If you are asked how many moons Saturn has by someone, the correct answer would be "a lot".
An alternative answer that is more accurate would be "at least 82."
The best answer is probably "150 plus/minus 30" if we're talking about larger than 2.8 km in diameter, and not counting irregular ones.
Yes, it's quite precise. That last bit is due to new research that suggests that this number could be extrapolated from information about the population on Saturn's moons.
It is often asked how many moons a planet contains. This is especially true when there are a few new ones discovered around one planet. For example, astronomers found a dozen around Jupiter and 20 around Saturn. It also simplifies the situation: Saturn's rings, made up of many small pieces of water ice, have trillions of moons. What size object would you consider a moon?
It ignores the fact that these moons are known. This is the problem with these planets: It's difficult to find those moons. They are small and faint. They are located next to brighter planets. The planets move against the background stars night to night. (The word planet is Greek for "wanderer"), and the moons orbit the planet. This is difficult.
Cassini mosaic showing Saturn's strange moon Phoebe. It is a 220km wide sphere, heavily battered and orbits Saturn retrograde (roughly) relative to Saturn’s spin. It is likely to be a captured object from outside the solar system. Credit: NASA/ESA/JPL/SSI
It's difficult to determine if the moving blip that you see in your observation is something nobody's seen before, or if it's a new discovery. Many of Saturn's moons that are faint were first observed some time ago, but their orbits weren’t sufficiently accurate to determine their precise positions in the future.
Both problems were solved by astronomers who used the Canada-France Telescope (a 3.6-meter telescope in Hawaii) to search deeper for fainter moons. They observed large areas of sky to the east and west of Saturn for faint moons over the course of the two nights.
Not just irregular moons, but all faint moons. Although it is difficult to define, regular moons orbit in the same way Saturn spins (called retrograde) and have an orbit tilted less that a few degrees from Saturn's equator. Major moons are either regular satellites or formed when large regular satellites were hit by impacts. The resulting debris then coalesced to form smaller moons.
They orbit in a retrograde orbit, meaning they orbit backwards. These are most likely captured planetesimals (larger chunks of material formed from the disk of material around the Sun when the planets were forming). However, it is not clear how they were captured. They cannot just fall to Saturn's orbit; they must have some way to slow down. For example, they may have been trapped in Saturn's gravity and slowed down by the material surrounding the planet while it was still being formed. If the planetesimal were binary, one could orbit Saturn and the other could collide with Saturn; the second takes away enough energy to allow the first one to be captured.
This is another reason astronomers searched for these moons. The more we find them, the better our understanding of them.
S A T U N. Credit: NASA/JPL/Space Science Institute/ GordanUgarkovic Photograph: NASA/JPL/Space Science Institute/ GordanUgarkovic
To minimize the smearing caused by a moon's orbital movement, they took many short exposures and then used some clever techniques to search for moons. The first image was used as a reference. They then adjusted the images to account for the tilted retrograde moon's movement between exposures. The stars will become blurred or streaked but the moons will be brighter when such an approach is used. The team then searched for possible moons by eye, literally having 2 astronomers inspect each image visually. This is a fun and tedious process, but it's really enjoyable once you get used to it.
Then they started over, shifting the images with a different value of orbital motion, tilt, and then looking again. And again. And again.
They found 120 moving objects that matched their criteria, with 74 being very firm detections. These were then compared against known moons. They found 34 of the 42 they expected to see. They didn't see eight because they were too close to the edge of the image or in images with poor observing conditions. They are confident that they would have seen all the moons that they could.
Several dozen more were still found. These moon candidates are not yet confirmed, but they have been considered. They believe there are 150 irregular moons (from the number they found to how faintly they could see) that range in size from 2.8-km to +-30. This is three times more than Jupiter.
It's cool to discover them, but scientists want more. Scientists want to understand why and how they are doing it, as well as what it means for the understanding of Saturn's system.
Phoebe, which is approximately 200 km across and 8 million kilometers from Saturn, is Saturn's largest irregular Moon*. They found other moons that have similar orbits to Phoebe's and are therefore likely remnants from past collisions with this moon.
However, many of them had orbits that were not unlike Phoebe's so it is unlikely they could be related to it. Astronomers believe they formed after a larger retrograde moon collided with a large amount of debris, which in turn created these moons. They believe this collision occurred within the last few hundred million years, based on their size distribution. Over time, more collisions reduce the moons to smaller sizes. This is quite recent! Keep in mind that the solar system is only 4.6 billion years old.
Over time, small impacts on Saturn's Moon Phoebe have created a dense but fragile ring of dust around Saturn. This image shows its size and extent with Saturn as a dot (left) and a small portion of it seen by the Spitzer Space Telescope, infrared. Credit: NASA/JPL-Caltech/Univ. Credit: NASA/JPL-Caltech/Univ.
There's more. There is more. It's too large: Phoebe's impact alone cannot explain how some particles got so far away from Phoebe. New research suggests that there may be many more moons orbiting in a similar way to Phoebe's, but further out than Saturn. Impacts on these moons could also contribute to the material found in the ring.
All of this proves that Saturn's moon system can be complex and there are many moons, depending on which moon you refer to. Since there isn’t one, I don’t worry about the exact number. I am more interested in how many are above a certain size, how they formed, how they orbit Saturn, and what they have done to contribute to the strange and wild environment around Saturn.
This is far more important than any single number that changes every time we look at Saturn. Or any other planet.
Do not sweat the small details. Saturn is incredible.
*Assuming that it isn't an alien vehicle loaded up with protomolecule.