Jupiter’s Moons Guide: How Many Are There & What Are the Four Largest
Jupiter is not alone: it is surrounded by a whole family of moons, from tiny irregular satellites to the giant ones. The four largest are the Galilean moons: Io, Europa, Ganymede, and Callisto. Best of all, you can spot them yourself even with ordinary binoculars! Open Sky Tonight, point your device at Jupiter, and the app will show you which moon is which.
Contents
- How many moons does Jupiter have (as of 2026)?
- What are the names of Jupiter's moons?
- How to see Jupiter’s Galilean moons?
- The largest Jovian moons: quick facts
- Io: Jupiter’s volcanic moon
- Europa: Jupiter’s icy moon with a hidden ocean
- Ganymede: the largest moon in the Solar System
- Callisto: Jupiter’s cratered moon
Jovian moons FAQ
- How did the Galilean moons form?
- How many satellites does Jupiter have?
- Why does Jupiter have so many moons?
- Does Jupiter have the most moons in our Solar System?
- What is the biggest moon of Jupiter?
- Can I see Jupiter’s moons?
- Do Jupiter’s moons have an atmosphere?
- Can we colonize Jupiter’s moons?
- Is the water on Europa drinkable?
- Jupiter’s moons: key facts
How many moons does Jupiter have (as of 2026)?
As of 2026, 115 Jupiter moons have been confirmed. The number keeps growing as astronomers study Jupiter’s system in greater detail – and it doesn’t even include the smallest meter-sized moonlets orbiting the planet. Most of Jupiter's moons are small, less than 10 kilometers (6.2 miles) in diameter, and were discovered in the late 20th and beginning of the 21st century. The largest Jovian moons, however, were spotted back in the 17th century using a homemade telescope. They were the first moons of Jupiter ever discovered. They’re (in descending-size order) Ganymede, Callisto, Io, and Europa.
What are the names of Jupiter's moons?
As of 2026, only 57 of Jupiter’s moons are officially named. Their names come from the lovers, favorites, and descendants of the Greek god Zeus, the counterpart of the Roman god Jupiter. The rest are listed under provisional designations, such as S/2017 J 12, or numerical designations without proper names. Apparently, even Zeus didn't have enough favorites to name all of Jupiter's moons.
The four most famous moons of Jupiter are Ganymede, Callisto, Io, and Europa. In Greek mythology, Ganymede was a beautiful Trojan prince, Callisto was a nymph of Artemis, Io was a priestess of Hera, and Europa was a Phoenician princess. You can find all official Jupiter moon names in NASA’s satellite discovery list.
Why are the largest Jupiter’s moons called Galilean moons?
The four largest moons of Jupiter were discovered in 1610 by Italian astronomer Galileo Galilei – that’s why they are also called the Galilean moons. At first, his telescope could not clearly resolve Io and Europa as separate objects, so he saw only three objects instead of four. Moreover, he mistook them for fixed stars. Only later did he notice that they were not standing still but orbiting Jupiter.
Galileo’s discovery was revolutionary. It revealed that not all celestial bodies revolve around the Earth, contrary to the geocentric theory that was in use at that time. Moreover, it showed the importance of a telescope for astronomical observations: Galileo could see the Jovian satellites only when he improved the design of his homemade telescope.
Initially, Galileo named the moons “Cosimo’s stars” (later – “Medicean stars”) after his patron Cosimo Medici. In his notebooks, he referred to the moons as Jupiter I (Io), Jupiter II (Europa), Jupiter III (Ganymede), and Jupiter IV (Callisto). The names we use now were chosen by the astronomer Simon Marius, who discovered the moons nearly at the same time as Galileo and decided to name them after the Greek mythological characters. Let's see what moons were first seen near Jupiter.
Read also: What Was Galileo Famous For: His Achievements and Inventions
How to see Jupiter’s Galilean moons?
To see Jupiter’s Galilean moons, simply point your optics at Jupiter. The four largest moons — Io, Europa, Ganymede, and Callisto — are easy to spot even with a small telescope. A decent pair of binoculars can also show them as tiny points of light lined up on one or both sides of the planet, though a telescope will show them more clearly.
If you want to tell the moons apart and check where each one is at the moment you’re observing, use the Sky Tonight app. Open the app, type a moon’s name (for example, “Io”) in the search bar, open the result, and tap the target icon — the app will show the moon’s current position.
