Jupiter: The Planet That Could Have Been a Star

Jupiter is not just the largest planet in the solar system. It is larger than all the other planets combined, twice over. If you hollowed it out, you could fit more than 1,300 Earths inside. It has the strongest magnetic field, the fastest spin, the most violent storms, and a collection of moons so diverse that it resembles a miniature solar system of its own. And yet, for all that size and power, Jupiter missed its chance at greatness. Had it gathered roughly 80 times more mass during its formation, it would have ignited into a star. Instead, it became something arguably more interesting: a failed star that shaped the fate of every other planet around it.

King of the Planets

Jupiter's numbers are difficult to absorb. Its diameter is about 143,000 kilometres, more than 11 times the width of Earth. Its mass is 318 times ours. It sits roughly 778 million kilometres from the Sun, five times further out than Earth, and takes nearly 12 years to complete a single orbit. But it spins on its axis in under 10 hours, making its day the shortest of any planet. That furious rotation bulges its equator outward and drives the immense atmospheric patterns visible even through a modest backyard telescope.

Jupiter is made almost entirely of hydrogen and helium, in roughly the same proportions as the Sun. Beneath the swirling cloud tops, pressure and temperature increase so dramatically that the hydrogen is squeezed into a liquid, and deeper still into a metallic fluid that conducts electricity. This metallic hydrogen, combined with Jupiter's rapid spin, generates a magnetic field up to 20,000 times stronger than Earth's. Its magnetosphere is so vast that, if it were visible to the naked eye from Earth, it would appear larger in our sky than the full Moon.

The Great Red Spot (and a Thousand Other Storms)

Jupiter's atmosphere is a canvas of violence. Its cloud bands, the pale zones and dark belts that stripe the planet, are streams of ammonia and water vapour racing in opposite directions. Where they meet, storms form. Lots of them.

The most famous is the Great Red Spot, a storm so large that Earth could fit inside it with room to spare. It has been raging for at least 190 years, and possibly much longer, with wind speeds reaching up to 640 kilometres per hour at its edges. It is an anticyclone, spinning counter-clockwise in Jupiter's southern hemisphere, and it has been slowly shrinking for as long as we have been watching it. Nobody knows whether it will eventually disappear or stabilise. For now, it remains the longest-lived storm in the known solar system, which is rather unsettling when you consider that nothing is holding it together but sheer atmospheric momentum.

And the Great Red Spot is not alone. Jupiter is covered in smaller storms, some of them still larger than Earth. New ones appear, merge, and vanish constantly. The planet's atmosphere is a laboratory for extreme weather on a scale that makes anything on Earth look gentle.

A Miniature Solar System

Jupiter has at least 95 confirmed moons, but it is the four largest that transform it from a planet into something like a neighbourhood. These are the Galilean moons, named after Galileo Galilei, who spotted them through his telescope in 1610 and, in doing so, helped overturn the idea that everything in the universe revolved around the Earth.

Each of the four is a world in its own right.

Io is the most volcanically active body in the solar system. Its surface is constantly being reshaped by hundreds of erupting volcanoes, driven by the immense tidal forces that Jupiter's gravity exerts on its interior. The lava flows are so fresh and so frequent that Io has almost no impact craters. Its surface looks like a pizza, blotched in sulphur yellows and oranges, and it is genuinely one of the most alien landscapes we have ever photographed.

Europa is, by many scientists' estimation, the most promising place to search for life beyond Earth. Beneath a smooth shell of ice lies a saltwater ocean that may contain twice as much liquid water as all of Earth's oceans combined. The ice surface is cracked and shifted in patterns that suggest the ocean below is in contact with a rocky seafloor, which could provide the chemical energy needed for life. NASA's Europa Clipper spacecraft, launched in October 2024, is on its way to investigate. It will arrive at Jupiter in April 2030 and conduct 49 close flybys of Europa, looking for the ingredients that could support biology.

Ganymede is the largest moon in the solar system, bigger than the planet Mercury, and the only moon known to generate its own magnetic field. It, too, is thought to harbour a subsurface ocean.

Callisto, the outermost of the four, is one of the most heavily cratered objects in the solar system, a frozen record of billions of years of impacts.

Four moons. Four completely different stories. All orbiting one planet.

The Solar System's Bodyguard

Jupiter's gravity does not just hold its moons in place. It shapes the entire solar system. Its immense pull is thought to have prevented a planet from forming in the asteroid belt, scattering the material there into fragments instead of letting it coalesce. It deflects or captures many of the comets and asteroids that drift through the inner solar system, absorbing impacts that might otherwise have struck Earth. In 1994, the world watched as Comet Shoemaker-Levy 9 broke apart and slammed into Jupiter's atmosphere in a series of explosions, some of which left scars larger than Earth that were visible for months.

The idea that Jupiter acts as a shield for the inner planets is debated among scientists. Some models suggest it deflects as many objects toward us as away from us. But what is not debated is that Jupiter's gravity has profoundly influenced the architecture of the solar system since its earliest days. It was the first planet to form, pulling together its massive bulk within just a few million years of the Sun's birth, and its gravitational influence helped determine where and how the other planets settled into their orbits.

Rings (Yes, Really)

Jupiter has rings. They are faint, thin, and made mostly of dust kicked up by meteoroid impacts on its small inner moons. They were discovered in 1979 by Voyager 1 and are essentially invisible from Earth without powerful instruments. They are nothing like Saturn's spectacular ice rings, but they exist, and they are a reminder that almost everything about Jupiter contains a surprise if you look closely enough.

Talking About Jupiter With Your Kids

Jupiter is the planet that makes children's eyes go wide, because everything about it sounds made up. It is a planet with no surface. A storm bigger than Earth that has been raging for centuries. A moon with erupting volcanoes and another with a hidden ocean that might contain life. At five, six, seven, eight, this does not sound like science. It sounds like a story. That is exactly the right way in.

Start with size. Tell your child that more than 1,300 Earths could fit inside Jupiter. Then find a basketball and a peppercorn. The basketball is Jupiter. The peppercorn is Earth. Hold them side by side. That single comparison tends to produce a silence followed by a lot of questions.

Use the storm. Tell them Jupiter has a storm so big that the whole Earth could fit inside it, and that it has been going for longer than anyone has been alive. Longer than their grandparents. Longer than their great-great-great-grandparents. Children at this age are still building their sense of time, and the idea of a storm that never stops is genuinely thrilling to them.

Let the moons do the work. You do not need to cover all four Galilean moons. Pick the one that suits your child. For the child who loves volcanoes, there is Io, a moon so volcanic that its surface is constantly melting and remaking itself. For the child who loves the ocean, there is Europa, a moon with a sea of saltwater hidden under ice, where scientists think something might be alive. For the child who loves records and facts, there is Ganymede, a moon bigger than the planet Mercury. Let them choose which story to follow.

Try the bodyguard idea. Tell them Jupiter is so massive that it pulls dangerous space rocks toward itself before they can reach us. Some scientists think Jupiter has been protecting Earth for billions of years. Whether or not that is exactly true, the image of a giant planet shielding a tiny one is the kind of thing that sticks with a child.

Save Europa for when they are ready. The idea that there might be an ocean under the ice of one of Jupiter's moons, and that a spacecraft is on its way right now to find out if something could be living in it, is one of the most powerful facts you can share with a child. Do not rush it. Let it arrive when the conversation is already alive. And when they ask, "Will they find something?" the honest answer is: nobody knows yet. That is the best kind of answer.

Jupiter is enormous in every sense, and the temptation is to keep going. Resist it. A conversation about the storm today, the moons next week, the ocean under Europa whenever they are ready. Let them come back. They will.