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ExploreSpace and Earth

The world, a little closer

The Solar System.

One star, eight planets, and a lot of empty space. Run the clock and see why the inner planets race while the outer ones crawl.

Explore the scene Interactive article

01 / The experiment

One star, eight worlds.

Run the clock, then scrub to true distances to see how empty the system really is.

Interactive scene

Preparing your experiment…

The explanation below is available while the scene loads.

Orbital periods are exact. Orbits are drawn as circles, which is very nearly right for all eight planets.Planet and Sun sizes are exaggerated enormously. At true scale, with Neptune at this distance, Earth would be smaller than a pixel.

02 / Three kinds of planet

Rock, gas, ice.

Where a planet formed decided what it is made of.

Inner four01

Rocky planets

Small, dense, with solid surfaces. Close to the Sun, where its heat drove the light gases away before they could gather.

Members
Mercury, Venus, Earth, Mars
Made of
Rock and metal
Year
88 days to 1.9 years
Jupiter and Saturn02

Gas giants

Vast worlds with no surface, mostly the same two gases the Sun is made of. Jupiter alone outweighs all the other planets together.

Members
Jupiter, Saturn
Made of
Hydrogen and helium
Year
12 and 29 years
Uranus and Neptune03

Ice giants

Smaller, colder, and built around ices rather than gas. So far out that sunlight takes hours to arrive and the Sun looks like a bright star.

Members
Uranus, Neptune
Made of
Water, ammonia, methane
Year
84 and 165 years
The big idea

The farther a planet is from the Sun, the longer its path and the weaker the pull along it, so its year grows faster than its distance.

T² ∝ a³

One star, eight worlds

The Solar System is the Sun and everything held by its gravity: eight planets, their moons, and a vast number of smaller bodies. The Sun contains more than 99.8 per cent of all the mass. Everything else, planets included, is a rounding error orbiting it.

All eight planets go round the Sun in the same direction, and very nearly in the same flat plane, a leftover of the spinning disc of gas and dust they formed from four and a half billion years ago.

Inner and outer

The four inner planets, Mercury, Venus, Earth and Mars, are small and rocky, with solid surfaces you could stand on. Beyond Mars the system changes character. Jupiter and Saturn are gas giants, enormous worlds of hydrogen and helium with no surface at all. Uranus and Neptune are ice giants, smaller and colder, built around water, ammonia and methane. The line between the two groups was drawn by the young Sun: close in, its heat drove off the light gases, and only rock and metal were left to build planets from.

Why the far planets are slow

Watch the scene for a while. Mercury flashes round the Sun while Neptune barely moves. Two things are at work. Neptune’s orbit is much longer, thirty times the circumference of Earth’s. And Neptune travels along it more slowly, because the Sun’s gravity is far weaker out there and does not need to pull as hard to keep it on its path.

Put together, the pattern is exact and was found by Kepler four centuries ago: the square of the year grows with the cube of the distance. Move a planet four times farther out and its year gets eight times longer.

Try it: set the speed to Season and count how many times Mercury goes round while Earth completes one lap. It should be a little over four.

How far is far

Distances in the Solar System are so large that kilometres stop being useful. Astronomers use the astronomical unit, the distance from Earth to the Sun, about 150 million kilometres. Light covers it in eight minutes. Neptune is thirty units out, so sunlight takes over four hours to reach it, and the Sun in its sky is just a very bright star.

Scrub to True distances and the scene has to shrink the whole inner system into a huddle near the Sun to fit Neptune in. That huddle, and the emptiness around it, is what the Solar System really looks like.

Not just planets

Between Mars and Jupiter lies the asteroid belt, millions of rocky bodies that never gathered into a planet. Beyond Neptune is a wider belt of icy worlds, including Pluto, which was counted as a planet until 2006 and is now called a dwarf planet along with several others. Farther still, comets drift in a cold cloud that may reach halfway to the nearest star, and a few of them fall inward every year to grow tails in the sunlight.