The moons don’t always appear as dots beside Jupiter. Depending on their positions, you may also notice:
- Transit: a bright dot crossing Jupiter’s disk.
- Shadow transit: a small dark spot moving across Jupiter — the moon’s shadow on Jupiter’s cloud tops.
- Occultation: a moon passing behind Jupiter, so it disappears for some time from your point of view.
To find out when these events will happen, search for a Galilean moon in Sky Tonight (for example, “Ganymede”), open its object card, and check the Events tab. There you’ll see when the moon passes in front of Jupiter, behind it, or casts a shadow, as seen from Earth.
The most notable events are double transits, when two Galilean moons (or their shadows) cross Jupiter at the same time. Find them in the Calendar window by tapping the Calendar icon at the bottom of the main screen. There, look for the planet icon with horizontal bands and a small dot on its disk.
The best time to observe Jupiter and its moons is around opposition, when the planet is closest and brightest. Learn more about upcoming planetary oppositions in our dedicated article.
The largest Jovian moons: quick facts
| Moon | Order from Jupiter | Diameter | What it’s known for |
|---|---|---|---|
| Io | 1 | 3,642.98 ± 1.00 km (2,263.6 ± 0.6 mi) | Volcanoes |
| Europa | 2 | 3,121.60 ± 0.60 km (1,939.7 ± 0.4 mi) | Possible ocean and habitability |
| Ganymede | 3 | 5,262.40 ± 3.40 km (3,269.9 ± 2.1 mi) | Largest moon in the Solar System |
| Callisto | 4 | 4,820.60 ± 3.00 km (2,995.4 ± 1.9 mi) | Ancient cratered surface |
Io: Jupiter’s volcanic moon
- Mass: 0.015 Earths
- Diameter: 3,642.98 ± 1.00 km (2,263.6 ± 0.6 mi)
- Equatorial circumference: 11,445.5 km (7,111.9 mi)
- Orbital speed: 17.334 km/s (10.770 mi/s)
- Surface temperature: -183 °C to -143 °C (-297 °F to -225 °F)
- Apparent magnitude: 5.02
- Named after: priestess of Hera, Zeus’ lover
When the Voyager scientists saw Io for the first time, they described it as “moldy pizza”. What causes the moon’s unusual look? Io is the most geologically active object in the Solar System – it has more than 400 active volcanoes. Io’s silicate surface is covered with sulfur dioxide frost, dark dots of erupting volcanoes, and lava flows. That’s why we see black, white, orange, yellow, and green spots over Io.
These spots make Io look just like a cheese pizza — but what about other space objects? Look at AI-generated images of delicious meals and try to guess which space object each image represents! Can you match them all?
Europa: Jupiter’s icy moon with a hidden ocean
- Mass: 0.008 Earths
- Diameter: 3,121.60 ± 0.60 km (1,939.7 ± 0.4 mi)
- Equatorial circumference: 9,807 km (6,093.8 mi)
- Orbital speed: 13.743 km/s (8.5 mi/s)
- Surface temperature: -223 °C to -148 °C (-369 °F to -234 °F)
- Apparent magnitude: 5.29
- Named after: Phoenician noblewoman, Zeus’ lover
Europa is the smallest and smoothest of the Galilean moons. It is notable for potentially having an ocean of water underneath its roughly 15–25 kilometers thick icy cover. If it’s true, this ocean may contain twice as much liquid water as all the Earth’s oceans combined. More importantly, it might have suitable conditions for extraterrestrial life to evolve, making Europa a key target for astrobiologists.
Europa is only one of the potentially habitable places in our Solar System. See other possible targets in our infographic about life in the Universe!
Ganymede: the largest moon in the Solar System
- Mass: 0.025 Earths
- Diameter: 5,262.40 ± 3.40 km (3,269.9 ± 2.1 mi)
- Equatorial circumference: 16,532 km (10,272.5 mi)
- Orbital speed: 10.880 km/s (6.8 mi/s)
- Surface temperature: -203 °C to -121 °C (-333 °F to -186 °F)
- Apparent magnitude: 4.61
- Named after: Zeus’ cupbearer
Ganymede looks similar to the Earth’s Moon but is 1.5 times larger (this is how it would look if Ganymede orbited Earth). It is the only moon known to have its own magnetic field. Ganymede is also thought to have an underground salty ocean, which might mean that the moon harbors living organisms.
