Jupiter and Saturn are the two largest planets in the Solar System, and both are surrounded by enormous families of moons. Jupiter currently has more than 100 reported moons, while Saturn, which is also encircled by an extensive ring system, has more than 280.
The makeup of those moon systems is very different. Jupiter has four especially large moons, including Ganymede, the biggest moon in the Solar System. Saturn, by comparison, is dominated by Titan, the Solar System’s second largest moon.
A Longstanding Moon Formation Mystery
Because Jupiter and Saturn are both gas giants, astronomers have struggled to explain why their major moons developed so differently. Existing theories of satellite formation offer several possible answers, but recent research into stellar magnetic fields suggests that some of those ideas may need to be reconsidered.
One unresolved question involves magnetic accretion and the formation of satellites. Researchers have debated whether Jupiter’s circumplanetary disk could have developed an empty inner region. A circumplanetary disk is the rotating collection of material around a young planet from which moons can form.
A single physically consistent theory capable of explaining both Jupiter’s and Saturn’s moon systems could also help scientists understand planets and moons outside the Solar System. That possibility led researchers from institutions in Japan and China, including Kyoto University, to create a new 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.
Simulating Young Jupiter and Saturn
The researchers used numerical simulations to examine the internal structures and thermal evolution of Jupiter and Saturn when the planets were young. This allowed them to estimate how the planets’ magnetic fields may have changed over time.
They also modeled the circumplanetary disks surrounding both worlds. In addition, the team ran N-body simulations to track the formation of moons and the gradual movement of their orbits. The calculations were carried out using the PC cluster at the Center for Computational Astrophysics, National Astronomical Observatory of Japan.
Jupiter’s Magnetic Field Created a Safe Zone
The simulations indicate that the contrasting moon systems of Jupiter and Saturn may have emerged from differences in the structures of their circumplanetary disks. Those differences, in turn, appear to have been controlled by the strength of each planet’s magnetic field.
Young Jupiter had a powerful magnetic field that created a magnetospheric cavity inside its circumplanetary disk. This inner gap likely helped capture and preserve Io, Europa, and Ganymede as they moved through the disk.
Young Saturn’s magnetic field was not strong enough to create a similar cavity. Without that protected region, migrating moons could not survive inside Saturn’s disk.
Predicting Moon Systems Beyond the Solar System
The findings may help guide future observations of exomoons and the disks surrounding young gas giants. According to the model, planets as large as Jupiter or larger should tend to develop compact systems containing several moons.
Gas planets closer to Saturn’s size, however, may typically end up with only one or two moons.
The researchers now plan to apply their theory to additional moons and to possible exomoon systems around distant planets.