4  Planet vital statistics

In previous sections you have covered sizes and distances in the Solar System; now you will look at how astronomers measure and discuss the properties of planets.

If you’ve ever taken a long-haul flight, you will know that the Earth is big. The usual terminology for measuring size and mass was designed for talking about things we usually deal with on Earth, like the distance of a car journey or weighing out ingredients for a cake. These measures quickly become inadequate when talking about astronomy.

You’ve already seen how astronomers use the distance between the Earth and Sun when discussing distances within the Solar System. Similarly, it is very convenient to measure all rocky planets in terms of ‘Earths’ and all gas giants and ice giants in terms of ‘Jupiters’. With this trick, instead of trying to quote a planet’s mass in billions of billions of kilograms, you can simply say it is a certain number of Jupiter masses. In the case of our Solar System this number will be a fraction of course, but this will become particularly useful when we encounter exoplanets that are even more massive than Jupiter!