How One Astronomer Helped to Discover Nearly 200 Moons of Saturn

Scientific American

22-05-2025

A mere decade ago, astronomers knew of just 62 moons around Saturn. Today the ringed planet boasts a staggering 274 official satellites. That's more than any other world in the solar system—and far too many for most people to keep track of. Astronomer Edward Ashton is no exception, even though he has helped to discover 192 of them—he thinks that's the total, anyway, after pausing to do some mental math.
Ashton is now a postdoctoral fellow at the Academia Sinica Institute of Astronomy and Astrophysics in Taiwan. He fell into hunting for Saturn's moons in 2018, when his then academic adviser suggested the project for his Ph.D. at the University of British Columbia. It has been a fruitful search. Most recently, in March, Ashton and his colleagues announced a batch of 128 newfound Saturnian satellites.
Scientific American spoke with Ashton about the science of discovering so many relatively tiny moons—most of them just a few kilometers wide—using vast amounts of data gathered by the Canada-France-Hawaii Telescope (CFHT), located in Hawaii.
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[ An edited transcript of the interview follows.]
How have you found these moons?
To detect the moons, we use a technique known as shifting and stacking. We take 44 sequential images of the same patch of sky over a three-hour period because, in that time frame, the moons move relative to the stars at a rate similar to Saturn. If we just stack the images normally, then the moon appears as a streak across the images, and that dilutes the signal of the moon.
So what we do is: we shift the images relative to one another at multiple different rates near that of Saturn, and then we basically blink between the different shift rates. If the shift rate is not quite at the rate of the moon, then it's going to be slightly elongated. As you get closer to the rate of the moon, then it slowly combines into a dot. And then, as you get faster than the moon's rate, it expands again. So basically, we look at the images and then quickly blink through the different rates, and you can see the moon coalescing.
That's for a single night. But just seeing an object moving at a Saturn-like rate near Saturn doesn't guarantee that it is a moon. It's highly likely that the object is a moon, but that hasn't been confirmed. So what we need to do is track the objects to show that they are in orbit around the planet. To do that, we repeat the [shift and stack] process multiple times over many months and years.
Why did this happen now? Did you need new techniques and observatories to do this work?
The technique and the technology have been there for a while—the same technique has been used to find moons of Neptune and Uranus. But the sky area around those planets where moons can exist is a lot smaller, so it takes less time to search through the data. One of the reasons why this hadn't been done for Saturn is because it's very time-consuming.
Why do those other planets have less space where moons could be than Saturn does?
Those planets are less massive, so the stable orbits that moons can have are smaller.
I had been wondering if this technique works for other planets, and clearly the answer is yes. But do you think there are other moons that have yet to be found around Saturn or other planets with the method?
We did find moon candidates around Saturn that we weren't able to track long enough to be able to confirm them. So if you redo this technique again, you will be able to find more moons around Saturn, but this is a case of diminishing returns. If you use a larger telescope [than the CFHT], then you'd be able to see fainter moons, so you'd be able to find more.
At the moment, if you use the same technique for Jupiter, you will be able to find fainter moons. The problem is: the amount of sky that moons of Jupiter can occupy is significantly larger than [the amount of sky that can be occupied by moons of] Saturn, so the method is even more time-consuming for Jupiter. And Jupiter is much brighter than Saturn and the other planets, so there's a lot of scattered light that makes it harder to see the moons.
So it's even harder to find satellites around Jupiter, and as you mentioned, other groups have already done this work for Uranus and Neptune. Does that mean we're sort of 'maxed out' on moons until we have better observations?
Yeah, you probably have to wait until better technology comes along.
Is there something being built or planned right now that could be that 'better technology'?
There currently are telescopes that can see deeper [than the CFHT], such as the James Webb Space Telescope (JWST). The problem is: JWST's field of view is very small, so you have to do quite a few observations to be able to cover the required area. But there is a telescope that's set to launch pretty soon, the Nancy Grace Roman Space Telescope, that has quite a large field of view. So that'll be a good telescope to use for hunting more moons.
What do we know about these new moons?
You basically can only get the moons' orbits and approximate sizes. But if you look at the distribution of the orbits, you can understand a bit more about the history of the system. Moons that are sort of clumped together in orbital space are most likely the result of a collision, so you can see what moons come from the same parent object.
Is seeing so many moons around Saturn unusual?
What's unusual is how many there are. It appears that the planets have more or less equal numbers of the larger moons. But when you get down to the smaller ones that we're discovering, Saturn seems to shoot up in terms of the numbers. So that's quite interesting. This could just be because there was a recent collision within the Saturnian system that produced a large number of fragments.
Do you get to name them all? Do you have to name them all?
I guess I don't have to. Some of these new moons, they've been linked back to observations by a different group from more than 10 years ago. That's maybe 20 to 30 of them. For the rest, we get full discovery credit, which, I think, means we get the right to name them. But they can't be named just yet; first, they're just given a number when they have a high-precision orbit, and I'm not sure how long that's going to take.
Do you have more moon-hunting observations to analyze?
No, I'm taking a little break from moons! I've got other projects to work on, relating to trans-Neptunian objects. They're quite far away. They're hard to see. There are some mysteries about them at the moment. It's interesting to understand their structure and how it relates to planet formation.