Jupiter and Saturn are the two largest planets in our solar system, and both are surrounded by huge groups of moons. Currently, Jupiter has more than 100 reported moons, while Saturn, which is also surrounded by a vast ring system, has more than 280 moons.
The compositions of these lunar systems vary widely. Jupiter has four particularly large moons, including Ganymede, the solar system’s largest moon. By comparison, Saturn is dominated by Titan, the second largest moon in the solar system.
The long-standing mystery of moon formation
Because Jupiter and Saturn are both gas giants, astronomers have struggled to explain why their major moons developed so differently. Existing theories about satellite formation offer several possible answers, but recent research on stellar magnetic fields suggests that some of those ideas may need to be reconsidered.
One unresolved question concerns magnetic accretion and satellite formation. Researchers have debated whether Jupiter’s circumplanetary disk may have developed an empty interior region. A circumplanetary disk is a collection of material that rotates around a young planet where moons form.
A single physically consistent theory that can explain both Jupiter’s and Saturn’s moon systems could also help scientists understand planets and moons outside our solar system. Considering that possibility, researchers from institutions in Japan and China, including Kyoto University, have created a new model.
“Testing theories of planet formation is somewhat difficult because we only have the solar system to refer to, but there are several satellite systems near us that allow us to observe detailed characteristics,” says lead author Yuri I. Fujii.
Simulation of young Jupiter and Saturn
The researchers used numerical simulations to examine the internal structure and thermal evolution of the planets Jupiter and Saturn when they were young. This allowed them to estimate how the planet’s magnetic field changed over time.
They also modeled the circumplanetary disk surrounding both worlds. Additionally, the team ran N-body simulations to track the formation of the satellite and the gradual movement of its orbit. The calculations were performed using a PC cluster at the National Astronomical Observatory of Japan’s Center for Computational Astrophysics.
Jupiter’s magnetic field created a safe zone
Simulations show that the contrasting satellite systems of Jupiter and Saturn may have emerged from differences in the structure of their circumplanetary disks. These differences appear to be controlled by the strength of each planet’s magnetic field.
Young Jupiter had a strong magnetic field, forming a magnetospheric cavity inside the disk around it. This internal gap may have helped capture and preserve Io, Europa, and Ganymede as they moved through the disk.
Young Saturn’s magnetic field was not strong enough to form a similar cavity. Without that protected area, moving satellites cannot survive within Saturn’s disk.
Predicting the lunar system beyond the solar system
The discovery could help researchers observe the disks surrounding future exomoons and young gas giant planets. According to this model, planets as large as Jupiter or larger should tend to develop compact systems containing multiple moons.
However, gas giants close to the size of Saturn typically may only have one or two moons.
The researchers now plan to apply their theory to additional moons and possibly exosatellite systems around distant planets.
