What a kid should know about the solar system

Five things. That is genuinely all of it, and every one of them is something you can hold in your head without a single big number.

1. How far apart everything is the one that surprises people

Start with one distance and measure everything else against it: from the Sun to the Earth. Call that one. Now watch what the other planets do.

Jupiter is the fun one. Five — hold up one hand, that is how many Earth-distances out it sits. And from there the numbers get easier, not harder:

• Saturn: ten. The nicest number there is.
• Uranus: twenty.
• Neptune: thirty.

Mercury is about a third of the way out — imagine half of Earth's distance, then a bit closer in. Venus is about three quarters — half again, then a bit closer to us. So the whole solar system, all eight planets, is: a third, three quarters, one, one and a half, five, ten, twenty, thirty. Not so bad!

Three more things that live out there

• Bennu is a small rocky asteroid that stays close to us, so it is about the same distance from the Sun as we are — one, near enough.
• Most asteroids sit between Mars and Jupiter, in the gap between one and a half and five. There is a lot of room in that gap.
• Most dwarf planets — Pluto, Eris, Makemake — are further out than Neptune, past thirty. (One of them, Ceres, is not: it lives in the asteroid belt with the rocks.)

2. How big they are a completely different order

Distance and size are two different lists, and they do not match. Here are the planets drawn on one shared scale — the same picture, all eight:

• Jupiter is the biggest. Comfortably. All the other seven put together would still not match it.
• Mercury is the smallest.
• Earth and Venus are almost exactly the same size. If you saw them side by side you would have to look twice.

3. How long a year is one simple rule

The further from the Sun, the longer the year. That is the whole rule, with no exceptions at all. Mercury goes round in 88 days. Earth takes 365. Neptune takes 165 years — it was discovered in 1846 and only finished its first lap in 2011.

4. How long a day is no rule whatsoever

You might expect the same trick to work twice. It does not. Day length is all over the place, and it depends on what happened to each planet long ago — something big hit it, or knocked it over, or slowed it down.

• Jupiter, the biggest planet, has the shortest day: under 10 hours.
• Venus takes 243 Earth days to turn round once — longer than its own year — and it turns the wrong way.
• Mars takes 24 hours and 37 minutes. Almost exactly ours, for no reason at all.

So: years are tidy, days are a mess. Both of those are worth knowing.

5. Moons some have none, some have hundreds

• Mercury and Venus have none. Not one.
• Earth has one — the Moon, and it is a big one for a planet our size.
• Mars has two, and they are tiny: Phobos and Deimos, both probably captured asteroids.
• Jupiter, Saturn, Uranus and Neptune have many — dozens each, and Saturn has more than two hundred. Four of Jupiter's are big enough to see through a small telescope from your own garden, in a line beside it.