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SYFY WIRE Bad Astronomy

Another score for Saturn: 20 newly discovered moons for the ringed planet

By Phil Plait
Schematic showing the orbits of 20 newly discovered moons around Saturn.

Until recently, we knew Saturn had a lot of moons. However, now we know it has a whole lot more.

Astronomers just announced that they’ve found 20 new moons orbiting the ringed planet. Twenty! That’s quite a load. It makes Saturn the record-holder for number of moons with 82; a dozen new moons were recently discovered around Jupiter, giving it 79. Saturn has that easily beat… for now.

These moons are all small — roughly 5 kilometers across — and orbit Saturn at great distances. The closest one in is over 17 million km from the planet! The farthest is over 26. For comparison, the farthest discrete ring of Saturn (the F ring) is 140,000 or so km from the planet’s center. Earth’s Moon orbits 385,000 km from our planet’s center. So these moons are incredibly distant.

Schematic showing the orbits of 20 newly discovered moons around Saturn.

That makes their discovery all the more remarkable. As I researched this I noted curiously that they’re all designated S/2004 SXX or S/2005 (where the first S means satellite, 2004 or 2005 is the year of discovery, and SXX is a number giving each moon’s designation as a moon of Saturn and the discovery order).

The years surprised me. I asked Scott Sheppard, who made these discoveries, why they have years from so long ago. He told me that over this past summer he went back over old images taken of Saturn with the Subaru 8.2-meter telescope. These were deep images planned to look for faint moons, and many were discovered right away at the time. Sheppard suspected more, very faint moons were in there, so he dug around the images and looked (by eye!) for objects that moved.

Even then they couldn’t be announced. The motions betrayed their status as Saturnian moons, but later observations were needed for confirmation. He had to calculate their orbits and predict where they’d be in later years — not a trivial task, since they move around Saturn and Saturn moves around the Sun. Nevertheless, he was able to do so, and the 20 new moons were then confirmed by the International Astronomical Union (or IAU), the official designator of cosmic things.

The moons of Saturn form groups according to their inclination (orbital tilt). The Norse group orbit retrograde at high angle (top), while the Gallic and Inuit groups orbit prograde at lower inclination.

The moons are interesting. Out of the 20, 17 orbit Saturn backwards, or retrograde, which means in the opposite sense of Saturn’s rotation. They also all have about the same inclination angle, meaning their orbits are all tilted to Saturn’s equator by roughly the same amount (between 140° and 180°, where 0° would be prograde — orbiting in the same sense as Saturn — directly above its equator, and 90° would be a perfectly polar orbit). This is a strong indicator they all had the same origin, probably two moons colliding or one moon getting hit by a comet or asteroid long ago. Such a catastrophic collision would shatter the moons, creating lots of smaller ones, all orbiting with roughly the same inclination.

Astronomers call collections of objects like this groups. This one is called the Norse group (and the moons in it have Norse names). The most distant moon of Saturn is one of these new ones, and falls in this group. Another in this group is Phoebe, which is over 200 km across, and is likely what’s left of the moon that got whacked (and, if it’s name sounds familiar, is the source of the protomolecule in The Expanse), and is also likely to be a captured object from the Kuiper Belt, a ring of icy objects orbiting the Sun out past Neptune (and of which Pluto is the largest member).

Animation showing the motion of the newly discovered Saturn moon S/2004 S24. Credit: Scott Sheppard

Of the three remaining moons, which orbit prograde, two are in the Inuit group (with inclinations of 45° to 50°) and the other in the Gallic group (about 36°). That last one, at the moment called S/2004 S24, is very interesting indeed. While it clearly is in the Gallic group due to its tilt, it’s much farther out from Saturn than the rest of the group. It’s not clear why. Perhaps some odd gravitational interaction pulled it away from the planet, or perhaps the others moved closer in after forming while this one didn’t.

All of these moons tell us more about Saturn, or at least hint at its environment and conditions in the past. The more moons found, the more complete a picture that is. I’ll note, too, that this makes Sheppard and his team the most successful moon hunters in history (they found that recent dozen-moon passel around Jupiter, too). Quite an achievement.

How many moons may orbit Saturn? Well, it depends on what you mean by “moon”; at the lower size end there’s no real definition. But even if you draw the line at, say, 1 km, this means there are a lot more out there. These moons, at 5 km across, are pushing hard at the limit of what we can do from Earth right now; ones smaller would be too faint to see (after all, they’re over 1.3 billion kilometers away from us at best!). But in general, collisions make a few big objects, a few more middling ones, and lots of smaller ones. If these are 5 km wide, I’d wager there are hundreds at 1 km. It’ll be a while before those are found, though.

So I wouldn’t get too hung up on Saturn holding the record now, or Jupiter having it until recently. There could be many hundreds, even thousands more out there is you include Uranus and Neptune.

Oh, one more thing. These 20 new moons… how’d you like to name them?

Because you can! Sheppard and his team are holding a contest to suggest names for this new score of satellites. You can read all about it online. There are some rules, though. The biggest is that the names have to be appropriate for their specific group, so Inuit, Gallic, and Norse. Moreover, given Saturn’s moons being named after Titans, the new moons all have to be named after giants in those mythologies. There are rules from the IAU, too, so be sure to read those (even further, you can’t repeat any names of asteroids or other objects, so check those as well).

Got a name? Then you can suggest it to Sheppard’s group on Twitter, @SaturnLunacy (include the hashtag #NameSaturnsMoons).

Quite a few suggestions have already been made, and the contest ends on 6 December, 2019, so think fast!