Solar System for Kids | mysimulator.uk

Eight planets, one star, and a LOT of empty space

The Solar System has one star (the Sun) at its centre and eight planets orbiting it: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, in order of distance outward. The four closest to the Sun — Mercury, Venus, Earth and Mars — are small, dense and rocky, and are called the terrestrial planets. The four farthest — Jupiter, Saturn, Uranus and Neptune — are enormous, made mostly of gas and ice, and are called the gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune).

live demo · the Sun and eight planets orbiting to scale in time● LIVE

Almost everything in the Solar System — 99.8% of its entire mass — is the Sun itself. Every planet, moon, asteroid and comet combined makes up the remaining sliver. Jupiter alone is more massive than all the other seven planets put together.

Distances are so big that models always lie a little

To fit on a screen or in a classroom, almost every solar system model shrinks the distances between planets far more than it shrinks the sizes of the planets themselves — otherwise you would need a model kilometres wide to see the outer planets at all. Astronomers avoid writing out huge numbers of kilometres by using the astronomical unit (AU): one AU is the average Earth-Sun distance, about 150 million kilometres. Using AU, the picture becomes much easier to compare:

Mercury   0.39 AU        Jupiter   5.2 AU
Venus     0.72 AU        Saturn    9.5 AU
Earth     1.0 AU (definition)   Uranus    19.2 AU
Mars      1.5 AU        Neptune   30.1 AU

If Earth's distance from the Sun were shrunk to 1 metre, Neptune would sit 30 metres away — about the length of three school buses parked end to end. That is why realistic scale models of the Solar System are almost always built outdoors, stretched across a park or a long stretch of road.

Why the planets keep orbiting instead of flying off or falling in

Each planet is in a constant tug-of-war between two things: its own sideways motion (inertia, carrying it in a straight line) and the Sun's gravity (constantly pulling it inward). Neither one wins outright — gravity keeps bending the planet's path inward just enough to keep it curving around the Sun instead of flying off in a straight line, but the planet's sideways speed keeps it from ever actually falling into the Sun. The result is a stable, repeating elliptical orbit. This is the same balance that keeps a satellite circling Earth, or the Moon circling us.

Planets closer to the Sun orbit faster and complete a year in less time; planets farther out orbit more slowly and take much longer. Mercury completes an orbit in just 88 Earth days, while Neptune takes about 165 Earth years — a relationship first worked out precisely by Johannes Kepler in the early 1600s, well before anyone understood gravity itself.

Sizes, side by side

Jupiter is so large that more than 1,300 Earths could fit inside its volume, and its Great Red Spot — a giant, centuries-old storm — is itself wider than Earth. At the small end, Mercury is only slightly larger than Earth's own Moon. Saturn is famous for its bright rings, made of countless chunks of ice and rock ranging from dust-sized grains to house-sized boulders, all orbiting the planet independently — but Jupiter, Uranus and Neptune have fainter ring systems too, just much harder to see.

Frequently asked questions

Why do outer planets take so much longer to orbit the Sun?

Because they have farther to travel around a much bigger orbit, and they also move more slowly than inner planets — the Sun's gravitational pull is weaker at greater distance, so less orbital speed is needed to balance it. Both effects combine to stretch Neptune's year to about 165 Earth years, compared to Mercury's 88 days.

Why can't a model show planet sizes and distances both to the same scale?

Because the real distances between planets are enormous compared to the planets themselves — even a marble-sized Earth would need to sit tens of metres from a basketball-sized Sun to be to true scale. Fitting both a readable planet size and a true-to-scale distance on one screen or in one room is essentially impossible, so most models scale distances down far more aggressively than sizes.

What keeps a planet from either flying away or falling into the Sun?

A constant balance between its sideways orbital speed, which would carry it off in a straight line, and the Sun's gravity, which continuously pulls it inward. Neither force wins, so the planet's path bends into a stable, repeating ellipse instead.

Try it live

Everything above runs in your browser — open Solar System for Kids and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.

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The Solar System for Kids: Sizes, Orbits and Scale