Hover over Jupiter: Saturn adds 62 extra moons to its tally

In the red corner, Jupiter, the largest planet orbiting our sun, which formed our solar system with its gravitational mass.

In the blue corner, Saturn, the magnificent ringed world with bewildering hexagonal storms at the poles.

These two giant worlds are late in their battle 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 will recognize 62 more moons of Saturn based on a series of objects discovered by astronomers. The small objects will give Saturn 145 moons – eclipsing Jupiter’s total of 95.

“They both have a lot of moons,” said Scott Sheppard, an astronomer at the Carnegie Institution for Science in Washington, DC. But Saturn “appears to have considerably more,” he said, for reasons that aren’t entirely clear.

Saturn’s newly discovered moons look nothing like the bright object in Earth’s night sky. They are irregularly shaped, like potatoes, and no more than a mile or two in diameter. They also orbit far from the planet, between six million and 18 million miles, compared to larger moons, such as Titan, which usually orbit within a million miles of Saturn. Still, these small irregular moons are fascinating in their own right. They are usually clumped together in groups and may be the remnants of larger moons that shattered during their orbit around Saturn.

“These moons are quite important for 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 happened in the early solar system.”

The growing number of moons also points to possible debates over what constitutes a moon.

“The simple definition of a moon is that it is an object orbiting a planet,” said Dr. Sheppard. The size of an object does not matter at this point.

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. The group of Dr. Sheppard used the Subaru telescope in Hawaii in the mid-2000s to hunt for more moons around Saturn.

In March, Dr. Sheppard also responsible for the find 12 new moons of Jupiter, which temporarily brought it into the fray above Saturn to be the largest 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 search more of Saturn’s moons and discover some of Dr. Sheppard to verify. To authenticate a moon, it must be observed multiple times to “make sure the sightings are a satellite and not just an asteroid that happens to be near the planet,” said Mike Alexandersen, who is responsible for is for the official confirmation of moons on the International Astronomical Union.

Most of Saturn’s irregular moons orbit the planet in what astronomers call the Inuit, Norse, and Gallic groups. The objects of each group may be the remnants of larger moons, up to 150 miles across, that once orbited Saturn but were destroyed by asteroid or comet impacts, or collisions between two moons. “It shows that there is a history of major collisions around these planets,” said Dr. Sheppard.

Those original moons may have been captured by Saturn “very early in the solar system,” said Dr. Ashton, perhaps in the first few hundred million years after its formation 4.5 billion years ago. However, not all moons in these groups orbit in a retrograde direction, i.e. opposite to the orbits of the other moons.

“We don’t know what happens to those retrograde moons,” said Dr. Sheppard. Dr. Ashton suspects they are the remains of a more recent collision.

Learning more about the new moons is difficult because of their small size and remote orbits. They seem to be a special class of objects, different from asteroids that formed in the inner solar system and comets in the outer solar system. But much more is not known.

“These objects may be unique,” said Dr. Sheppard. “They could be the last remnants of what formed in the giant planet area, probably very icy objects.”

NASA’s Cassini spacecraft managed to observe about two dozen of the moons around Saturn until its demise in 2017. While not close enough to study in detail, the data allowed scientists to determine “the rotational period” of some moons, the spin axis and “even the shape,” said Tilmann Denk of the German Aerospace Center in Berlin, which led the observations. Cassini also found abundant ice on the surface of one of the larger irregular moonsPhoebe.

Closer observations of Saturn’s small moons could give scientists a glimpse into a tumultuous time in the early solar system. During that period, collisions became more frequent and the planets jostled around their position, with Jupiter thought to have migrated from closer to the sun and further away to its current orbit. “That gives you additional information about the formation of the solar system,” said Dr. Think.

But the irregular moons we see so far may be just the beginning. “We estimate there may be thousands,” said Dr Ashton around Saturn and Jupiter. Uranus and Neptune may also have many such irregular moons, but their immense distance from the sun makes them difficult to detect.

Saturn, despite being smaller than Jupiter, appears to have many more irregular moons. It may have three times as many as Jupiter, up to about two miles in size. The reason is unclear, Dr. Ashton said.

Jupiter’s original moons may have been larger and less likely to shatter. Or Saturn may have captured more objects in its orbit than Jupiter. Or Saturn’s moons may have been in orbits that were more likely to overlap and collide, creating smaller, irregular moons.

Whatever the reason, the outcome is clear. Jupiter is hanging on the ropes and is unlikely to regain its title of planet with the most moons. As astronomers’ abilities to find ever-smaller satellites improve, “Saturn will gain miles,” said Dr. Alexandersen. “I don’t think it’s a competition anymore.”