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An introduction to exoplanets
An introduction to exoplanets

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2  The scale of the Solar System: everything is big!

Probably the most obvious difference between a gas giant planet such as Jupiter and a terrestrial planet such as Earth is their size. Jupiter, the largest planet in our Solar System, is around ten times the size of Earth. In Week 1 you learned that stars are much bigger than planets; for instance, the Sun is nearly ten times the size of Jupiter and therefore about one hundred times the size of the Earth! When referring to the ‘size’ of a planet or star we could think of its diameter, measured through the middle from one side across to the other. Astronomers usually use the ‘radius’ though, which is the distance between the centre and the surface, and so is half the diameter.

Because everything is so big, it can be very difficult to picture the differences in size between the Solar System planets, and to imagine the vast distances that separate them. The next two activities should help you to do this.

Big numbers!

We said that the lifetime of the Sun is about 9 billion years. A billion is clearly a huge number but it’s very difficult to get a feel for just how big it is.

How long do you think it would take to count to one thousand if you say one number each second? And what about one million? Does it take much longer to count to one billion in this way?

You might be surprised by the answer:

One thousand seconds is 17 minutes, one million seconds is nearly 12 days, and one billion seconds is nearly 32 years! It’s easy to forget just how much bigger billions are than millions.

Incidentally, one trillion seconds is nearly 32 000 years!