The solar system

The solar system consists of the Sun and everything held by its gravity: eight planets and their moons, together with dwarf planets, asteroids, comets and countless smaller bodies. The Sun, an ordinary star, contains by far most of the system's mass, and its gravity keeps the planets moving along their orbits. Animations are a clear way to present this arrangement, because the planets move at very different speeds and distances, and seeing them in motion conveys the scale and order of the system better than a fixed chart.

The planets fall into two groups. The four inner planets — Mercury, Venus, Earth and Mars — are relatively small, rocky worlds close to the Sun. The four outer planets — Jupiter, Saturn, Uranus and Neptune — are much larger gas and ice giants, several of them encircled by ring systems and many moons. Between the rocky and giant planets lies the asteroid belt, a region of rocky debris, and far beyond Neptune are icy bodies including the dwarf planet Pluto. Comets, made largely of ice and dust, travel on long orbits and grow bright tails when they near the Sun.

The motion of these bodies follows Kepler's laws: each planet moves in an elliptical orbit with the Sun at one focus, travelling faster when nearer the Sun and slower when farther away, and the planets farther out take much longer to complete one circuit. Moons orbit their planets by the same rules. By showing these orbits playing out together, interactive solar-system models make plain how the inner planets race around in months or a few years while the distant giants take decades, and how the whole system is organised around the Sun.

Frequently asked questions

How many planets are in the solar system?

There are eight planets: the four rocky inner planets Mercury, Venus, Earth and Mars, and the four giant outer planets Jupiter, Saturn, Uranus and Neptune.

What is the asteroid belt?

It is a region of rocky debris orbiting the Sun mostly between Mars and Jupiter, separating the inner rocky planets from the outer giants.

Why do outer planets take longer to orbit the Sun?

They are much farther from the Sun and travel along longer paths at slower speeds, so completing one orbit takes them many more years than the inner planets.