Jupiter and Saturn may seem similar as gas giants, yet their vastly 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 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 , 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.
