Jupiter and Saturn, the two largest planets in our Solar System, also host the most extensive systems of moons. Jupiter is currently known to have more than 100 moons, while Saturn, along with its prominent ring system, has more than 280.
Despite these large numbers, their moon systems are very different. Jupiter has four major moons, including Ganymede, the largest moon in the Solar System. Saturn, on the other hand, is dominated by a single standout moon, Titan, which ranks as the second largest.
Because both planets are gas giants, scientists have long tried to understand why their satellite systems developed so differently. Existing theories of moon formation offer some explanations, but recent research on stellar magnetic fields suggests those ideas may need revision. One key question involves magnetic accretion and whether an inner cavity can form in Jupiter’s circumplanetary disk, the accumulation of material orbiting a planet from which satellites may form.
Building a Unified Model
Researchers from Japan and China, including a team at Kyoto University, set out to create a model that could explain both systems using the same physical principles. Such a model could also help scientists understand moon formation in planetary systems beyond our own.
“Testing planet formation theory is somewhat difficult because we have only our Solar System for reference, but there are multiple satellite systems close to us whose detailed characteristics we can observe,” says first author Yuri I. Fujii.
To explore how Jupiter and Saturn evolved, the team ran numerical simulations of their internal structures during their early stages. This allowed them to track changes in temperature and magnetic field strength over time. They also modeled the circumplanetary disks surrounding each planet and carried out N-body simulations to study how moons formed and migrated. These calculations were performed using a PC cluster at the Center for Computational Astrophysics at the National Astronomical Observatory of Japan.
Magnetic Fields Shape Moon Systems
The simulations show that the contrasting moon systems of Jupiter and Saturn can be traced to differences in the structure of their surrounding disks, which are influenced by magnetic field strength.
Jupiter’s strong magnetic field likely created a magnetospheric cavity within its disk. This region may have helped trap and preserve major moons such as Io, Europa, and Ganymede.
In comparison, Saturn’s early magnetic field appears to have been too weak to produce a similar cavity, making it harder for migrating moons to survive within the disk.
These findings offer a framework for studying exomoons and circumplanetary disks around distant gas giants. According to the model, planets similar in size to Jupiter or larger are expected to form compact systems with multiple moons, while Saturn-sized planets may end up with only one or two. The researchers plan to extend this work to better understand other moon systems, including those beyond the Solar System.
Reference: “Different architecture of Jupiter and Saturn satellite systems from magnetospheric cavity formation” by Yuri I. Fujii, Masahiro Ogihara and Yasunori Hori, 2 April 2026, Nature Astronomy.
DOI: 10.1038/s41550-026-02820-x
Funding: Japan Society for the Promotion of Science, National Natural Science Foundation of China, Kyoto University
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1 Comment
I wonder if that’s why Earth has a large Moon while Venus doesn’t? Despite the fact that it appears both had a collision with something very large early in their formation.