How Many Moons Does Jupiter Have? We're Losing Count

Earth only has one moon, but dozens of natural satellites orbit Jupiter, the biggest planet in our solar system. And new members in the Jupiter posse are still being discovered.

Back in December 2022, a team of astronomers published orbits for 12 previously unreported moons around the planet. This discovery brought the tally of satellites around Jupiter to 95, briefly putting it in the lead for most moons orbiting a planet. (A few months later, 62 new satellites were found orbiting Saturn, making it the moon king again.)

The new Jupiter moon count became official Jan. 20, 2023, and was announced in February 2023. The new additions were appended to the roster maintained by the International Astronomical Union’s Minor Planet Center. The search for these moons took place through observations conducted via telescopes in Hawaii and Chile in 2021 and 2022, followed by additional observations confirming their orbits.

Scott Sheppard of the Carnegie Institution, an integral part of the discovery team, expressed the desire to obtain close-up images of these newfound moons to delve into their origins further. These recently uncovered moons exhibit sizes ranging from 0.6 to 2 miles (1 to 3 kilometers), as stated by Sheppard.

The astronomer is also leading a search for new objects in the distant Kuiper Belt, an enormous ring of debris that lies beyond Neptune, and was part of the team that found these moons. Sheppard and his colleagues also found 12 previously unknown moons around Jupiter in 2018, as well as 20 new satellites around Saturn in 2019. To date, he has contributed to the discovery of approximately 70 moons encircling Jupiter.

Sheppard anticipates continuous augmentation to the moon counts of both Jupiter and Saturn. He postulates that these gas giants host numerous small moons, believed to be fragments resulting from past collisions between larger moons, comets or asteroids.

While similar moon-spawning dynamics apply to Uranus and Neptune, their remoteness complicates detection. For context, Uranus possesses 27 confirmed moons, Neptune boasts 14, Mars harbors two and Earth has one moon. Intriguingly, Venus and Mercury stand devoid of moons.

With regard to the newfound moons orbiting Jupiter, naming has yet to be undertaken. Sheppard highlights that only half of these distant moons surpass the size threshold of at least 1 mile (1.5 kilometers) to merit a formal name.

In April 2023, the European Space Agency launched a spacecraft from French Guiana to investigate Jupiter and its substantial icy moons. In 2024, NASA's Europa Clipper mission will explore Jupiter's moon of the same name, with potential implications for harboring an ocean beneath its icy surface.

In 1610, the great astronomer Galileo Galilei noticed four heavenly bodies that appeared to revolve around Jupiter. Named Io, Europa, Ganymede and Callisto, these are Jupiter's biggest moons by far (Ganymede being the largest moon). These Galilean satellites were also the first to be discovered.

As stargazing technology grew more sophisticated, it became clear that the quartet had lots of company. These latest moons are small and have orbits of one or two years, unlike the "big four," which are huge and have orbits of less than 17 days.

Jupiter had always had the most moons in the solar system, until 2019 when Saturn temporarily took over that crown with 82 moons — then took it back again in 2023. (Currently, Saturn has 146 moons that we know of.)

There's a reason why Jupiter has so many satellites while other planets — ours, for instance — have so few. It all comes down to gravity.

Astronomers divide the planets within our solar system into two categories. Mercury, Venus, Earth and Mars are the so-called "terrestrial" or "inner" planets while Jupiter, Saturn, Uranus and Neptune have been classified as "outer planets." (All the planets in the Jovian system were once classified as gas giants, but Uranus and Neptune have since been reclassified as ice giants.)

The size gap between those factions is quite considerable; although Uranus is the smallest outer planet, it's still 14.5 times more massive than Earth, the largest of the inner planets. None of the other planets can compete with Jupiter in terms of sheer bulk, however. You'd need more than 300 duplicates of our puny home world to equal Jupiter's colossal mass. It's an absolute monster.

Now, as Isaac Newton observed, there's a positive correlation between the mass of an object and the strength of its gravitational field. Because the gas giants are so massive, they're able to attract more satellites.

But that's not the only reason why planets like Jupiter have such large moon collections. Our solar system's gas giants are relatively far away from the sun. In contrast, some stars outside of our solar system have massive, Jupiter-like exoplanets called "Hot Jupiters." Basically, these are gas giants which orbit in close proximity to their stars. (Imagine if Saturn switched places with Mercury.)

A 2010 paper by French astronomer Fathi Namouni argues that Hot Jupiters have few, if any, moons. These planets are thought to originate in distant parts of their solar systems and then migrate inward.

Along the way, their moons get caught in a game of celestial tug of war. Gas giants may be big, but stars are much bigger. As such, they've got far stronger gravitational fields. So, when a Hot Jupiter gets too close to its star, the star will eventually steal its moons.

Distance offsets this ability: The further you travel from the sun, the weaker its gravitational pull on you becomes. Therefore, if Namouni is correct, the real Jupiter has 95 moons and counting because it's a massive planet that's far enough away from the sun to avoid lunar theft.

A Jovian moon, also known as a Galilean moon, refers to any of the four largest moons of Jupiter: Io, Europa, Ganymede and Callisto. The four Galilean moons are significant due to their substantial sizes and their distinctive characteristics. Io is loaded with active volcanoes; there's a hidden ocean on Europa that might harbor alien life; and at two-thirds the size of Mars, Ganymede is the biggest satellite in the entire solar system.

These three moons, along with Callisto, probably formed in tandem with Jupiter itself. The big planet likely started out as a disc of gasses and dust that eventually became the gas giant we know today. While Jupiter took shape, some of the material swirling around it coalesced into the four moons Galileo spied in 1610. A young Saturn may have helped move the process along.

Other satellites weren't necessarily home-grown. Scientists think that many of Jupiter's moons started out as drifting chunks of rock that became ensnared by the planet's gravitational pull.

"Jupiter can be thought of as a sort of mini-solar system itself, because its gravity controls thousands of small bodies," Sheppard said in a press release. "Only the sun has had more influence than Jupiter on the shape of our planetary neighborhood."

Before wrapping things up, we should talk about lunar behavior. Many of the Jovian moons orbit in the same direction in which Jupiter spins. But there are those with retrograde orbits, meaning they go the opposite way. With so many bodies revolving in different directions, collisions are inevitable.

Moons that crash into one another might well be destroyed in the process. Just as Jupiter acquires new moons, it's finding ways to lose some of the older ones.

This latest discovery is not the end of the moon hunt. New technology has made it easier to find faint objects that move against background stars. Sheppard and his colleagues believe there are many more moons to be found around both Jupiter and Saturn, as well as Neptune and Uranus, though their extreme distance from Earth (and our lonesome, only moon) makes those harder to confirm.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.