Free Astronomy Magazine July-August 2026

7 T he Solar System’s two largest gas giant planets, Jupiter and Saturn, have extensive but very different families of moons orbiting them. New sim- ulations conducted on the PC cluster at the Center for Computational Astrophysics (CfCA), National Astronomical Observa- tory of Japan (NAOJ) showed that the planet’s magnetic field plays a role in cre- ating an environment where the new moons can survive and grow, thus shaping the evolution of the system. Jupiter has more than 100 reported moons, including four large ones (Ganymede, Cal- listo, Io, and Europa). Saturn has more than 280 reported moons, but only one large one (Titan). So it is a puzzle why Saturn managed to cultivate more moons, but fewer large moons than Jupiter. A team led by Kyoto University, including researchers from institutes in Japan and China, used the PC cluster at CfCA, NAOJ, to simulate the formation of the moon sys- tems around Jupiter and Saturn. This sim- ulation recreated the planets’ internal structure to calculate the thermal evolu- tion of Jupiter and Saturn and how their magnetic fields have varied over time. Moons form from material in a “circum- planetary disk” of gas and dust orbiting the young planet. The disk nurtures the young moons, but interactions with the disk may cause them to fall into the planet. The simulations showed that young Jupiter generated a strong planetary magnetic field that created a safe “cavity” around the planet where its young large moons were prevented from migrating too close to their host planet. Young Saturn lacked a strong magnetic field, so only one large moon managed to survive. “Testing planet formation theory is some- what 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 Yuri I. Fujii, primary author of the re- port announcing these findings. Next, the team is interested in expanding their the- ory to other moons and potential exo- moon systems. ! A rtist’s impression of the simulations conducted in this re- search. Jupiter (lower left) has a strong mag- netic field which cre- ates a cavity in its circumplanetary disk. Saturn (upper right) lacks a strong magnetic field so its circumplan- etary disk evolves without a cavity. [Yuri I. Fujii/L-INSIGHT (Kyoto University), Shinichiro Kinoshita]