Jupiter and Saturn: The Giant Planets

Meet the giants of the Solar System

Beyond Mars and the asteroid belt lie the two largest planets in our Solar System: Jupiter and Saturn. They are so big that all the rocky planets — Mercury, Venus, Earth and Mars — could fit inside Jupiter many times over. These worlds are completely unlike Earth. They are gas giants: vast, swirling balls of gas with no solid ground, ferocious storms, faint or spectacular rings, and whole families of moons orbiting them like miniature solar systems.

Studying them helps scientists understand how planets form, what other star systems might look like, and even where else in our own neighbourhood life might be hiding.

What makes a "gas giant"

Jupiter and Saturn are made mostly of hydrogen and helium — the two lightest and most common elements in the Universe, and the main ingredients of the Sun itself. This is a big clue: it suggests the giant planets grew by gathering huge amounts of the leftover gas from the cloud that formed the Sun, while the small inner planets formed from heavier rock and metal.

Because they are mostly gas, there is no surface to stand on. If you tried to descend into Jupiter, the gas would simply get thicker, hotter and more crushing the deeper you went, until the pressure became unimaginable. Deep inside, scientists think the hydrogen is squeezed so hard it behaves like a liquid metal, and there may be a dense core of rock and ice at the very centre.

Both planets spin astonishingly fast. A day on Jupiter — one full spin — lasts under 10 hours, even though it is the biggest planet. That rapid spin flings their gas outward, so both planets bulge slightly at their equators.

Jupiter: king of the planets

Jupiter is the largest planet, more than 11 times the width of Earth and more than twice as massive as all the other planets put together. Its powerful gravity acts like a giant vacuum cleaner, pulling in or flinging away comets and asteroids — some scientists think it has helped protect the inner planets, including Earth, from extra impacts.

Through a telescope, Jupiter shows beautiful bands of cream, orange and brown clouds. These are belts of gas blowing in opposite directions at hundreds of kilometres per hour. The most famous feature is the Great Red Spot, a gigantic storm — a swirling hurricane wider than the whole Earth — that has been raging for at least 350 years, since astronomers first turned telescopes on the planet. Imagine a storm that has lasted longer than any human has been alive.

Jupiter also has the strongest magnetic field of any planet and dramatic auroras far brighter than Earth's northern lights.

Saturn and its glorious rings

Saturn is the second-largest planet and, for many people, the most beautiful object in the Solar System because of its magnificent rings. Galileo first saw them in 1610 but could not work out what they were; his telescope was too weak to show their shape.

The rings are not solid. They are made of billions of separate pieces of ice and rock, from grains the size of dust up to boulders the size of houses, all orbiting Saturn in a vast, thin disc. The rings are enormous across — far wider than the planet — yet astonishingly thin, often only tens of metres thick. They may be the leftover bits of a moon or comet torn apart by Saturn's gravity, or material that never managed to clump into a moon.

Saturn is also famously light for its size. It is the only planet less dense than water — if you could find a bathtub big enough, Saturn would float. That is because it is so puffed up with light hydrogen gas.

Worlds within worlds: the moons

Each giant planet is circled by dozens of moons — Jupiter and Saturn each have over 90 confirmed. Some of these moons are among the most exciting places in the whole Solar System.

• Europa (a moon of Jupiter) has a smooth, cracked crust of ice. Beneath it lies a deep, salty ocean of liquid water — possibly containing more water than all of Earth's oceans. Where there is water, there may be life, which is why a spacecraft is on its way to study it.
• Io (also Jupiter's) is the most volcanic body in the Solar System, constantly squeezed and heated by Jupiter's gravity until its surface erupts with sulphur.
• Ganymede (Jupiter's) is the largest moon in the Solar System — bigger even than the planet Mercury.
• Titan (Saturn's) is the only moon with a thick atmosphere, and it has rivers, rain and lakes — but made of liquid methane, not water. It is one of the strangest, most Earth-like worlds we know.

The four big moons of Jupiter — Europa, Io, Ganymede and Callisto — are called the Galilean moons because Galileo discovered them in 1610. Seeing them proved that not everything orbits the Earth, which helped change our whole view of the Universe.

Try it yourself: spot the giants with binoculars

Both Jupiter and Saturn are easy to find, and even simple binoculars reveal something amazing.

1. Find out when they are visible. Use a stargazing app or website to see if Jupiter or Saturn is in your evening sky tonight. Both are bright and shine with a steady light, brighter than most stars.
2. Look at Jupiter through binoculars. Steady your elbows on a wall or fence. Beside Jupiter you should see up to four tiny points of light in a line — the Galilean moons, the very ones Galileo saw. Watch them change position from night to night as they orbit.
3. Try for Saturn's rings. Binoculars may show Saturn looking slightly oval or "eared". To see the rings clearly you need a small telescope — but spotting that the shape is not a simple dot is a thrill in itself.
4. Compare their colours and steadiness. Notice how planets shine steadily while stars twinkle. That steadiness is a quick way to tell a planet from a star.

Want to go further? Explore the other worlds in Planets of the Solar System, and learn how astronomers study them in Telescopes and Studying Space.