How Many Moons Does Jupiter Have?

How many moons does Jupiter have? More importantly, does it have the most moons of any planet in our solar system? Well, seeing as how it is the largest planet by a wide margin, it stands to reason that it should. Interestingly enough, Saturn held that record for a long time.

However, it was recently discovered that Jupiter may actually have more moons than Saturn. As it currently stands at the time of this article’s publication, Saturn has 83 moons, whereas Jupiter has about 95.

These numbers include moons within the orbit of the planets and do not count the various moonlets located within Saturn’s rings, or the hundreds of pieces of debris found within the space immediately around either planet. Nevertheless, Jupiter, the largest planet in the solar system, has claimed another crown. Let’s take a look at the moons of Jupiter.

Moons of Jupiter: The Galilean Moons

The first discovery of the moons of Jupiter was by none other than Galileo way back in 1610. While looking through his legendary telescope, he noticed 3 stars on either side of Jupiter that couldn’t be seen with the naked eye. As the stars moved, Galileo realized he was looking at objects that were orbiting Jupiter, otherwise known as moons.

It was definitive scientific proof that not everything in the universe revolved around the Earth, a groundbreaking discovery at the time. The following week, he noticed a fourth one and documented them all. To this day, those four moons of Jupiter are known as the Galilean moons and, if you wanted to, you could spot them with a telescope of your own.

All four of the Galilean moons are massive. It should be noted that even though Galileo needed a telescope to see them, all of these moons would be visible to the naked eye if the light from Jupiter weren’t drowning them out.

All four of these moons are planet-sized and would be considered such if they revolved around the Sun instead of another planet. But the gas giant Jupiter’s mass is so great that it draws planet-sized objects into its orbit. 

Ganymede

Measuring 2270 km across, Ganymede is the largest of Jupiter’s moons and it’s also the largest moon in the entire solar system. Even more impressive is the fact that it is larger than the planet Mercury.

Truly, if it had not been sucked into Jupiter’s orbit, Ganymede could easily have been a planet. But it has more than size going for it; Ganymede is also quite rocky, icy, and most likely has a liquid-iron core, making it even more planet-like than most moons.

All that rock and iron provide it with its own magnetic field, further helping it impersonate a planet. Its surface is marked with large grooves of unknown origins.

A current hypothesis claims that the grooves were created by the gravitational pull from other moons as they pass in orbit, much like our own moon’s effect on our tides. Speaking of tides, Ganymede has oceans of water located under its surface.

Callisto

With a diameter of 4800 km, Callisto is the second biggest of Jupiter’s moons (after Ganymede). It shares a few similarities with Ganymede in that it is mostly made of rock and ice. The rocky core is most likely surrounded by a layer of mixed rock and ice.

Much like Ganymede, it has its own magnetic field, despite most likely lacking a metallic core. The icy surface shows little evidence of volcanoes, which suggests little to no tectonic activity. That means that the surface is very old, and may have changed very little since the formation of Callisto.

Interestingly, Callisto has an atmosphere, but it is so thin it’s only one-one billionth the pressure of our own. Out of the large moons, Callisto has the orbit that is furthest away at about 2 million km from Jupiter.

Io

An odd name, but Io is probably the closest thing to our own moon. It is just barely bigger and, despite Jupiter’s massive size, its orbit is so close to the planet that it only takes about a day and a half to complete.

Unlike our moon, however, Io is oddly colored. Orange, yellow, red, and black could all be observed by the Voyager 1 space probe when it took photos of Io in 1979. Even odder is the fact that Io doesn’t appear to have any impact craters.

The reason for this could be that Io is the most volcanic object in the entire solar system. Io’s more than 400 active volcanoes erupting constantly explain why no craters can be found on its ever-changing surface. 

The volcanic activity also accounts for the odd coloring of this moon, as the sulfur levels in the ejected materials are incredibly high. Interestingly enough, much of Io’s volcanic activity can be attributed to the other moons of Jupiter. As they pass Io in orbit, the tides flex the moon, creating friction and, thereby, heating Io’s interior.

Interestingly, a lot of the sulfur from Io gets picked up by the very powerful magnetic field of the massive Jupiter and is accelerated as it passes the planet. This has created a large ring-shaped radioactive belt around Jupiter.

This radioactive belt would be deadly to humans that passed through it without any protection. Of course, so would the vacuum of space, so there’s no practical application to that fact; it’s just cool to think about.

