A new study explains why Jupiter has multiple large moons while Saturn has only one. Researchers suggest Jupiter’s strong magnetic field created regions in its early disk where moons could form, while Saturn’s weaker field did not.
Artist's impression of the simulations conducted in this research.
Photo Credit: NASA
The two largest planets in the solar system, Jupiter and Saturn, have massive belts of moons. However, there is a notable contrast between them: Jupiter has four big moons, Io, Europa, Ganymede, and Callisto, and Saturn, with all the moons of Saturn totaling more than 280, has only one big moon, Titan. What is the reason behind this contrast? Finally, in April 2026, researchers at Kyoto University published a new study in Nature Astronomy that provides a convincing answer.
According to the research, the reason behind this is the existence of magnetic fields around these two planets. In the beginning, when these two planets were formed, they both had a ring of gas and dust around them; in other words, circumplanetary disks. The strong magnetic field of Jupiter cut off a part of this ring, forming an isolated area where the moons could be formed. However, in the case of Saturn, no such area could be created because of its weak magnetic field, so migrating moons could not be formed around it.
These findings extend beyond Jupiter and Saturn. The model of the research team predicts that any gas giant that is as large as Jupiter or larger would have the formation of a similarly compact, multi-moon system, and smaller Saturn-sized planets would have only one or two major moons. This has important implications for the discovery of exomoons, which are moons around planets in other star systems. Our solar system satellite systems, as stated by the author of the paper, Yuri Fujii, are unique, observable test beds of the theory of planetary formation that would otherwise be extremely difficult to test anywhere in the universe.
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