Astro Bob: 92 moons and counting — Meet Jupiter's growing family

Recent discoveries put Jupiter back on top as "moon king."

Jupiter and Galilean moons
Most of us know four moons of Jupiter, called the Galilean moons because they were first discovered by Galileo in 1610. Named for figures closely associated with Jupiter from Greek mythology, Io, Europa, Ganymede and Callisto are all bright enough to see in the smallest of telescopes. The other 88 Jovian moons are much smaller and fainter.
Contributed / NASA, JPL, Space Science Institute with image data processing and layout Kevin M. Gill

Two hundred nineteen. That's the current tally of known moons in the solar system. Let's check who has what:

Earth — 1
Mars — 2
Jupiter — 92
Saturn — 83
Uranus — 27
Neptune — 14

If Pluto were still considered a planet, we could add five more! Jupiter was only recently crowned "moon king" after astronomer Scott Sheppard (Carnegie Institute for Science) turned up a dozen new moons from observations he made in 2021 and 2022 using the monster 6.5-meter (256-inch) Magellan telescope in Chile and 8-meter (315-inch) Subaru telescope atop Mauna Kea in Hawaii, among others.

Prior to 2023, Saturn stood atop the heap with 83 satellites. The new discoveries now make Jupiter, the solar system's largest planet, undisputed champion again. But as we know from human affairs, victories are sometimes temporary. Despite getting pretty much whatever it wants thanks to its irresistible gravitational sway, smaller Saturn may soon surpass this Goliath again. More on that in a moment.

Jupiter's inner moons
Orbiting even closer to Jupiter than the four Galilean satellites, Metis, Thebe, Amalthea and Adrastea (not pictured), make up the planet's innermost moons. They're so close to the planet they circle it rapidly — about 7 hours for Metis and Adrastea, 12 hours for Amalthea and 16 hours for Thebe.
Contributed / Stellarium

Jupiter's four brightest moons — Ganymede, Callisto, Europa and Io — are similar in size to our own moon, and orbit the planet from 1.8 to 16.7 days. According to Sheppard, who maintains his own Moons of Jupiter site, all the new objects are much smaller with diameters from 0.6 to 2 miles (1-3 km) and take more than 340 days to orbit the planet. Nine of them are among Jupiter's most remote moons, with orbital periods of more than 550 days.


All nine revolve around the planet backwards in retrograde motion, a good indication that they were probably once asteroids snared by the mighty planet's gravity. Once wild and free, they're now working for the man.

Jupiter moon families
This 2018 diagram shows how Jupiter's moons are broadly divided into those that circle the planet in the normal or prograde direction and those that orbit in the opposite or retrograde direction. Retrograde moons are likely captured asteroids. Valetudo, discovered in 2016, is the "oddball," with an orbit that crosses the retrogrades.
Contributed / Roberto Molar Candanosa, courtesy of Carnegie Institution for Science

The four Galilean moons, which orbit in the same direction as Jupiter rotates, have prograde orbits just like our own moon. The remaining three new finds also orbit in the prograde direction but lie between the Galilean moons and the retrograde crowd. Moons that formed from the whirling gas cloud that gravitationally collapsed to birth Jupiter typically share its spin.

The new discoveries, especially the closer-in prograde moons, were incredibly difficult to find given their tiny size, faintness and proximity to the planet's glare. Another hurdle in identifying new moons is that astronomers must track a potential suspect for a complete orbit to make sure it's not a stray asteroid. Many of the fainter moons take a year or two to circle the planet, requiring patience and precision.

Jupiter and many moons
This diagram showing both the Galilean and outer satellites demonstrates the richness and diversity of Jupiter's family of moons.
Contributed / NASA

None of the dozen newbies have names yet, and the smaller ones may never have. Given that many more will likely be discovered as telescopes and imaging technology improve, the group responsible for naming new planetary satellites has decided to no longer give the tiniest of them mythological names unless they're scientifically interesting. Too bad. I love the names. The world is a richer place with Eukelade (named for one of the Muses, daughters of Zeus) and Valetudo (Roman goddess of health), just two of Jupiter's lesser minions.

Despite the difficulties in clawing them from the glare of their host planets, Sheppard is certain that a lot more moons await discovery not just around Jupiter and Saturn but also at the more remote planets Uranus and Neptune.

Another team of astronomers has been watching a swarm of small bodies moving in step with Saturn. They're almost certain they belong to the planet and are likely fragments from a moon-on-moon collision that occurred several million years ago. The group estimates that about 150 of the objects (plus or minus 30) have diameters of 1.8 miles (3 km) and up — three times the number of Jupiter's moons down to that size. Astronomers would only need to discover a fraction of that number for Saturn to reclaim its status as mega-moon collector. Listen up, Jupiter!

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"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune. You can reach him at
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