Jupiter
Two and a half times the mass of everything else in the solar system put together. Where it is right now, how big and how heavy, what goes round it, and what is still unexplained.
Jupiter on its axis
Turning here at one rotation every 12 seconds, which is about 3,000 times real speed. What is not sped up: which way it turns, how far the axis leans, and how its day compares with every other planet's. The belts and zones are at their real latitudes and the Great Red Spot at 22 degrees south, one and a quarter Earths wide. The blue-grey plumes along the equator are festoons, hanging off the south edge of the North Equatorial Belt where they really hang. Where each sits in longitude is schematic, and the white ovals stand for a set that changes every few years.
Wed, August 26, 2026
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Distances from the sun are in AU — one AU is the Earth’s average distance, 149,597,870 km — and the angle is where the body sits around its orbit, measured from the March equinox direction.
Jupiter by the numbers
The mass is worked out from Jupiter’s gravitational parameter and the gravity from that and its radius; the year comes from Kepler’s third law and the width is the same figure the simulator draws with. None of them is typed in beside the picture, so none of them can disagree with it.
The moons of Jupiter
Jupiter has 97 confirmed moons, of which the 17 below are large enough to be worth drawing. The rest are mostly a few kilometres of captured rubble on distant, tilted orbits. Every distance and period here is real, and they are not independent: each pair has to satisfy Kepler’s third law against Jupiter’s mass, which is how this table is checked rather than trusted.
| Moon | Distance | Orbit | Width | |
|---|---|---|---|---|
| Metis | 127,690 km | 7.1 h | 43.0 km | Inside the main ring, and feeding it |
| Adrastea | 128,690 km | 7.2 h | 16.4 km | The dust knocked off these two IS the main ring |
| Amalthea | 181,366 km | 12.0 h | 167 km | Red, potato-shaped, and radiating more heat than it receives |
| Thebe | 221,900 km | 16.2 h | 98.6 km | Its dust makes the outer gossamer ring |
| Io | 421,700 km | 1.77 d | 3,643 km | The most volcanically active body known — squeezed by Jupiter and by Europa |
| Europa | 671,034 km | 3.55 d | 3,122 km | A saltwater ocean under an ice shell, and more liquid water than Earth has |
| Ganymede | 1,070,412 km | 7.15 d | 5,268 km | The largest moon in the solar system — bigger than Mercury — and the only one with its own magnetic field |
| Callisto | 1,882,709 km | 16.7 d | 4,821 km | The most cratered surface known: nothing has resurfaced it in four billion years |
| Himalia | 11,451,000 km | 250.6 d | 140 km | A captured asteroid, 27 times further out than Io |
| Themisto | 7,393,216 km | 129.9 d | 9.0 km | Alone between the Galileans and the Himalia group - lost for 25 years after its discovery |
| Leda | 11,187,781 km | 241.8 d | 21.5 km | The smallest of the Himalia group |
| Lysithea | 11,700,800 km | 259.2 d | 42.0 km | Himalia group: same orbit, same colour, same parent body |
| Elara | 11,712,320 km | 259.6 d | 79.9 km | Second largest of the group - Lysithea and Elara sit barely 12,000 km apart in orbital radius |
| Ananke | 21,454,952 km | 641.9 d ↺ | 29.1 km | Leader of a retrograde group of about two dozen fragments |
| Carme | 23,404,000 km | 734.2 d ↺ | 46.7 km | Another shattered capture - the Carme group are all the same dark red |
| Pasiphae | 23,624,000 km | 743.6 d ↺ | 57.8 km | The largest retrograde moon Jupiter has |
| Sinope | 23,939,000 km | 758.9 d ↺ | 35.0 km | Nearly 57 times further from Jupiter than Io, and running backwards |
↺ marks a moon going round backwards. This system is the picture at the top of this page — press Play and watch these periods run.
The moons of Jupiter, one at a time
The table above has every figure — how far out, how long a lap takes, how wide. What follows is what those figures cannot tell you: what each one is actually like, who found it and when, and what is still argued about it.
Amalthea
The reddest object in the solar system, and less dense than water.
- Its red colour is thought to be sulfur thrown out by Io and swept up.
- It is less dense than water ice, so it is a loose pile with a great deal of empty space in it.
Io
The most volcanically active body in the solar system, by a wide margin — and none of that heat comes from the sun.
