The planets — Astronomy, 4–6 years
Our Solar System has eight very different worlds travelling around the Sun. Astronomy, 4–6 years.
Eight different worlds
A planet is a large, round world travelling through space around a star. Our Solar System has eight planets, and they are not all alike: some are rocky, some have thick gas around them, and some are much bigger than Earth. They all move around the Sun.
Why sort the planets?
When people watched the sky, they needed to tell moving worlds apart from bright points of light and from our Moon. Naming and sorting the planets made it easier to share observations and find them again. Scientists still use groups such as rocky planets and giant planets because the groups reveal how worlds differ.
Counting the planets
Start at the Sun and point to each planet in order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Touch one name for each world. When you reach Neptune, you have touched eight names, so our Solar System has eight planets.
They are not all alike
A picture may show every planet as a neat little ball, so it is easy to think they are all similar. The mistake makes sense because a small drawing cannot show their true sizes, surfaces or thick gases. Jupiter is far wider than Earth, while Mercury is much smaller, and some planets have rings.
Choosing a space destination
The differences between planets matter when people plan space missions. A spacecraft going to Mars needs different equipment from one studying the giant planet Jupiter. Scientists compare each planet’s size, surface, air and distance before deciding what to send.
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