The discovery of 20 tiny moons circling Saturn has knocked Jupiter out of the top spot in the moon race; the new additions bring Saturn’s total to 82 moons, while Jupiter has just 79.
Observers discovered the new moons using the Suburu telescope at the top of Mauna Kea in Hawaii and operated by the National Astronomical Observatory of Japan. All of the newly identified moons are only about three miles in diameter and are very far from the planet itself, according to a Carnegie Institution for Science press release. Scientists report that 17 moons have retrograde orbits, meaning they travel in the opposite direction of the planet’s rotation. Another three have prograde orbits, meaning they orbit in the same direction as the planet’s spin. Of those, two moons are closer to the planet, taking about two Earth-years to complete an orbit. Most of the other distant new moons take about three years to complete on orbit.
Two of the prograde moons are located in a cluster of previously discovered moons with 46 degree inclinations called the Inuit group, named after characters in Inuit mythology. Moons in the Inuit group are likely remnants of a much larger moon that broke into smaller pieces. The retrograde moons also have orbits similar to moons found in the Norse group; these are probably also fragments of a larger moon. One of those newly discovered prograde moons orbits the farthest from Saturn of any known natural satellite. That moon is believed to belong to the Gallic group—though it is possible it’s an oddball with a unique origin story.
Ian Sample at The Guardian reports that the researchers didn’t press their eye to the telescope lens to find the little planetoids. Instead, the team, led by Carnegie astronomer Scott Sheppard, used algorithms to examine images of Saturn taken between 2004 and 2007 captured by the Suburu scope. By comparing images over time, the team could identify which dots of light were stationary stars and galaxies versus which were moons orbiting Saturn.
While it’s cool to catalogue all the moons orbiting Saturn, finding these relatively tiny rock nuggets also helps researchers understand the formation of the planets in the solar system. In the release, Sheppard explains that these moons couldn’t have been created in Saturn’s youth, when a large rotating disc of gas and debris circled the planet. If the moons had formed during that time, the friction between the moons and debris would have sent them hurtling into Saturn.
“The fact that these newly discovered moons were able to continue orbiting Saturn after their parent moons broke apart indicates that these collisions occurred after the planet-formation process was mostly complete and the disks were no longer a factor,” says Sheppard.
Sheppard tells Michael Greshko at National Geographic that it’s likely we’ll get to see one of these moons close up in the near future. “One of the more exciting things about these outer moons is that there’s always missions going,” he says. “There’s so many of these moons now, there’s almost guaranteed to be one of these moons somewhere near where the spacecraft enters the Jupiter or Saturn environment.”
These are not the only moons in the solar system to avoid detection in the 21st century. In 2018, Sheppard and his colleagues announced the discovery of 12 new moons orbiting Jupiter. The team also ran a contest soliciting names for the new moons from the public. Sheppard tells Greshko that it’s likely we’ll find more moons when a new generation of telescopes go live in the first half of the 2020s. Current telescopes can only find moons larger than three miles in diameter orbiting Saturn and larger than one mile in diameter around Jupiter. The new scopes will be able to locate moons smaller than this.
But the biggest challenge now is giving Saturn’s new moons names. The team is currently taking suggestions from the public, but with a few stringent guidelines. You probably won't be able to name a moon after your cat, unless they're named from a character from Inuit, Norse or Gallic mythology.