Why the number keeps changing

In early 2023 Jupiter briefly held the moon crown at 95. Then fresh batches of confirmations — 4 announced by the IAU Minor Planet Center on 16 March 2026 and 14 more on 9 April 2026, most found by Scott Sheppard’s survey team at Carnegie Science — pushed the tally to 115. Yet Jupiter has been decisively overtaken: Saturn now leads with 292 confirmed moons, after a single March 2025 announcement added 128 at once and further batches in 2026 kept piling on.

Almost all the new finds are just a few kilometres across, circling far out on tilted, looping paths — too faint for older telescopes. Deep survey cameras spot them by stacking long exposures, so the count will keep rising. Jupiter is more than twice as massive as every other planet combined, and that gravity is forever catching passing asteroids and comets.

Why most of them have no name

Only 57 of the 115 have proper names. A new find starts life as a provisional tag like S/2021 J 1 — the year, J for Jupiter, and a counter. Once its orbit is pinned down it earns a permanent Roman numeral, and only then can a name be proposed. The proposal usually comes from the discovery team, but the sign-off belongs to one body: the International Astronomical Union’s Working Group for Planetary System Nomenclature. Not the discoverers alone, and not the public.

The rules are strict and oddly poetic. Jovian moons must be named for lovers, favourites or — since 2004 — descendants of Zeus/Jupiter, in 16 characters or fewer. And the last letter is a secret code: names ending in a or o mark prograde moons (orbiting with Jupiter’s spin), while e marks retrograde ones (orbiting backwards). You can read a moon’s direction of travel straight off its name. The oddball Valetudo — a prograde moon ploughing through Jupiter’s retrograde swarm — was described by its discoverer Scott Sheppard as “basically driving down the highway in the wrong direction”.

So why the backlog of 58 nameless rocks? Volume. With kilometre-scale moons now arriving by the batch, the IAU decided the smallest finds no longer get mythological names automatically — only those of real scientific interest. The public’s one recent chance came in 2019, when Sheppard’s team ran an open naming contest that christened five moons, including Pandia and Ersa. Commercial “name a moon” registries, for the record, have no official standing whatsoever.

Could any of them host life?

The secret ingredient is tidal heating: Jupiter’s gravity kneads its moons, generating heat that keeps Io erupting and, crucially, keeps hidden oceans liquid. Here is how the four big moons rank as places life might exist:

Which moons have we visited?

No spacecraft has landed on a Jovian moon, but seven missions have flown through the system — and two more are on their way to study the icy ocean worlds up close.

When does a rock stop being a moon worth counting?

The moon census is inflating for reasons that have little to do with Jupiter and everything to do with cameras. Scott Sheppard and Edward Ashton — the two most prolific moon-hunters alive, with over 200 discoveries each — find these objects by stacking hours of telescope exposures until 2-kilometre rocks emerge from the noise. Every one of them counts, one-for-one, exactly the same as Ganymede, a world bigger than Mercury. The headline number is a census of debris, not a ranking of worlds.

The IAU’s own naming policy is the quiet admission. When the body in charge of labels decides most new moons don’t merit a name, it is conceding that the word “moon” is being stretched past usefulness — Saturn’s 292 moons include just 64 with names. There is no minimum size for a moon, and sooner or later that will have to change, or the counts will simply stop meaning anything.

Expect the tallies to keep climbing and the definition fight to arrive within the decade — astronomy has been here before, and last time it ended with Pluto losing its planet card in 2006. The number that will still matter in ten years isn’t 115. It’s four.

Frequently asked questions

Sources

• IAU Minor Planet Center — Jupiter at 115 confirmed moons after batches announced 16 March and 9 April 2026 (Scott Sheppard’s team, Carnegie Science); Saturn at 292 as of 9 April 2026.
• IAU MPC, 11 March 2025 — the 128-moon Saturn announcement (Edward Ashton’s team, Canada–France–Hawaii Telescope) that took Saturn to 274 and past Jupiter for good.
• NASA — Europa Clipper (launched 2024, arrives 2030) and the Galileo, Juno and Voyager findings.
• ESA — JUICE mission, arriving at Jupiter in 2031 to study Ganymede, Europa and Callisto.
• Europa ocean volume ~2× Earth’s; a Jan 2026 study tempering its seafloor habitability.
• IAU Working Group for Planetary System Nomenclature — Jovian naming conventions (Zeus/Jupiter mythology, 16-character limit, a/o prograde vs e retrograde endings) and the policy against auto-naming the smallest moons.
• Carnegie Science, 2019 — Scott Sheppard’s public naming contest (five moons named, incl. Pandia and Ersa) and his “wrong direction” description of Valetudo, reported by CBC News.

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