Jupiter and Saturn are the two largest planets in our solar system, and both are surrounded by dozens—even hundreds—of moons.
But their moon systems look very different. Jupiter has four large, well-known moons, including Ganymede, the biggest moon in the entire solar system.
Saturn, on the other hand, is dominated by just one giant moon, Titan, while most of its other moons are much smaller.
This difference has puzzled scientists for years.
A new study published in Nature Astronomy offers a fresh explanation. It suggests that the key factor shaping these moon systems may be something invisible but powerful: the planets’ magnetic fields during their early formation.
Scientists have long believed that moons form from disks of gas and dust that surround young planets, similar to how planets form around stars.
These disks, called circumplanetary disks, provide the material that eventually gathers into moons. However, existing theories have struggled to explain why Jupiter ended up with several large moons while Saturn did not.
To investigate this mystery, a team of researchers from Japan and China created detailed computer simulations of how Jupiter and Saturn evolved when they were young.
They modeled the planets’ internal structures, how heat moved inside them, and how their magnetic fields developed over time.
They also simulated the disks of material around the planets and tracked how moons formed and moved within those disks.
The results revealed a key difference between the two planets. Jupiter had a much stronger magnetic field early in its history.
This strong magnetic field created a kind of gap, or “cavity,” in the disk of gas and dust surrounding the planet.
This cavity acted like a safe zone where growing moons could avoid spiraling into Jupiter and being destroyed. As a result, several large moons—such as Io, Europa, and Ganymede—were able to survive and remain in stable orbits.
Saturn, in contrast, had a weaker magnetic field when it was forming. Without a strong magnetic cavity, its growing moons were more likely to drift inward and be lost to the planet. This made it much harder for multiple large moons to survive. Instead, Saturn ended up with one dominant large moon, Titan, and many smaller ones.
This new model helps explain not only the differences between Jupiter and Saturn, but also how moon systems might form around other planets beyond our solar system. Scientists believe that gas giants similar in size to Jupiter are more likely to develop multiple large moons, while Saturn-sized planets may only form one or two.
The findings also open the door to studying “exomoons,” or moons orbiting planets outside our solar system. As technology improves, astronomers hope to observe these distant systems directly and test whether the same magnetic processes are at work.
In the end, the study suggests that a planet’s magnetic field—something we cannot see—may play a crucial role in shaping the worlds that orbit it.
