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Author: Laura Nicole Driessen, Postdoctoral researcher in radio astronomy, University of Sydney
Original article: https://theconversation.com/saturn-now-has-274-moons-but-exactly-what-makes-something-a-moon-remains-unclear-252033
Earlier this week, Saturn gained a whopping 128 new official moons, as the International Astronomical Union recognised discoveries from a team of astronomers led by Edward Ashton at the Academia Sinica in Taiwan. The sixth planet from the Sun now has a grand total of 274 moons, the most of any planet in the Solar System.
The discovery has raised a lot of questions. How do you spot moons, and why hadn’t anybody seen these ones already? Doesn’t Jupiter have the most moons? What are they going to call all these moons? Are there more out there? And what exactly makes something a moon, anyway?
NASA/JPL-Caltech/Space Science Institute
These new discoveries cement Saturn’s place as the winner of the Solar System’s moon competition, with more confirmed moons than all of the other planets combined. But it hasn’t always been this way.
Jupiter’s four largest moons – Io, Europa, Ganymede and Callisto – were the first ever discovered orbiting another planet. They were spotted by Galileo Galilei more than 400 years ago, in 1610. Saturn’s first known moon, Titan, was discovered by Dutch astronomer Christiaan Huygens 45 years later.
NASA/JHU-APL/Southwest Research Institute
The new batch of 128 moons was discovered by stacking images from the Canada France Hawaii telescope. Some of Saturn’s other moons were discovered by space voyages, and some during what are called “ring-plane crossings”.
When the Voyager 1 spacecraft passed by Saturn, it took images that were used to discover the moon Atlas. The Cassini Mission later discovered seven new Saturnian moons.
A ring-crossing is where Saturn’s rings seem to disappear from our point of view here on Earth. This is when Saturn is at just the right angle so we’re looking at the rings exactly side-on (that is, when we can only see the edge of the rings).
Titan was discovered during a ring-plane crossing, and so were 12 other moons. Saturn’s rings will be edge-on twice in 2025, in March and November.
Reta Beebe (New Mexico State University), D. Gilmore, L. Bergeron (STScI), NASA/ESA, Amanda S. Bosh (Lowell Observatory), Andrew S. Rivkin (Univ. of Arizona/LPL), the HST High Speed Photometer Instrument Team (R.C. Bless, PI), and NASA/ESA
The moon race
From 2019 to 2023, Jupiter and Saturn were fighting for first place in the moon race.
In 2019, Saturn surpassed Jupiter with the discovery of 20 new moons. This took the count to 82 for Saturn and 79 for Jupiter.
Just a few years later, in February 2023, Jupiter took the lead with 12 new moons, beating Saturn’s 83 moons at the time. Only a short time later, still in 2023, the same astronomers who discovered the recent 128 moons discovered 62 moons orbiting Saturn. This placed the ringed planet firmly in the lead.
Elsewhere in the Solar System, Earth has one moon, Mars has two, Jupiter has 95, Uranus has 28 and Neptune has 16, for a total of 142 moons. We only need to discover ten more moons around Saturn to give it double the number of all the other planets combined.
Regular or irregular?
The newly discovered moons are all small. Each one is only a few kilometres across. If something that small can be a moon, what really counts as a moon?
NASA tells us “naturally formed bodies that orbit planets are called moons”, but even asteroids can have moons. We crashed a spacecraft into an asteroid’s moon in 2022. Earth has had a few mini-moons, some only a couple of metres in size. The line of what is and isn’t a moon is a bit hazy.
Moons orbiting planets in the Solar System can be either “regular” or “irregular”. The new moons are all irregular.
NASA/JPL
Regular moons are formed around a planet at the same time as the planet itself forms. Irregular moons are thought to be small planets (planetesimals) that are captured by a planet as it finishes forming. They are then broken into pieces by collisions.
Regular moons tend to orbit their planets in nice, circular orbits around the equator. Irregular moons typically orbit in big ovals further away from planets, and at a range of angles. Saturn has 24 regular moons and 250 irregular moons.
Studying these moons can tell us about how moons form, and reveal clues about how the Solar System formed and evolved.
Saturn’s rings are made of small chunks of ice and rock. Astronomers think they formed out of pieces of comets, asteroids and moons that were torn apart by Saturn’s gravity.
So for Saturn in particular, irregular moons can tell us more about the formation of its beautiful rings.
What’s in a name?
Names of astronomical objects are governed by the International Astronomical Union (IAU). Originally, all moons in the Solar System were given names from Greco-Roman mythology.
But the large number of moons, particularly of Saturn and Jupiter, means the IAU has expanded to giants and gods from other mythology. And it’s all about the details. If binary moons are discovered, they are required to be given names of twins or siblings.
Saturn’s first seven moons were given numbers instead of names. In 1847, John Herschel named them after the Greek Titans. After they ran out of titans and Greek mythological giants, they expanded the naming system to include Inuit and Gallic gods and Norse giants.
Discoverers get to suggest names for moons, and the names they suggest are given priority by the IAU. In the past, there have been competitions to name new moons of Jupiter and Saturn.
With 128 new moons for Saturn, it might take a while to come up with names that follow the IAU rules. Maybe we’ll even see the addition of different mythologies. We’ll have to wait and see. Until then, each moon has a name made of a string of numbers and letters, such as “S/2020 S 27”.
NASA/JPL-Caltech/Space Science Institute
Will we find more moons?
Without a solid definition of what a moon is, it’s hard to say when (or if) we will ever finish finding them. Everyone agrees we shouldn’t call every single chunk of rock in Saturn’s rings a moon, but exactly where to draw the line isn’t clear.
That said, there is probably a limit to the number of moon-like objects astronomers are likely to want to add to the list. Edward Ashton, who led the discovery of the new moons, doesn’t think we’ll be finding too many new moons until our technology improves.