Perhaps, one day, we will build a spacecraft that will take humans to Jupiter. We’ll have to plan for all kinds of situations that are inhospitable to human life.

Europa

She may be the smallest of the Galilean moons, but in many ways, Europa might be the most interesting. For a long time, Europa’s claim to fame was being extremely reflective. This led to the hypothesis that it was covered in water ice.

When Voyager 1 finally got there, the images were surprising. The surface looked smooth, much like Io’s continually reformed surface. What was strange, however, was that, unlike Io, Europa did not appear to have volcanic activity.

What could have happened that resurfaced Europa? The answer came in the form of large ridges. These ridges were made up of water from a massive ocean that lies in the moon’s interior.

This water wells up under the surface, erupts, and freezes on the icy surface, creating a new landscape, not dissimilar to the way lava erupts and cools into rock. The amount of water on Europa is estimated to be more than that of all the oceans on Earth. 

Life on Europa?

Even more interesting than all of that is the hypothesis that Europa has all the elements required to create life. The oceans are believed to be salty due to their interaction with the silicate rock that makes up much of Europa.

Earth’s life originated in salty water and, if Europa does contain the same carbon-based molecules as the ones here on Earth, it isn’t a stretch to imagine that life could possibly take hold on Europa.

Astrobiologists have proposed sending a probe to Europa to look for signs of life on the microbial level. It’s fascinating to think that a moon in our solar system, particularly one located so far from the heat of the Sun, might contain life.

The idea of extraterrestrial life was once a thing of science fiction, then a real possibility that was always imagined in some remote galaxy, and now a possibility in our own backyard (cosmically speaking). But, until such a discovery is potentially made, we will just have to marvel at the possibilities.

Moons of Jupiter: Other Moons

Other moons have been discovered since the time of Galileo. In fact, new moons are discovered all the time. In order to be considered a moon, an object must be observed making a complete orbit around Jupiter. Sometimes, the light from Jupiter is so bright it hinders our Earthly observations. Here are some of the other moons that have been discovered.

Amelthea

The next largest moon after the four Galilean moons is Amelthea, which is 250 km (155.3 miles) across its largest dimension. It’s phrased that way because, unlike some other moons, Amelthea is not perfectly spherical, but rather irregularly shaped.

The surface of this moon is red and scientists hypothesize that its color is the result of sulfur pollution from Io. Amelthea has a very tight orbit to Jupiter, often finding itself only 100,000 km from Jupiter’s cloud tops.

If someone were to stand on the surface of Amelthea, Jupiter’s mass and proximity would make the planet dominate the horizon, filling half the sky.

Irregular and Distant Moons

Going by their size, the moons of Jupiter just get smaller and more irregularly shaped after Amelthea. Their names are Himaila, Thebe, Elara, Pasiphae, Hegemone, Kale, and so on. Most of these small moons are no bigger than hills.

Interestingly, many of them have backward orbital paths. These backward orbits (at least compared to the other, larger moons) are also known as retrograde orbits. This irregularity of orbit may give clues as to their origins.

Perhaps they were asteroids at one point, sucked into Jupiter’s massive gravitational pull, or perhaps they were once a larger object that had been broken up. To support the latter theory, there are several families of moons that orbit Jupiter in similar patterns.

Moons of Jupiter: 2023 Discovery

In February of 2023, the MPC, or Minor Planet Center, operated by the Smithsonian, discovered a dozen new moons around Jupiter. This is a 15% increase in the number of total moons and was enough to nudge Jupiter past Saturn in the leaderboard for the most moons in the solar system.

When it comes to declaring an orbital object a moon, there can be a bit of a delay between observation and confirmation, partly because astronomers must compile enough data to prove that the object in question is, in fact, orbiting the entire planet, thereby qualifying it for lunar status.

Of the 12 new moons, 9 are very far and have very long orbits, some taking longer than 550 days. Still, there is no rule for how quickly a moon must orbit its planet in order to be considered a moon.

Moons of Jupiter: Conclusion

It should be noted that there is some debate about what exactly constitutes a moon. Both Jupiter and Saturn have potentially billions of pieces of rock orbiting them. Should the pieces that are no bigger than a car be considered a moon? How about no bigger than a grapefruit?

When does a moon stop being debris and become a moon? The answer to that is most likely best left up to the experts. In the meantime, we’ll just have to be satisfied with Jupiter being the temporary winner in the solar system-wide competition for the total number of moons.

The image featured at the top of this post is ©Dotted Yeti/Shutterstock.com.