Jupiter: The Giant Planet and Its Great Red Spot

Jupiter is the king of the planets.

It is bigger and more massive than everything else that orbits the Sun combined,

and through even a small telescope its striped clouds and bright moons make it one of the most rewarding sights in the night sky.

It is also a place of extremes: a world with no solid surface,

crushing pressures and a storm wider than Earth that has raged for well over a century.

Here is a guide to how Jupiter works, what makes the Great Red Spot so remarkable and how you can watch this giant yourself.

How Big Is Jupiter, Really?

Numbers about Jupiter can be hard to picture, so a few comparisons help:

• Jupiter is about 11 times as wide as Earth, and more than 1,300 Earths could fit inside it.
• It has about 318 times the mass of Earth and roughly two and a half times the mass of all the other planets put together.
• It orbits about 778 million km (484 million miles) from the Sun, around five times Earth's distance, and takes about 12 Earth years to complete one lap.
• Despite its size, it spins faster than any other planet, completing a day in just under 10 hours.

That rapid spin flattens Jupiter noticeably.

Its equator bulges outward, so the planet is visibly wider than it is tall, a detail you can even notice through a telescope.

What Jupiter Is Made Of

Jupiter is a gas giant, built mostly from hydrogen and helium, the same main ingredients as the Sun.

There is no solid ground to stand on.

Instead, the atmosphere gets thicker, hotter and more compressed the deeper you go.

Thousands of kilometers down, the pressure is so great that hydrogen behaves like a liquid metal that can conduct electricity.

Currents flowing through this vast metallic layer, combined with the planet's fast spin, are thought to generate Jupiter's magnetic field, the strongest of any planet.

Near the center there is probably a core of heavier material,

though measurements suggest it may be spread out and blended with its surroundings rather than a tidy, compact ball.

Jupiter also gives off more heat than it receives from the Sun.

Much of that extra heat is left over from the planet's formation, slowly leaking out as Jupiter continues to cool and contract.

Stripes, Jet Streams and Storms

Jupiter's famous stripes are bands of clouds pulled around the planet by its rapid rotation.

The lighter bands are called zones and the darker ones are called belts.

They are separated by powerful jet streams that blow in alternating directions, east in one band and west in the next.

The clouds come in layers.

The top layer is mostly ammonia ice, with deeper layers thought to contain other compounds and water.

Chemicals mixed into the clouds, altered by sunlight, add the creams, tans, browns and oranges,

though exactly which substances create each color is still not fully understood.

Where the bands rub against each other, the atmosphere churns into swirls, plumes and storms.

Spacecraft have seen lightning flashing inside Jupiter's storm clouds, and brilliant auroras glow around its poles.

The Great Red Spot

The most famous feature on Jupiter is the Great Red Spot, a giant storm in the planet's southern half.

It is an anticyclone, a high-pressure system that spins counterclockwise, with winds around its edge blowing at several hundred kilometers per hour.

A Storm Watched for Generations

Astronomers have tracked the Great Red Spot continuously since the 1800s,

and there are reports of a large spot on Jupiter going back to the 1600s,

although it is not certain those early observers saw the same storm.

Either way, no storm on Earth comes close to that kind of lifespan.

Measurements from spacecraft show that it extends far below the visible clouds, reaching hundreds of kilometers deep.

A Shrinking Giant

The Great Red Spot is still wider than Earth, but it is not as large as it used to be.

Observations over more than a century show it has shrunk considerably, becoming smaller and rounder.

No one knows whether it will keep shrinking, stabilize or eventually fade, which is one reason astronomers watch it so closely.

Its reddish color is also something of a mystery.

It may come from chemicals lifted from deep in the atmosphere and then altered by sunlight, but the exact recipe has not been pinned down.

Rings, Radiation and a Family of Moons

Jupiter has rings, but they are faint and made of fine dust rather than the bright ice of Saturn's.

They were discovered by the Voyager 1 spacecraft in 1979.

The planet is also surrounded by intense belts of radiation, trapped by its magnetic field, that can damage spacecraft electronics.

Jupiter has more than 100 known moons, and new small ones are still being found.

The four largest, Io, Europa, Ganymede and Callisto, were discovered by Galileo Galilei in 1610.

They are worlds in their own right, from volcanic Io to ice-covered Europa, and they deserve a full tour of their own.

Visitors From Earth

Pioneer 10 made the first flyby of Jupiter in 1973.

The two Voyager spacecraft followed in 1979 and revealed the planet's dynamic clouds and faint rings.

The Galileo orbiter arrived in 1995 and dropped a probe into the atmosphere,

and NASA's Juno spacecraft entered orbit in 2016, studying the planet's interior, magnetic field and poles.

In 1994, astronomers also watched fragments of Comet Shoemaker-Levy 9 slam into Jupiter, leaving dark scars in its clouds that were larger than Earth.

It was the first time people had observed a collision between two solar system bodies as it happened.

How to Observe Jupiter

1. Find it. Jupiter is usually the brightest object in the night sky after the Moon and Venus, shining with a steady, creamy white light. It is best placed around opposition, which comes about every 13 months.
2. Use binoculars for the moons. Brace them against something solid and you can spot up to four tiny points of light beside the planet. Their positions change from night to night.
3. Use a small telescope for the stripes. Even modest magnification shows the two main dark belts, and larger telescopes reveal more detail.
4. Time your look for the Great Red Spot. Because Jupiter rotates so quickly, the spot is only on the side facing us for part of each rotation. Many astronomy apps list when it crosses the middle of the disk.

Frequently Asked Questions

Is Jupiter a failed star?

Not really.

Jupiter is made of similar material to the Sun, but to ignite hydrogen fusion in its core it would need roughly 80 times more mass.

It formed as a planet, from the same disk of gas and dust as the rest of the solar system.

Could a spacecraft land on Jupiter?

No.

There is no solid surface to land on, and any probe that descends is eventually crushed and heated as the pressure rises.

The Galileo probe sent back data for about an hour as it fell through the atmosphere before contact was lost.

Does Jupiter protect Earth from impacts?

It is often said that Jupiter's gravity shields Earth by capturing or deflecting comets.

The real picture is more complicated: Jupiter can fling some objects away, but it can also nudge others toward the inner solar system.

Scientists still debate whether its overall effect helps or hurts.

Keep Watching the Giant

Jupiter changes from week to week, as its moons shift position and its clouds swirl and evolve.

Few sights in astronomy offer so much with so little equipment.

Set up a pair of binoculars on the next clear night when it is up,

and you will be looking at the same moons that helped change how people understood the universe.