🧲 A quirk about Jupiter's moons explained by... a magnetic cavity

Jupiter and Saturn, these two giant and gaseous planets, are similar in size but present a striking contrast: their moon families are very different. While Jupiter is surrounded by a multitude of satellites, including four large ones like Ganymede, Saturn sees its moon Titan largely dominating the others. Why such a divergence?

Japanese and Chinese teams, including researchers from Kyoto University, have developed a unified explanation. Their model relies on detailed numerical simulations of the internal structures of the planets in their youth, tracing the evolution of temperatures and magnetic intensity. These calculations, conducted on a computer cluster in Japan, also represented the circumplanetary disks, these reservoirs of matter where moons are born.


The results show that the strength of the magnetic field played a determining role. Jupiter, benefiting from a powerful field, likely formed a magnetic cavity in its disk. This zone could trap and preserve moons like Io, Europa, and Ganymede. Conversely, Saturn, endowed with a less intense initial field, did not generate such a cavity.

This model provides a new approach for examining exomoons, these satellites orbiting distant planets. It suggests that gas giants of size comparable to Jupiter or larger could develop compact systems with several moons, while those of Saturn's size would tend to host only one or two. Scientists plan to apply this work to other systems, including beyond our neighborhood.

The influence of planetary magnetic fields

The magnetic fields of planets, produced by the movements of liquid metals in their cores, act as invisible shields interacting with their environment. For gas giants like Jupiter, this field is very powerful, capable of repelling the solar wind and shaping the surrounding matter. This magnetic force can generate specific regions, like cavities, in the gas and dust disks that surround the young planet.

These magnetic cavities serve as refuge zones where particles can accumulate without being scattered. For the genesis of moons, this implies that blocks of matter can aggregate and grow more easily sheltered from disturbances. The model indicates that this protection was necessary to allow several large moons of Jupiter to maintain themselves and evolve.

In contrast, a weaker magnetic field, like that of Saturn in its early ages, fails to establish such a cavity. The materials of the circumplanetary disk are then more exposed to forces that push them to scatter. This reduces the possibilities for several large-sized moons to form and persist.

Astronomers can now search for comparable signatures in the disks observed around exoplanets, paving the way towards the identification of new extrasolar satellites.