In the red corner, Jupiter, the largest planet orbiting our sun, which shaped our solar system with its gravitational bulk.
These two giant worlds are late in their bout for satellite-based supremacy. But now the fight over which planet has the most moons in its orbit has swung decisively in Saturn’s favor.
This month, the International Astronomical Union is set to recognize 62 additional moons of Saturn based on a batch of objects discovered by astronomers. The small objects will give Saturn 145 moons — eclipsing Jupiter’s total of 95.
“They both have many, many moons,” said Scott Sheppard, an astronomer from the Carnegie Institution for Science in Washington, D.C. But Saturn “appears to have significantly more,” he said, for reasons that are not entirely understood.
The newly discovered moons of Saturn are nothing like the bright object in Earth’s night sky. They are irregularly shaped, like potatoes, and no more than one or two miles across. They orbit far from the planet too, between six million and 18 million miles, compared with larger moons, like Titan, that mostly orbit within a million miles of Saturn. Yet these small irregular moons are fascinating in their own right. They are mostly clumped together in groups, and they may be remnants of larger moons that shattered while orbiting Saturn.
“These moons are pretty key to understanding some of the big questions about the solar system,” said Bonnie Buratti of NASA’s Jet Propulsion Laboratory in California and the deputy project scientist on the upcoming Europa Clipper mission to Jupiter. “They have the fingerprints of events that took place in the early solar system.”
The growing number of moons also highlights potential debates over what constitutes a moon.
“The simple definition of a moon is that it’s an object that orbits a planet,” Dr. Sheppard said. An object’s size, for the moment, doesn’t matter.
The new moons were discovered by two groups, one led by Dr. Sheppard and the other more recently by Edward Ashton of the Academia Sinica Institute of Astronomy and Astrophysics in Taiwan. Dr. Sheppard’s group, in the mid-2000s, used the Subaru Telescope in Hawaii to hunt for more moons around Saturn.
In March, Dr. Sheppard was also responsible for finding 12 new moons of Jupiter, which took it temporarily above Saturn in the scuffle to be the biggest hoarder of moons. That record was short-lived, it seems.
Dr. Ashton’s group, from 2019 to 2021, used the Canada France Hawaii Telescope, a neighbor of the Subaru Telescope on Mauna Kea, to look for more of Saturn’s moons and to verify some of Dr. Sheppard’s discoveries. For a moon to be authenticated, it must be spotted multiple times to “be sure the observations are a satellite and not just an asteroid that happens to be near the planet,” said Mike Alexandersen, who is responsible for officially confirming moons at the International Astronomical Union.
Most of Saturn’s irregular moons orbit the planet in what astronomers call the Inuit, Norse and Gallic groups. Each group’s objects may be the remains of larger moons, up to 150 miles across, that once orbited Saturn but were destroyed by impacts from asteroids or comets, or collisions between two moons. “It shows there’s a big collision history around these planets,” Dr. Sheppard said.
Those original moons may have been captured by Saturn “very early on in the solar system,” Dr. Ashton said, perhaps in the first few hundred million years after its formation 4.5 billion years ago. Not all orbit in these groups, however, with a few rogue moons orbiting in a retrograde direction — that is, opposite to the orbits of the other moons.
“We don’t know what’s happening with those retrograde moons,” Dr. Sheppard said. Dr. Ashton suspects they may be remnants of a more recent collision.
Learning more about the new moons is difficult owing to their small size and remote orbits. They appear to be a special class of object, different from asteroids that formed in the inner solar system and comets in the outer solar system. But not much more is known.
“These objects might be unique,” Dr. Sheppard said. “They might be the last remnants of what formed in the giant planet region, likely very icy-rich objects.”
NASA’s Cassini spacecraft managed to observe about two dozen of the moons around Saturn up to its demise in 2017. While not close enough to study in detail, the data did allow scientists to “determine the rotation period,” of some of the moons, the spin axis and “even the shape,” said Tilmann Denk from the German Aerospace Center in Berlin, who led the observations. Cassini also found abundant ice on the surface of one of the larger irregular moons, Phoebe.
Closer observations of Saturn’s tiny moons could give scientists a window into a tumultuous time in the early solar system. During that period, collisions were more common and the planets jostled for position, with Jupiter thought to have migrated from nearer the sun farther out to its current orbit. “That gives you additional information on the formation of the solar system,” Dr. Denk said.
Yet the irregular moons we are seeing so far may only be the beginning. “We estimated that there are potentially thousands,” around Saturn and Jupiter, Dr. Ashton said. Uranus and Neptune, too, may have many such irregular moons, but their vast distance from the sun makes them difficult to discover.
Saturn, despite being smaller than Jupiter, appears to have many more irregular moons. It may have three times as many as Jupiter, down to about two miles in size. The reason is unclear, Dr. Ashton said.
Jupiter’s original moons may have tended to be larger, and less likely to shatter. Or Saturn may have captured more objects into its orbit than Jupiter. Or Saturn’s moons may have been on orbits that were more likely to overlap and collide, producing smaller, irregular moons.
Whatever the reason, the outcome is clear. Jupiter is on the ropes, and it is unlikely to recover its title as the planet with the most moons. As astronomers’ capabilities to find smaller and smaller satellites improve, “Saturn will win by miles,” Dr. Alexandersen said. “I don’t think it’s a contest any more.”