Abstract: Jupiter and Saturn are both gas giant planets, yet their satellite systems are strikingly different. This contrast between their satellite systems may have arisen from differences in the strength of the planets’ magnetic fields during their formation. A research group including Prof. Masahiro Ogihara of Tsung-Dao Lee Institute at Shanghai Jiao Tong University has proposed a new scenario to explain this difference through computer simulations. The results of this study were published online in Nature Astronomy on April 2, 2026.
Our Solar System contains two gas giant planets: Jupiter and Saturn. More than 100 moons are known around Jupiter, among which the four Galilean satellites—Io, Europa, Ganymede, and Callisto—stand out as particularly massive. Saturn, by contrast, is known to have more than 280 moons, but more than 95% of the total mass of its satellite system is contained in a single giant moon, Titan. In addition, while the Galilean satellites orbit close to Jupiter, Titan orbits at a somewhat greater distance from Saturn. Why did Jupiter and Saturn end up with such different satellite systems?
A key to this mystery lies in how the satellites formed. Large moons orbiting gas giant planets are thought to have formed within a gaseous disk surrounding the planet as the planet itself was forming. The properties of the resulting satellites are strongly influenced by the structure and composition of that disk. In turn, the disk structure is determined by planetary properties such as density, temperature, and mass. A planet’s magnetic field is also an important factor shaping the structure of the disk. However, few previous studies have examined the role of magnetic fields in explaining the differences between the satellite systems of Jupiter and Saturn.
In this study, the researchers first carried out computer simulations to investigate the internal structure of gas giant planets, and they calculated the magnetic field strength at the planetary surface. They found that although the magnetic field strength inside the planets differs by only a factor of a few, the field at the surface of Jupiter is estimated to be about 100 times stronger than that at the surface of Saturn.
How, then, does this difference in magnetic field affect the structure of the gas disk around the planets and the formation of satellites? To address this question, the team carried out a detailed analysis of the gas disk structure taking into account the basic physical processes operating in the disk, and then used the results in computer simulations of satellite formation and orbital migration. They found that around Jupiter, whose magnetic field is strong, “magnetospheric accretion” occurs, in which gas flows onto the planet along magnetic field lines. This creates a gas-poor cavity in the region close to the planet. Satellites forming in the gas disk gradually move inward through interactions with the surrounding gas, but their migration is halted at the edge of this cavity, allowing them to remain in stable orbits near the inner edge of the disk. In contrast, around Saturn, whose magnetic field is weak, magnetospheric accretion does not take place. As a result, satellites formed close to Saturn gradually spiral inward and are eventually lost to the planet. Only satellites that grow farther from Saturn can survive. In other words, the results suggest that the difference in magnetic field strength between Jupiter and Saturn may have produced the observed differences in their satellite systems.
This study is the first to provide a unified explanation for the differences between the satellite systems of Jupiter and Saturn using simulations that consistently combine models of giant-planet interiors with the key physical processes in the surrounding gas disk. These findings not only deepen our understanding of the diversity of satellite systems within the Solar System, but also offer important insights for future searches for moons orbiting exoplanets.
An artist’s impression of satellites forming within disks around gas giant planets. Around Jupiter (lower left), gas flows onto the planet along magnetic field lines. Around Saturn (upper right), gas accretes along the equatorial plane.
(Credit: Yuri I. Fujii/L-INSIGHT [Kyoto University], Illustrator: Shinichiro Kinoshita)
Comments:
Prof. Masahiro Ogihara of the Tsung-Dao Lee Institute, Shanghai Jiao Tong University, developed and carried out the computer simulations of satellite formation and evolution in this study, and also contributed to writing the paper with the other two co-authors. Masahiro Ogihara commented: “Jupiter’s Galilean satellites and Saturn’s Titan are among the best-known moons in the Solar System, yet how such different systems came to be has remained a longstanding mystery. In this study, we were able to present one possible answer to that question.” He added, “This work has implications not only for future surveys of moons around exoplanets, but also for understanding the broader circumplanetary environments that may have influenced the development of the ring systems of Jupiter and Saturn.”
Article Link: https://www.nature.com/articles/s41550-026-02820-x