What is the solar system?

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 more than a year to cross it.

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.

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 in the solar system 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.

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 most of the known dwarf planets live. Far beyond both, a vast, thick shell of sleeping comets — the Oort Cloud — is thought to mark 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 — ʻOumuamua, Borisov and 3I/ATLAS 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 bubble of the Sun’s wind for good, carrying golden records out among the stars.

Hypothetical

Two things in the solar system’s story have never been seen by anybody. The Oort Cloud is worked out backwards from where long-period comets keep arriving from; Planet Nine is worked out backwards from the way the orbits of a handful of distant rocks are bunched and tilted. Both may turn out to be there, and both may turn out to be a mistake — which is a perfectly ordinary thing for astronomy to be in the middle of.

The 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 eyeEarthplanetYou are hereMarsplanetNaked eyeJupiterplanetNaked eyeSaturnplanetNaked eyeUranusplanetBinocularsNeptuneplanetBinoculars

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. Mars is plainly redder than every star around it, and binoculars show Venus going through phases.

Moons

Four of Jupiter’s moons 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.

DimorphosmoonBeyond a 150 mm telescopeThe MoonmoonNaked eyePhobosmoonBeyond a 150 mm telescopeDeimosmoonBeyond a 150 mm telescopeIomoonBinocularsEuropamoonBinocularsGanymedemoonBinocularsCallistomoonBinocularsTitanmoonBinocularsEnceladusmoonBeyond a 150 mm telescopeTritonmoonBeyond a 150 mm telescope

The leftovers

The belts of rubble, and the dwarf planets that live in them:

The asteroid beltbelt of small bodiesThe Kuiper Beltbelt of small bodiesThe Oort Cloudsuspected, not yet foundCeresdwarf planetBinocularsPlutodwarf planetBeyond a 150 mm telescopeHaumeadwarf planetBeyond a 150 mm telescopeMakemakedwarf planetBeyond a 150 mm telescopeErisdwarf planetBeyond a 150 mm telescope

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 gaps are true, and every body is drawn at its true size as soon as you are close enough to see it as a disc, because you zoom in powers of ten instead of sliding a scale.

How anyone knows any of this

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

🌙 Plan an evening outsideEvery object, one by one