Orbiting Bodies (Cambridge O Level Physics)

Revision Note

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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 Tip

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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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.