Callisto: Jupiter’s cratered moon
- Mass: 0.018 Earths
- Diameter: 4,820.60 ± 3.00 km (2,995.4 ± 1.9 mi)
- Equatorial circumference: 15,144 km (9,410 mi)
- Orbital speed: 8.204 km/s (5.09 mi/s)
- Surface temperature: -193 °C to -108 °C (-315 °F to -162 °F)
- Apparent magnitude: 5.65
- Named after: a nymph, Zeus’ lover
Callisto is one of the most heavily cratered objects in the Solar System. One of the largest craters is named after Valhalla – the majestic hall for the dead warriors to rest in the afterlife. Callisto has been considered “dull”, as it didn’t show any volcanic or tectonic activity. But in the 1990s, data collected by NASA’s spacecraft revealed that there might be an ocean underneath its surface. Since then, Callisto is also on the list of potentially habitable worlds.
Jovian moons FAQ
How did the Galilean moons form?
Scientists believe that Jupiter’s four largest moons formed from the disk of material left over after Jupiter itself took shape. Just like the other planets in the Solar System, Jupiter formed from a disk of gas and dust surrounding the young Sun.
Once Jupiter formed, the leftover material that remained in its orbit started to clump together and grow. Eventually, it formed Jupiter’s largest moons, the Galilean moons. These moons are likely about 4.5 billion years old — as old as the Solar System itself.
How many satellites does Jupiter have?
As of May 2026, Jupiter is known to have 115 natural satellites. However, this number is growing: small, distant moons are still being discovered, tracked, and officially confirmed, so the count may rise in the future.
Why does Jupiter have so many moons?
Jupiter is big – it’s twice as massive as all the other Solar System planets combined. Therefore, its gravitational field is strong enough to attract so many satellites.
Does Jupiter have the most moons in our Solar System?
Although Jupiter is famous for its many moons, it doesn’t hold the record. The true king of satellites is Saturn, which, as of May 2026, has 292 moons! Want to learn more amazing facts about the ringed planet? Take our quiz!
What is the biggest moon of Jupiter?
The biggest moon of Jupiter is Ganymede. It is also the largest moon in the entire Solar System – even larger than the planet Mercury in diameter. Ganymede is one of Jupiter’s four Galilean moons, along with Io, Europa, and Callisto.
Can I see Jupiter’s moons?
Yes, but they are almost impossible to spot with the naked eye. Better use modern binoculars that are optically superior to Galileo’s homemade telescope. To locate them in the sky, use the Sky Tonight app: launch the app and type a moon’s name in the search bar.
If you don't have binoculars, you can still explore the Galilean moons up close with the Solar Walk 2 app. There, you can see their real-time positions, check out their surfaces, compare their sizes with Jupiter, and learn lots of interesting facts about each one.
Do Jupiter’s moons have an atmosphere?
Io has an atmosphere consisting primarily of sulfur dioxide. Callisto’s atmosphere is composed of carbon dioxide. Both Europa and Ganymede have an oxygen atmosphere, but it’s too tenuous for us to breathe.
Can we colonize Jupiter’s moons?
The Jovian system, in general, is not ideal for colonization, mostly because of its severe radiation environment: for example, after spending a few days on Europa, you can get a lethal dose of radiation. Of all the moons of Jupiter, Callisto is the least affected by radiation and is considered the most likely place for building a human base.
Is the water on Europa drinkable?
No one has drunk it yet, but due to chemical reactions with the rock, it most likely tastes like water in the Earth's oceans. This means it is too salty to drink.
Jupiter’s moons: key facts
Jupiter is surrounded by a huge family of moons – from tiny distant satellites to four giants that are fascinating enough to feel like planets of their own. The largest four are the Galilean moons: Io, Europa, Ganymede, and Callisto. You don’t need professional equipment to start exploring them. When Jupiter is visible in the sky, open Sky Tonight, find the planet, and check the real-time positions of its moons. The app will help you see which bright dot near Jupiter is Io, Europa, Ganymede, or Callisto. Start your adventure across Jupiter’s incredible moons tonight!
To learn even more surprising facts about Jupiter, the largest planet in the Solar System, take our dedicated quiz!
