They are called the Galilean moons: Io, Europa, Ganymede and Callisto. Four worlds, four totally different stories — all circling the same giant planet.
This page is an overview of all four moons together. To dive into any single moon, follow the links inside each section, or see the full Jupiter facts for kids pillar page.
The Galilean moons are Jupiter’s four biggest moons: Io, Europa, Ganymede and Callisto. Galileo Galilei discovered them in January 1610 — the first moons ever seen orbiting a planet other than Earth. Each one is a unique world: Io has hundreds of volcanoes, Europa has a hidden ocean, Ganymede is the biggest moon in the solar system, and Callisto has one of the oldest and most heavily cratered surfaces in the solar system.
You can see the Galilean moons tonight with basic binoculars! Look at Jupiter after sunset and steady your hands. You should see up to four tiny dots in a line beside the planet. Sketch where each one is, then look again tomorrow night — they will have moved. That is exactly what Galileo did in 1610.
Quick reference — all four at a glance
Here is a side-by-side comparison of the four Galilean moons. Each column links to the full page for that moon.
| Io | Europa | Ganymede | Callisto | |
|---|---|---|---|---|
| Position | 1st (closest) | 2nd | 3rd | 4th (outermost) |
| Diameter | 3,643 km | 3,121 km | 5,268 km | 4,821 km |
| Best known for | Volcanoes | Hidden ocean | Biggest moon | Most craters |
| Orbital period | 1.77 days | 3.5 days | 7.15 days | 16.7 days |
| Hidden ocean? | No known global ocean | Strong evidence | Strong evidence | Possible (from magnetic data) |
| Magnetic field? | No (induced) | No (induced) | Yes (its own) | No (induced) |
| Relative radiation | Extreme | Very high | High | Low |
| Estimated surface age | Often very young in resurfaced areas | About 40–90 million years | Ancient dark + younger grooved terrain | About 4 billion years |
Meet the four moons
Each Galilean moon has its own personality. Here’s a quick introduction to all four — click through to any one of them for the full story.
| 3rd moon out — Ganymede — The biggest moon in the solar system |
|---|
| Size: 5,268 km across — bigger than the planet Mercury Ganymede is the biggest moon in the entire solar system — bigger than Mercury, bigger than Pluto, bigger than Earth’s Moon. It is also the only moon known to generate its own magnetic field. ESA’s JUICE mission is planned to begin orbiting Ganymede in December 2034 — if successful, the first spacecraft ever to orbit a moon other than Earth’s Moon. Read the full Ganymede page → |
How Galileo discovered the four moons
In late 1609, Galileo Galilei built one of the first astronomical telescopes. Telescopes were still new at the time, mostly used as toys or sailing tools — but Galileo had the idea of pointing one straight up at the night sky.
Galileo began observing Jupiter’s moons on 7 January 1610. On that first night he saw three little stars near the planet, lined up in a row. He was curious enough to look again the next night, and the next. The stars kept changing position. By 13 January 1610, a fourth one had appeared. By 15 January 1610, Galileo had worked out what was happening:
These were not stars. They were worlds orbiting Jupiter.
It was the first time in human history anyone had clearly observed objects orbiting another planet. That fact mattered enormously — because at the time, most people still believed every object in the night sky orbited Earth. Galileo’s tiny moons proved the old belief wrong.
Galileo wanted to name the moons the “Medicean Stars”, after his patrons the Medici family. The names we use today — Io, Europa, Ganymede and Callisto — came from a German astronomer called Simon Marius, who saw the moons at about the same time and proposed names from Greek mythology connected to Zeus.
Read more about the early Jupiter missions →
The hidden rhythm — the Laplace resonance
The three innermost Galilean moons — Io, Europa and Ganymede — are locked into a special orbital pattern. Their orbits are tied together by gravity so that they tick along in a steady rhythm:
- For every 4 orbits Io makes around Jupiter…
- Europa makes exactly 2 orbits…
- …and Ganymede makes exactly 1 orbit.
This is called the Laplace resonance, named after the French scientist Pierre-Simon Laplace who discovered it in 1798. The three moons have been keeping this 4:2:1 rhythm for billions of years — and the rhythm is what powers their wonderful, strange features.
