energy + gravitational potential energy) stays constant, since there is no friction in space
| Position in Orbit | Distance from Sun | Speed | Kinetic Energy | Gravitational PE |
|---|---|---|---|---|
| Closest (perihelion) | Minimum | Maximum | Maximum | Minimum |
| Furthest (aphelion) | Maximum | Minimum | Minimum | Maximum |
A planet moves fastest at perihelion (closest to the Sun) and slowest at aphelion (furthest from the Sun)
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