The solar system, for kids

Everything the Sun holds on to, from the rock you are standing on to a shell of sleeping comets so far out that light takes a year to reach it. This is the short version — the one worth knowing before you go and look at four of these with your own eyes tonight.

The one-sentence version

One ordinary star, eight planets, a few hundred moons, two belts of leftover rubble, and an enormous amount of empty space. That is it. Everything else is detail — good detail, but detail.

What is actually in it

The Sun

Not just the biggest thing here -- it practically IS the solar system: 998 parts in every 1,000 of all the material. Everything else on this list adds up to the last little sliver, and every bit of it orbits the Sun.

The planets

Eight, in two families. Four small rocky worlds near the warmth -- Mercury, Venus, Earth, Mars -- then four giants further out -- Jupiter, Saturn, Uranus, Neptune -- balls of gas and ice with no ground to stand on.

Moons

A planet can have companions of its own. Earth has one large Moon; Jupiter and Saturn have whole families; and even the asteroid Didymos owns a tiny moon, Dimorphos. Whatever a thing orbits, that is where you will find it in this list.

The belts

Two rings of leftovers from when the planets were built: the rocky asteroid belt between Mars and Jupiter, and the icy Kuiper belt beyond Neptune, where Pluto and the other dwarf planets live. Far beyond both, a thin shell of sleeping comets -- the Oort cloud -- marks the true edge of the Sun's neighbourhood.

Comets

Icy wanderers on long, stretched orbits. Each visit close to the Sun boils away a little of their ice into the glowing tail that makes them famous -- Halley's returns about once a human lifetime.

Visitors and messengers

Once in a while something falls through from another star entirely -- 'Oumuamua and Borisov were only passing, and never belonged to the Sun. And two machines of our own, Voyager 1 and 2, have now done the same journey in reverse: they have left the Sun's neighbourhood for good, carrying golden records out among the stars.

The eight planets, in order

Four small rocky ones close in, where it is warm enough that the ice boiled away while they were forming. Then a gap full of rubble. Then four giants, made of the gas and ice that survived out there in the cold. The gap is the story: it is why Earth is a rock and Jupiter is not.

MercuryplanetNaked eyeVenusplanetNaked eyeMarsplanetNaked eyeJupiterplanetNaked eyeSaturnplanetNaked eyeUranusplanetSmall telescopeNeptuneplanetSmall telescope

Five of them you can find with nothing but your eyes, on the right night — Mercury, Venus, Mars, Jupiter and Saturn. They look like stars until you watch them for a few weeks and notice they have moved.

Moons

A planet can have companions of its own. Earth has one large Moon; Jupiter and Saturn have whole families. Four of Jupiter's show up in any small telescope as a line of tiny dots that rearrange themselves from one night to the next — which is exactly how Galileo worked out that not everything goes round the Earth.

DimorphosmoonNot observableThe MoonmoonNaked eyePhobosmoonNot observableDeimosmoonNot observableIomoonSmall telescopeEuropamoonSmall telescopeGanymedemoonSmall telescopeCallistomoonSmall telescope

The leftovers

Two rings of rubble that never became planets: the rocky asteroid belt between Mars and Jupiter, and the icy Kuiper belt out past Neptune, where Pluto lives with the other dwarf planets. Far beyond both, a thin shell of sleeping comets marks the real edge of the Sun's neighbourhood.

The Asteroid Beltbelt of small bodiesNot observableThe Kuiper Beltbelt of small bodiesNot observableThe Oort Cloudsuspected, not yet foundNot observableCeresdwarf planetSmall telescopePlutodwarf planetWith effortHaumeadwarf planetNot observableMakemakedwarf planetNot observable

How big is it, really

Every picture you have seen of the solar system is a lie about distance — it has to be, or the planets would be invisible specks and most of the page would be blank. The app's Space view does not cheat: the sizes and the gaps are both true at once, because you zoom in powers of ten rather than sliding a scale.

How anyone knows any of this

Nobody flew out with a tape measure. The size of the Earth came from a stick and a shadow; the distance to the Sun came from watching Venus cross it from two continents at once. Each measurement stands on the one before it:

• How big is the Earth? — a stick, a shadow, and a walk between two towns
• How far away is the Moon? — a curved shadow during an eclipse
• What shape is the solar system? — two pins and a loop of string
• How far away is the Sun? — a transit of Venus, seen from two continents

Then go outside

The solar system is the one part of astronomy a child can verify personally. Jupiter's moons move overnight. Venus goes through phases. Mars turns visibly redder than everything around it. None of that needs equipment beyond patience and a dark-ish garden.