Because Io, Europa and Ganymede pull on each other gravitationally, their orbits get stretched into ovals instead of perfect circles. As each moon swings closer to and farther from Jupiter, it gets squeezed and stretched by Jupiter’s gravity. That squeezing creates heat inside the moons — what scientists call tidal heating.
LAPLACE RESONANCE — a special pattern where three moons keep their orbital periods in a steady ratio. Io, Europa and Ganymede are locked in a 4:2:1 Laplace resonance — for every 4 times Io goes around Jupiter, Europa goes around exactly 2 times and Ganymede exactly 1 time.
Imagine the inside of an old mechanical clock. Tiny gears mesh together — when the small one spins twice, the next one spins once, and the big one spins half as often. The three inner Galilean moons work like clockwork around Jupiter, locked in step with one another for billions of years.
Callisto — the outermost moon — is not part of the Laplace resonance. That is why Callisto is so different from the other three: less squeezing means less heat, no volcanoes, an icy surface that holds onto its craters.
You can see this pattern in action on the Io page (volcanoes powered by tidal heating) and the Europa page (hidden ocean kept liquid by tidal heating). Then compare with Ganymede and Callisto to see how much difference the squeezing makes.
Why are the four moons so different?
Same parent planet. Same star. Same age (roughly 4.5 billion years). Yet the four moons are completely different worlds. Why?
The big answer is how close each moon is to Jupiter.
- Io is closest, so it gets squeezed the hardest. Result: hundreds of volcanoes.
- Europa is the next out, with moderate squeezing. Result: a hidden ocean kept liquid by some tidal heating.
- Ganymede gets less tidal heating than Europa, but it is much larger — large enough to have a layered interior, a hidden ocean, and a liquid iron-rich core that creates its own magnetic field.
- Callisto is the farthest out, outside the Laplace resonance. Result: very little tidal heating, an ancient surface, and a hidden ocean buried very deep.
So the moons read like a ladder of activity: fiery Io at the bottom, frozen quiet Callisto at the top, Europa and Ganymede in between. The same idea — energy from gravity — explains all four.
Io: hundreds of volcanoes, surface less than 1 million years old
Europa: hidden ocean, young surface (40–90 million years old)
Ganymede: magnetic field, mix of old and newer terrain
Callisto: almost no activity, ~4-billion-year-old surface
The Galilean moons make a great teaching set: same parent planet, four different worlds. Ask students to predict what would happen if Callisto somehow joined the Laplace resonance. (More tidal heating → possible volcanic activity → fresher surface.) This is also a great chance to talk about how gravity is the same physics here on Earth, on Jupiter’s moons, and across the universe.
Missions to the Galilean moons
Since the 1970s, several spacecraft have visited Jupiter and seen the Galilean moons up close. Here are the highlights:
- Pioneer 10 (1973) — first ever close-up images of Jupiter’s big moons
- Pioneer 11 (1974) — more close-up moon images
- Voyager 1 (1979) — discovered active volcanoes on Io, captured the first sharp images of Europa’s cracks
- Voyager 2 (1979) — sharper images of all four Galilean moons
- Galileo orbiter (1995–2003) — circled Jupiter for 8 years, flew close to all four Galilean moons, discovered Ganymede’s magnetic field and Europa’s likely hidden ocean
- New Horizons (2007) — flew past Jupiter on its way to Pluto, took new images of the moons
- Juno — arrived at Jupiter in 2016; extended mission planned through September 2025. Close flybys of Ganymede (2021), Europa (2022), and Io (multiple in 2023–2024). Operational status should be checked before publishing.
- Europa Clipper (NASA, launched 2024) — on its way; scheduled to arrive at Jupiter in April 2030 and make 49 close flybys of Europa
- JUICE (ESA, launched 2023) — scheduled to arrive at Jupiter in 2031 and tour Europa, Callisto and Ganymede before — if successful — entering orbit around Ganymede in 2034
NASA’s Galileo orbiter did more than any single mission to change what we know about the Galilean moons. Between 1995 and 2003, Galileo discovered Ganymede’s magnetic field, found strong evidence for hidden oceans on Europa and Callisto, and watched Io’s volcanoes erupt in real time.
