Astronomers say they have discovered more than 100 new moons around Saturn, possibly the result of cosmic smashups that left debris in the planet’s orbit as recently as 100 million years ago.
The gas giant planets of our solar system have many moons, which are defined as objects that orbit around planets or other bodies that are not stars. Jupiter has 95 known moons, Uranus 28, and Neptune 16. The 128 in the latest haul around Saturn bring its total to 274.
“It’s the largest batch of new moons,” said Mike Alexandersen at the Harvard-Smithsonian Center for Astrophysics, an author of a paper announcing the discovery that will be published in the days ahead in Research Notes of the American Astronomical Society.
Many of these moons are rocks only a few miles across — small compared with our moon, which is 2,159 miles across. But as long as they have trackable orbits around their parent body, the scientists who catalog objects in the solar system consider them to be moons. That is the responsibility of the International Astronomical Union, which ratified the 128 new moons of Saturn on Tuesday.
The forthcoming paper’s lead author, Edward Ashton of the Academia Sinica Institute of Astronomy and Astrophysics in Taiwan, will have naming rights for the objects.
“Whoever discovers them gets the right to name them,” said Dr. Alexandersen, who works with the International Astronomical Union to confirm the existence of objects in the solar system. The current naming scheme for moons on Saturn is based on characters from Norse and other mythology.
“Maybe at some point they’ll have to expand the naming scheme further,” Dr. Alexandersen said.
The moons were discovered in 2023 using the Canada France Hawaii Telescope at Mauna Kea in Hawaii. Dr. Ashton and his colleagues observed patches of space near Saturn, and over time this allowed them to track the motion of previously unknown moons.
“You need to be able to prove that the object is in orbit around the planet,” said Dr. Ashton, who was also responsible for finding 62 new moons of Saturn two years ago.
All the moons are irregular, meaning they are small, orbit at a highly angled slope relative to Saturn’s equator, and often travel around the planet backward relative to the other major moons. Not much else can be gleaned about them because they are just faint dots of light in telescopic views. But they extend from about 6.5 million to nearly 18 million miles from the planet. For comparison, the planet’s rings extend to just 175,000 miles, and its major moons — including Titan and Enceladus — are up to two million miles away.
The existence of so many moons around Saturn hints at multiple dramatic collisions in space. Dr. Ashton and his team believe that the irregular moons were captured by Saturn at some point in its history. Some may be fragments of large objects that collided elsewhere in the solar system, while others may be further fragments of collisions between moons up to tens of miles in size that crashed together in Saturn’s orbit.
The team has grouped many of the moons, identifying potential families that may have come from the same collisions. “You’re trying to conclude what the great-great-grandparents were like, five generations later,” said Brett Gladman, an author of the paper at the University of British Columbia.
A particularly interesting subgroup is named Mundilfari, after a deity of Norse mythology, and includes 47 of the 128 new moons. The team thinks this subgroup might be the result of a collision within Saturn’s orbit as recent as 100 million years ago, which was not so long ago on cosmic time scales.
The group’s age could be a window into chaotic activity in the outer solar system, which has typically been assumed to be more calm in the past 100 million years.
“This is implying we could be having collisional events, and we’re seeing the shrapnel in the population of tiny moons,” said Michele Bannister, an astronomer at the University of Canterbury in New Zealand, who was not involved in the paper.
Learning more about these moons is difficult considering their small size, but astronomers might be able to study them with the James Webb Space Telescope, said Heidi Hammel, an astronomer at the Association of Universities for Research in Astronomy.
There may be even more moons around Saturn awaiting discovery, potentially in the thousands, Dr. Ashton said.
But he may leave those discoveries to others.
“I’m a bit mooned out at the moment,” he said.