The Solar System

Our Sun is a giant, blazing ball of burning gas — so big that more than a million Earths could fit inside it. And it does not travel through space alone. It has a whole family circling round it: eight worlds, plus a swarm of smaller bits and pieces, all held close and swept round and round for billions of years.

The Sun, the eight planets, their moons, and all the rocky and icy leftovers travelling with them make up our Solar System. Earth — our home, the world with oceans, forests and you — is just one member of the Sun's family. Let's meet the rest.

Eight worlds, in order out from the Sun

Starting right beside the Sun and stepping outward, the eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Earth is the third one out. Learning the order sounds hard until you use a little trick — a silly sentence where each word starts with the same letter as a planet:

My Very Easy Method Just Speeds Up Naming.

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Say it a few times and the order will stick for life.

Watch the family go round

Every planet travels its own huge circle round the Sun — its orbit. The planets nearest the Sun have a short track and whip round quickly; the far-out planets have an enormous track and crawl round slowly. That is why little Mercury laps the Sun again and again while distant Neptune barely seems to move. Earth is coloured brightly — see if you can spot how many times Mercury goes round before Earth gets back to where it started. Slide time forward and watch.

Two families: little rocks and giant gasballs

The eight planets split neatly into two groups. The inner four — Mercury, Venus, Earth and Mars — are the small, rocky ones. They are made of stone and metal, they have solid ground you could stand on, and they huddle in close to the Sun's warmth.

Way out beyond them, the outer four — Jupiter, Saturn, Uranus and Neptune — are the giants. These are gigantic balls of gas and liquid with no solid surface at all — try to land on one and you would simply sink and sink. Jupiter is the king of them, so big that all the other planets could fit inside it with room to spare, and Saturn wears its famous shining rings of ice and rock. Out there it is fantastically dark and freezing cold, because the Sun is so far away.

Nearer is hotter — and faster

The closer a planet sits to the Sun, the more of its heat it feels, so the nearer planets are hotter and the far ones are colder. Baking-hot Mercury and Venus are close in; frozen Uranus and Neptune are far out in the dark.

Distance changes something else, too: how long a year lasts. One year is a single full lap round the Sun. Because near planets have short tracks and race along, their year is short — a year on Mercury is over in just 88 Earth days. Far planets have gigantic tracks and dawdle, so their year is huge — one year on Neptune lasts about 165 Earth years. Nobody could ever live long enough to see a single Neptune birthday!

Why we don't drift off — and why we're here

What stops the planets flying off into the dark? The Sun's gravity — the very same invisible pull that holds you on the ground holds every planet on its orbit. The Sun is so stupendously heavy that its gravity reaches billions of kilometres, gently tugging each planet round and round like a ball whirled on a string. Without it, the planets would shoot off in straight lines and never come back.

Earth sits in a lucky spot — the third planet, in the just-right zone. Any closer to the Sun and it would be too hot, with the oceans boiling away; any further out and it would freeze solid. At exactly our distance the water stays liquid and the air stays gentle, which is why Earth is the one world in the family bursting with life.

The rest of the family

Planets aren't the only travellers round the Sun. There are also:

Moons — worlds that circle a planet instead of the Sun. Earth has one (our Moon); Jupiter and Saturn have dozens each.
Dwarf planets — small round worlds that are a bit too little to count as proper planets. Pluto is the most famous one.
Asteroids — millions of lumpy space rocks, most of them drifting in a wide band between Mars and Jupiter.
Comets — dirty balls of ice and dust that swoop in close to the Sun, then grow a long, glowing tail that streams across the sky.

All of it — planets, moons, dwarf planets, asteroids and comets — belongs to the Sun's enormous family.

Planets don't shine by themselves. A planet makes no light of its own — the only star that glows in our Solar System is the Sun. When you see a planet sparkling in the night sky, you're really seeing sunlight bouncing off it, just like moonlight is bounced-back sunlight.
The planets are almost never in a neat line. Pictures line them up in a tidy row to fit on the page, but out in space they are scattered all round their own orbits, usually on completely different sides of the Sun.
A "year" is different on every planet. A year isn't a fixed length — it's one lap round the Sun, so it's short for near planets and enormous for far ones.

In pictures the planets look cosy and close together. The truth is that space is almost entirely empty. Imagine shrinking the Sun down to the size of a beach ball and setting it on the floor of a big hall. Earth would be a tiny peppercorn — and it wouldn't be next to the beach ball, it would be a long walk across the room. Distant Neptune would be a small bead sitting right out of the hall, far down the corridor outside.

Between those specks there is nothing but vast, silent nothingness. Our fastest spacecraft take years just to crawl out to the giant planets. Space isn't a little crowd of worlds — it is a handful of specks scattered through a colossal, dark ocean of empty.

For a long time schoolbooks said there were nine planets, and the ninth, way out past Neptune, was little Pluto. Then astronomers with better telescopes started finding other icy worlds out there, some almost as big as Pluto. If Pluto was a planet, were all of these planets too? Suddenly the family might have dozens of members!

So in 2006 the world's astronomers made a rule for what counts as a proper planet — and Pluto didn't quite fit, because it's small and shares its region with lots of other icy chunks instead of sweeping its path clear. Pluto was moved into a brand-new group called dwarf planets. It didn't shrink and it didn't go anywhere — we simply gave it a more honest name. That's why we now say eight planets, not nine.