The two largest planets in our Solar System, Jupiter and Saturn, possess the most moons. Jupiter has over 100 moons, including four large members like Ganymede, the solar system's largest. Saturn has more than 280 moons, with its system dominated by Titan, the second largest.
The distinct differences in these satellite systems, despite both planets being gas giants, have been a subject of astronomical inquiry.
A collaborative team of researchers from institutions in Japan and China, including Kyoto University, developed a model to explain these differences. The team sought to create a physically consistent model applicable to various planetary and satellite systems.
Modeling Gas Giant Interiors and Disks
To understand the thermal evolution and magnetic field variations of Jupiter and Saturn over time, the team conducted numerical simulations of young gas giants' interior structures. They also modeled the circumplanetary disks of both planets and performed N-body simulations to track satellite formation and orbital migration.
Magnetic Fields: The Key to Distinct Satellite Systems
The study concluded that the differences in the large satellite systems around Jupiter and Saturn are explained by their distinct disk structures, which originate from the strength of their magnetic fields.
Jupiter's strong magnetic field caused the formation of a magnetospheric cavity in its circumplanetary disk, which likely captured moons such as Io, Europa, and Ganymede. In contrast, the young Saturn's magnetic field was too weak to form a cavity, preventing migrating moons from surviving in its disk.
Implications for Exomoon Discovery
This research provides a foundation for future observations of exomoons and circumplanetary disks around gas giants. The model predicts that gas giants the size of Jupiter or larger would evolve compact multi-moon systems, while Saturn-sized gas planets would form one or two moons. The team intends to expand their theory to other moons and potential exomoon systems.