4.1  Planet sizes

Earlier this week, you learned that the radius is used as a measurement of size in planets. Generally, if people need to calculate something that depends on a radius, they use the letter r to stand for the radius. For the purpose of this course, we will use a capital R for a planet’s radius (Figure 6) and a lower case r for other radii. The radius of the Earth is 6378 km.

Figure 6  The radius, R, of a planet is the distance from the centre to the surface

Show description|Hide description

This is an image of a planet with the core labelled, and a red dashed line going from the core to the outside edge of the planet. This line is labelled R.

Figure 6  The radius, R, of a planet is the distance from the centre to the surface

When astronomers discuss the sizes of planets, it is cumbersome to use kilometres. It is far more convenient to introduce the unit 1 Earth radius, written 1 R_E (or sometimes 1 R_⊕), when discussing terrestrial planets (Figure 7). It is obvious that a planet with a radius of say 1.1 R_E is slightly bigger than Earth, 10 per cent bigger to be exact. On the other hand, if a planet had a radius of 7020 km, you probably wouldn’t immediately know how much bigger or smaller than Earth it is.

Figure 7  The radius of the Earth, 1 R_E, is used as a unit to measure the sizes of terrestrial planets

Show description|Hide description

This is a diagram of the Earth, with a segment cut away, displaying the core. The core is labelled core of iron, and the next layer from the core, is labelled mantle. The outer layer is labelled atmosphere, and the layer between the atmosphere and the mantle is labelled crust. There is a dashed line running horizontally from the core to the edge of the Earth. This is labelled R_E.

Figure 7  The radius of the Earth, 1 R_E, is used as a unit to measure the sizes of terrestrial planets

Similarly, when discussing giant planets, it is most convenient to use Jupiter’s radius, so giant planet radii are generally given in units of R_J.