Orbiting Bodies (Cambridge (CIE) O Level Physics): Revision Note
Analysing Orbits
Over many years, data about all the planets, moons and the Sun have been collected
This is not just for general interest, but to indicate:
Factors that affect conditions on the surface of the planets
Environmental problems that a visit (using manned spaceships or robots) would encounter
Data for the planets in the Solar System
planet | orbital distance from Sun / million km | orbital period | density / kg/m3 | surface temperature / °C | surface gravitational field strength / N/kg |
|---|---|---|---|---|---|
Mercury | 57.9 | 88 days | 5427 | 350 | 3.7 |
Venus | 108.2 | 225 days | 5243 | 460 | 8.9 |
Earth | 149.6 | 365 days | 5514 | 20 | 9.8 |
Mars | 227.9 | 687 days | 3933 | –23 | 3.7 |
Jupiter | 778.6 | 11.9 years | 1326 | –120 | 23.1 |
Saturn | 1433.5 | 29.5 years | 687 | –180 | 9.0 |
Uranus | 2872.5 | 75 years | 1271 | –210 | 8.7 |
Neptune | 4495.1 | 165 years | 1638 | –220 | 11.0 |
Worked Example
State and explain the relationship between the distance of a planet from the Sun and its
(a) surface temperature
(b) orbital period
(c) density
Answer:
(a)
The relationship between distance from the Sun and surface temperature is...
The closer a planet is to the Sun, the hotter its surface temperature
This can be seen in the data as...
The planets closest to the Sun are the hottest e.g. Mercury has a surface temperature of 350°
(Note: Venus has the hottest surface temperature due to its dense atmosphere which traps heat)
The planets furthest from the Sun are the coldest e.g. Neptune has a surface temperature of −220°C
Explanation:
The planets nearer to the Sun receive a greater proportion of the emitted heat radiation compared to the further planets
(b)
The relationship between distance from the Sun and orbital period is...
The closer a planet is to the Sun, the shorter its orbital period
This can be seen in the data as...
The planets closest to the Sun have the shortest orbital periods e.g. Mercury completes one orbit in 88 days
The planets furthest from the Sun have the longest orbital periods e.g. Neptune completes one orbit in 165 years
Explanation:
The Sun's gravitational field strength is strongest at Mercury and decreases with distance
Therefore, the planets which are closer to the Sun travel faster than the planets which are farther away
So, the closest planets move faster and have a shorter distance to travel, meaning they complete orbits in a quicker time
(c)
The relationship between distance from the Sun and density is...
The 4 closest planets to the Sun have the greatest densities
The 4 furthest planets to the Sun have the lowest densities
This can be seen in the data as...
Mercury, Venus, Earth and Mars all have densities around 4000-5000 kg/m3
Jupiter, Saturn, Uranus and Neptune all have densities around 1000-2000 kg/m3
Explanation:
The four planets nearest to the Sun must have formed in the hotter inner regions of the early Solar System where higher density material (rocks & metals) collected
The four planets furthest from the Sun must have formed in the cooler outer regions of the early Solar System where lower density material (water and gases) collected
Examiner Tips and Tricks
Although you don't need to memorise any of the numbers in the table, you must be able to confidently analyse and interpret it.
Look out for trends such as one variable increasing whilst the other decreases (or also increases).
Think carefully about why that may be with what you have already learnt about the planets from this topic. For example, what is the planet made of? What is its distance from the Sun and how does this affect it?
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