Jupiter Reclaims Title of Planet With the Most Moons

Astronomers discovered 12 additional moons of Jupiter, raising its total number of known natural satellites to 92. The gas giant now holds the record for the most known moons of any planet in the solar system—for the time being, at least.

Saturn is the current runner-up with 83 known moons, and prior to this discovery, it had been the reigning leader for moon count since 2019. But if scientists could detect all moons at least three kilometers wide, “Saturn would have more moons than all the rest of the solar system,” Brett Gladman, an astronomer at the University of British Columbia in Canada who was not involved in the recent discoveries, says to Sky & Telescope’s Jeff Hecht.

Sometimes called the “king of planets,” Jupiter is more than twice as massive as the rest of planets combined. While people have known of Jupiter’s existence for at least a couple thousand years, the first detailed observations of the fifth-closest planet to the sun are credited to Galileo Galilei in 1610. He also discovered four of Jupiter’s moons at this time.

Over the next 400-odd years, further observations increased that tally to 80 known moons. Then, astronomers spotted these 12 additional candidates using telescopes in Hawaii and Chile in 2021 and 2022.

“We have been surveying for new moons around Jupiter serendipitously while our main survey is looking for planets in the outer solar system beyond Pluto,” Scott Sheppard, an astronomer at the Carnegie Institute for Science who is part of the group that found the new moons, tells Gizmodo’s Kevin Hurler via email.

While the original observations took place some time ago, astronomers needed to track each object’s entire orbit to confirm it was a moon, per Sky & Telescope. The new moons orbit Jupiter from a distance—all 12 take at least 340 days to orbit the planet, and nine take more than 550 days—so this proved to be a slow process. Follow-up observations confirmed their status as moons, clearing the way for them to be officially recognized by the Minor Planet Center, hosted by the Center for Astrophysics, Harvard & Smithsonian, between late December and late January.

“We want to have a detailed map of everything that is out there, so we know what to expect and can plan missions accordingly,” Mike Alexandersen, an astrophysicist at the Minor Planet Center who worked on publishing the discoveries, tells Smithsonian magazine in an email.

The new moons are all much smaller than Earth’s—they range from 0.6 to 2 miles in diameter. Jupiter and Saturn are thought to have many small moons because once-larger moons collided with other space objects and broke into smaller satellites. Astronomers expect to find more moons around both planets.

Scientists hope to learn more about Jupiter and its moons in the coming years. This spring, the European Space Agency plans to launch a spacecraft to observe Jupiter’s icy moons. And NASA is targeting an October 2024 launch of its Europa Clipper spacecraft, which will orbit Jupiter and closely study its moon Europa to learn whether it has conditions that could support life.

“I hope we can image one of these outer moons close-up in the near future to better determine their origins,” Sheppard told Marcia Dunn of the Associated Press via email.

Having a better understanding of how many moons orbit Jupiter can help protect spacecraft and increase the potential of upcoming missions, Alexandersen says.

“By mapping out the whole Jovian moon system and knowing of as many of the moons as possible, next time there is a mission to Jupiter, that mission can be planned to have close flybys (and thus cool photos) of as many of those small moons as possible,” he says.

Will Sullivan | | READ MORE

Will Sullivan is a science writer based in Washington, D.C. His work has appeared in Inside Science and NOVA Next.