Saturn still reigns supreme as moon king with 128 new moons

Sorry, Jupiter: Saturn has left its former rival in the dust with a new total of 274 moons, almost twice as many as all the other planets combined.

The International Astronomical Union recognized the discovery by astronomers from Taiwan, Canada, USA, and France on March 11.

The team used the Canada-France-Hawaii Telescope (CFHT) to repeatedly monitor the sky around Saturn between 2019 to 2021 in minute detail, combining multiple images to strengthen an astronomical object’s signal. This initial run yielded 62 moons — and an even larger number of other objects that, at that time, couldn’t be designated.

“With the knowledge that these were probably moons, and that there were likely even more waiting to be discovered, we revisited the same sky fields for three consecutive months in 2023,” said lead researcher Dr. Edward Ashton, postdoctoral fellow in the Institute for Astronomy and Astrophysics at Academia Sincia. “Sure enough, we found 128 new moons. Based on our projections, I don’t think Jupiter will ever catch up.”

Highly irregular, Captain

All of the 128 new moons are “irregular moons,” objects captured by their host planet early on in the history of the solar system. “These moons are a few kilometers in size and are likely all fragments of a smaller number of originally captured moons that were broken apart by violent collisions, either with other Saturnian moons or with passing comets,” said Dr. Brett Gladman, professor in the UBC Department of Physics and Astronomy.

A mystery within Saturn’s irregular moon system was a key motivator for the latest search: given the high number of small compared to large moons, there was likely a collision somewhere within the Saturn system within the last 100 million years — relatively recent in astronomical terms. Otherwise, says Dr. Gladman, any longer and these moons would have collided with each other and been blown into smithereens, which would preferentially reduce the ratio of small moons to bigger ones.

Indeed, most of the newly discovered moons are near the Mundilfari subgroup of Saturn’s moons which, given their size, number, and orbital concentration, is the likely the site of the collision. "Our carefully planned multi-year campaign has yielded a bonanza of new moons that tell us about the evolution of Saturn's irregular natural satellite population," Dr. Ashton said.

The team also includes Dr. Mike Alexandersen of the Harvard Smithsonian Centre for Astrophysics and Dr. Jean-Marc Petit of the Observatoire de Besancon.

No more moons

As for what’s next for the team, their moon-spotting days may be over, for now. “With current technology I don't think we can do much better than what has already been done for moons around Saturn, Uranus, and Neptune,” Dr. Ashton said.