Mooning over: Saturn now has 274 moons; but what makes something a moon?

Laura Nicole Driessen

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 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?

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.

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.

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.

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.

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.

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.

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.

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.

(Laura Nicole Driessen is a Postdoctoral Researcher in Radio Astronomy, University of Sydney)