A longstanding moon formation mystery
Jupiter and Saturn are the two largest planets in the Solar System, both surrounded by enormous families of moons. But the makeup of those systems is very different. Jupiter has four especially large moons — Io, Europa, Ganymede, and Callisto — while Saturn is dominated by Titan, with no other giant moons.
Because Jupiter and Saturn are both gas giants, astronomers have struggled to explain why their major moons developed so differently. A new study from Kyoto University, published in _Nature Astronomy_, offers a compelling answer: magnetic fields.
Simulating young Jupiter and Saturn
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 — the rotating material from which moons form.
Jupiter's magnetic field created a safe zone
The simulations indicate that the contrasting moon systems emerged from differences in the structures of their circumplanetary disks, 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 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 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 may typically end up with only one or two moons.
"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," said first author Yuri I. Fujii.
The researchers now plan to apply their theory to additional moons and to possible exomoon systems around distant planets.