Scientists have not found life on any of the Galilean moons. Europa and Ganymede have hidden oceans, and Callisto may have one too — but no spacecraft has ever sampled them. The upcoming missions (Europa Clipper and JUICE) will help us learn whether any of these worlds have the right conditions for life, not whether life is actually there.
Which Galilean moon should you explore first?
Not sure which moon to dive into? Pick the one that matches what you love.
- Love volcanoes? Start with Io — the most volcanically active world in the solar system.
- Curious about aliens or oceans? Start with Europa — the moon with a hidden ocean that may have the right conditions for tiny life.
- Like record-breakers? Start with Ganymede — the biggest moon in the solar system, with its own magnetic field.
- Love craters and ancient history? Start with Callisto — the time-capsule moon with a 4-billion-year-old surface.
Frequently asked questions about the Galilean moons
What are the Galilean moons?
The Galilean moons are Jupiter’s four biggest moons — Io, Europa, Ganymede and Callisto. They were discovered by Galileo Galilei in January 1610 and were the first moons ever seen orbiting a planet other than Earth.
Who discovered the Galilean moons?
Italian astronomer Galileo Galilei spotted them through an early telescope in January 1610. German astronomer Simon Marius observed them around the same time and gave them their current Greek-mythology names.
Why are they called “Galilean” moons?
They are named after Galileo Galilei. The names of the individual moons (Io, Europa, Ganymede and Callisto) all come from characters in Greek mythology connected to Zeus.
What is the biggest Galilean moon?
Ganymede — at 5,268 km across, it is bigger than the planet Mercury and is the biggest moon in our entire solar system.
What is the smallest Galilean moon?
Europa, at 3,121 km across — slightly smaller than Earth’s Moon.
Which Galilean moon is closest to Jupiter?
Io is the closest. It orbits Jupiter at about 421,700 km from the planet’s centre and completes one orbit every 1.77 Earth days.
Which Galilean moon is furthest from Jupiter?
Callisto. It orbits Jupiter at about 1,883,000 km, and takes about 16.7 Earth days to complete one orbit.
Are the Galilean moons bigger than Earth’s Moon?
Some are. Ganymede (5,268 km) and Callisto (4,821 km) are both substantially bigger than Earth’s Moon (3,474 km). Io (3,643 km) is slightly bigger. Only Europa (3,121 km) is smaller than our Moon — and not by much.
Which Galilean moon is most interesting?
They are interesting in different ways. Io has volcanoes, Europa may have a hidden ocean that could support tiny life, Ganymede is the biggest moon and the only one with its own magnetic field, and Callisto preserves ancient craters from the early solar system. Most scientists would say Europa is the most exciting for the search for life beyond Earth.
Which Galilean moon is safest?
Callisto has the lowest radiation of the four Galilean moons, so it is often considered the safest for spacecraft. But none of the moons is safe for humans without heavy protection — all four are extremely cold and have almost no breathable atmosphere.
Can you see the Galilean moons from Earth?
Yes! With basic binoculars or a small telescope you can see all four as tiny bright dots beside Jupiter. They change position every night. Use a sky-map app to identify which moon is which.
Could any of the Galilean moons have life?
Maybe. Europa is considered the strongest candidate because of its hidden ocean and the chemistry seen on its surface. Ganymede and Callisto may also have oceans, but they are buried deeper. Io is too hot and too radiation-blasted for life. No life has actually been found on any of them.
Why are the Galilean moons so different from each other?
Mostly because of distance from Jupiter. The closer a moon is, the more it gets squeezed by Jupiter’s gravity (and by the other moons), which produces internal heat. Io gets squeezed the most and is geologically wild; Callisto gets squeezed the least and is quiet and frozen.
Quick recap
- The Galilean moons are Jupiter’s four biggest: Io, Europa, Ganymede, Callisto
- Galileo Galilei discovered them in January 1610 — the first moons seen orbiting another planet
- Three of them (Io, Europa, Ganymede) are locked in a 4:2:1 Laplace resonance
- Distance from Jupiter explains most of why the moons are so different: more squeezing = more activity
- Io = volcano moon · Europa = ocean moon · Ganymede = biggest moon · Callisto = time-capsule moon
- NASA’s Europa Clipper (arriving 2030) and ESA’s JUICE (arriving 2031) will study them up close in the 2030s
Now pick your favourite moon to explore in detail — or head back to the main Jupiter pillar.