Kyoto, Japan -- The two largest planets in our Solar System, Jupiter and Saturn, also have the largest satellite systems, or the most moons. At present, Jupiter's reported moon count stands at more than 100 moons, and along with its many rings Saturn has more than 280 reported moons. Not all these moons are equal, however. Jupiter's moon family has four large members, including the largest moon in the solar system, Ganymede, while Saturn's family is dominated by one large moon, Titan, the Solar System's second largest.
Since both planets are gas giants, the reasons for the differences in these satellite systems have long puzzled astronomers. Satellite formation theories have proposed some possibilities, but recent studies on stellar magnetic fields have hinted at the need to rethink these theories. There is also a long-running debate surrounding magnetic accretion and satellite formation: specifically, whether an inner cavity can be formed in Jupiter's circumplanetary disk, the accumulation of material orbiting a planet from which satellites may form.
A physically consistent model that can explain multiple systems, like the satellite systems of Jupiter and Saturn, may be applicable to other planetary and satellite systems beyond the Solar System. This motivated a collaborative team of researchers from institutions in Japan and China, including Kyoto University, to develop such a model.
"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 understand the thermal evolution of Jupiter and Saturn and how their magnetic fields have varied over time, the team performed numerical simulations on the interior structures of young gas giants. The team also numerically modeled the circumplanetary disks of both planets, and performed N-body simulations to follow satellite formation and orbital migration using the PC cluster at the Center for Computational Astrophysics, National Astronomical Observatory of Japan.
The results revealed that the difference between the large satellite systems around Jupiter and Saturn can be explained by their differing disk structures, originating from the strength of their magnetic fields. Specifically, Jupiter's strong magnetic field caused the formation of a magnetospheric cavity in the circumplanetary disk around the young gas giant, which likely captured the moons Io, Europa, and Ganymede. In contrast, the young Saturn's magnetic field was too weak to form a cavity so the migrating moons cannot survive in the disk.
This study provides a foundation for future observations of exomoons and circumplanetary disks around gas giants. The team's model predicts that gas giants the size of Jupiter or larger would evolve compact multi-moon systems, while one or two moons would form around Saturn-sized gas planets. Next, the team is interested in expanding their theory to other moons and potential exomoon systems.