- More than 400 active volcanoes, with plumes reaching 500 km up. Its surface is repaved so fast that it has essentially no impact craters.
- The heat is tidal: Io is squeezed and released on every lap because Europa and Ganymede keep tugging it out of a circular orbit. That resonance is what powers it.
- It loses about a tonne of material every second to a doughnut of plasma around Jupiter, which is part of what drives Jupiter’s aurorae.
- The yellows, reds and whites are sulfur and sulfur dioxide frost, not lava colour.
Still unsettled
Is there a global magma ocean under the crust, or many separate chambers? The induced magnetic signature suggests a connected layer; the eruption pattern does not obviously agree.
The volcanoes are systematically offset from where tidal heating models say they should be, and nobody has a settled explanation.
Europa
A shell of ice over a salt-water ocean holding perhaps twice as much water as every ocean on Earth.
- Its surface is among the youngest in the solar system — tens of millions of years — which means something is actively resurfacing it.
- The dark lines crossing it are cracks that have opened and refrozen as the shell is flexed by Jupiter.
- Jupiter’s magnetic field induces a field in Europa, and only a conducting layer — salty liquid water — does that. It is the strongest evidence for the ocean and it is indirect.
- Europa Clipper launched in October 2024 and arrives in 2030, to fly past it about fifty times.
Still unsettled
How thick is the ice? Estimates run from a few kilometres to thirty, and that single number decides whether anything could ever reach the ocean.
Are the water plumes real? They have been reported more than once and confirmed never.
It has liquid water, chemistry and an energy source. Whether that is enough is the question the next decade is built around.
Ganymede
The largest moon in the solar system, and larger than the planet Mercury.
- It is the only moon known to generate its own magnetic field, which means it has its own aurorae inside Jupiter’s.
- Two kinds of ground: dark, ancient, heavily cratered terrain, and lighter ground grooved by tectonics.
- Those aurorae rock back and forth less than they should if there were no ocean underneath, which is how a subsurface ocean was inferred from a telescope in Earth orbit.
- ESA’s JUICE arrives in 2031 and will end up in orbit around it — the first spacecraft ever to orbit a moon other than ours.
Still unsettled
Why does it have a dynamo when Callisto, a similar size, does not?
Its ocean may be layered, with ice both above and below it. If so, water never touches rock, which matters for whether anything could live there.
Callisto
The most heavily cratered object anyone has ever photographed — a surface that has been left alone for four billion years.
- Valhalla is a bullseye of concentric rings about 3,800 km across, from an impact into ice that behaved like a liquid and then froze.
- It barely separated into layers: rock and ice are still substantially mixed inside, which means it never got hot enough to sort itself out.
- It orbits outside the worst of Jupiter’s radiation belts, which is why it turns up in nearly every serious sketch of a crewed outpost at Jupiter.
Still unsettled
It probably has an ocean too, and it has no tidal heating worth the name. What is keeping it liquid is not settled.
When Earth and Jupiter are next closest
Both planets are moving, so the gap between them swings enormously — and the closest approaches are not all equal, because the orbits are ellipses rather than circles. These are the next four, solved from the orbits rather than looked up.
Click a date to take the simulator there. These are minimum-distance moments, which fall near — but not exactly on — opposition.
Flying to Jupiter: when to leave
A rocket cannot point at Jupiter and fire. It has to leave Earth on an orbit around the sun whose far side touches Jupiter’s orbit — half an ellipse — and it has to leave at the moment when Jupiter will have arrived at that far side by the time the ship gets there. That is what a launch window is, and it is why they come round only every 12.9 months.
Set Launch to: Jupiter on the rocket launches page to see the ellipse, where Jupiter is on launch day, and where it will be on arrival. Click the date to fly it.
What the flight path is, and is not
It is a minimum-energy transfer: the cheapest possible path, half an ellipse with Earth’s orbit at one end and the target’s at the other, solved against where the planets really are — so the arrival lands on the planet’s real distance from the sun and its real position on the day it gets there, not on a circle standing in for its orbit.
It is not a mission plan, and three things are left out on purpose. The orbits are treated as flat, so the real inclinations — Mars is tilted 1.85°, Jupiter 1.3° — cost a plane change this ignores. Real missions trade fuel for a faster arrival, so they leave within days or weeks of these dates rather than exactly on them. And anything going past Jupiter usually steals speed from a planet on the way instead of buying it with fuel, which changes both the date and the path.
