Simulations by researchers from Japan and China suggest that Jupiter's strong magnetic field created a void in the disk around it and preserved large moons, while at Saturn a weaker field led to a more sparse system dominated by Titan.
![Artist's impression of the simulations performed in this study. Jupiter on the left, Saturn on the right. Credit: Yuri I. Fujii/L-INSIGHT [Kyoto University], Illustrator: Shinichiro Kinoshita](https://www.hayadan.org.il/images/content3/2026/04/image-15.jpeg.webp)
Credit: Yuri I. Fujii/L-INSIGHT [Kyoto University], Illustrator: Shinichiro Kinoshita
Jupiter and Saturn may look similar as gas giants, but their very different moon systems reveal a deeper story shaped by magnetic forces and planetary evolution.
Jupiter and Saturn, the two largest planets in our solar system, also host the most extensive moon systems. Jupiter currently has more than a hundred known moons, and Saturn, with its prominent ring system, has more than 280.
Despite their large numbers, their moon systems are very different. Jupiter has four large moons, including Ganymede, the largest moon in the solar system. Saturn, on the other hand, has one prominent moon, Titan, which is the second largest.
Because both planets are gas giants, scientists have long struggled to understand why their moon systems evolved so differently. Existing theories of moon formation offer several explanations, but new research on stellar magnetic fields suggests that these ideas need to be reconsidered. One important question involves magnetic repulsion and whether an inner cavity can form in a disk around a planet, the accumulation of material that orbits a planet and from which moons can form.
Building a unified model
Researchers from Japan and China set out to create a model that could explain both systems using the same physical principles. Such a model could also help scientists understand the formation of moons in planetary systems beyond our own.
"It's difficult to test theories of planet formation because we only have our own solar system to refer to, but there are several satellite systems close to us that we can observe in detail," says first author Yuri Fujii.
To study how Jupiter and Saturn evolved, the team ran numerical simulations of their internal structures in their early stages. This allowed them to track changes in temperature and genetic field strength over time. They also created models of the disks around the planets and performed N-body simulations to study how moons formed and migrated.
Magnetic fields shape lunar systems
The simulations show that the contrasting moon systems of Jupiter and Saturn can be attributed to differences in the structure of the disks around them, which are affected by the strength of the magnetic fields.
Jupiter's magnetic field likely created a magnetospheric cavity within its disk. This region may have helped capture and preserve large moons such as Io, Europa, and Ganymede.
In contrast, Saturn's early magnetic field appears to have been too weak to create a similar void, so it was more difficult for wandering moons to survive within the disk.
According to the model, planets at least the size of Jupiter are expected to form compact systems with many moons, while planets the size of Saturn will eventually have only one or two moons.
for the scientific article DOI: 10.1038 / s41550-026-02820-x
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