What it gets right is the thing worth teaching: why the window exists, why it comes round on the cadence it does, and why the cost of the trip is set by where two planets happen to be rather than by how far apart they are.
Jupiter: things worth knowing
- Jupiter does not orbit the centre of the Sun. The pair's balance point sits just outside the Sun's surface, so strictly the Sun and Jupiter both go round a point in empty space.
- It turns once in under ten hours, faster than any other planet, and the spin visibly squashes it: the equator is about 7% wider than the poles.
- The Great Red Spot is a storm that has been watched for at least 190 years and possibly 360. It is shrinking — it was more than twice Earth's width in the 1800s and is now about one Earth wide.
- Io, Europa and Ganymede are locked in a 1:2:4 resonance: Io goes round exactly twice for each Europa orbit and four times for each Ganymede orbit. The regular tug that produces is what keeps Io molten and Europa's ocean liquid. You can watch it happen on Jupiter's view here — the periods in the drawing are the real ones.
- Jupiter has rings. Voyager 1 found them in 1979, and they are made of dust kicked off the tiny inner moons Metis, Adrastea, Amalthea and Thebe by micrometeorite impacts — a ring system that has to be continuously resupplied to exist at all.
What we still don’t know about Jupiter
Every one of these is genuinely unsettled — not simplified for the page, not waiting on a textbook update.
Why does Jupiter have rings, and why are they so faint?
The mechanism is fairly well settled — dust knocked off four small inner moons, held briefly in orbit, spiralling in and being replaced. What is not settled is why the giants all have rings and the rocky planets have none, and why Jupiter's are dust while Saturn's are ice boulders. Ring systems may be a phase every giant goes through, in which case Saturn's are a snapshot of something Jupiter has already finished.
Is there a core in there?
Juno's gravity measurements do not fit a planet with a clean rocky core, nor one with none. They fit a 'fuzzy' core: heavy elements smeared through the inner half of the planet rather than concentrated. That is difficult to produce in formation models unless something enormous hit Jupiter early on, and it has not been explained.
How deep does the weather go?
Juno found that the belts and zones extend about 3,000 km down — far deeper than expected — before the interior starts rotating as a solid body. What sets that depth, and what the flows look like below the clouds, is unresolved.
Recently learned about Jupiter
Findings reviewed August 2026. Space science moves; a date on a finding is part of the finding.
The rest of the system
Inward: Mars. Outward: Saturn. Or step out: all the planets, a picture and a paragraph each, and the solar system simulator, where every orbit runs at once.
Make a link to a particular view
The date, the zoom, the span, the speed, the layers and any flight path are all in the address bar, so copying the URL shares exactly what is on screen. Set it up above, then take the link — it is the quickest way to hand a class one specific thing to look at.
The other simulator
This one is about the whole system. If the question is where the sun and the moon are from where you are standing — what time the sun comes up, why tonight's moon is the shape it is — that is the Sun, Earth & Moon movement simulator, which has a page for every city and a slider over a day, a week or a month. Between the two is the three bodies moving together — Earth going round the sun, the moon going round the Earth, on one screen and openly not to scale, keeping only the real ratio between the two periods.
And for why any of it stays up: the orbital velocity simulator takes one planet and lets you set its distance and its sideways speed by hand, so you can watch the balance that holds every orbit here — and break it, into an ellipse, an escape, or a fall into the sun.
Also: the classroom guide · sunrise & sunset by city · moon phase & moonrise · lunar eclipses · how the positions are worked out
Jupiter: questions without settled answers
Why does Jupiter have rings, and why are they so faint? The mechanism is fairly well settled — dust knocked off four small inner moons, held briefly in orbit, spiralling in and being replaced. What is not settled is why the giants all have rings and the rocky planets have none, and why Jupiter's are dust while Saturn's are ice boulders. Ring systems may be a phase every giant goes through, in which case Saturn's are a snapshot of something Jupiter has already finished.
Is there a core in there? Juno's gravity measurements do not fit a planet with a clean rocky core, nor one with none. They fit a 'fuzzy' core: heavy elements smeared through the inner half of the planet rather than concentrated. That is difficult to produce in formation models unless something enormous hit Jupiter early on, and it has not been explained.
How deep does the weather go? Juno found that the belts and zones extend about 3,000 km down — far deeper than expected — before the interior starts rotating as a solid body. What sets that depth, and what the flows look like below the clouds, is unresolved.