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We already know a decent amount about how planets form, but moon formation is another process entirely, and one we're not as familiar with. Scientists think they understand how the most important moon in our solar system (our own) formed, but its violent birth is not the norm, and can't explain larger moon systems like the Galilean moons around Jupiter. A new book chapter, also released as a preprint on arXiv, by Yuhito Shibaike and Yann Alibert from the University of Bern, discusses the differing ideas surrounding the formation of large moon systems, especially the Galileans, and how we might someday be able to differentiate them.

The Galilean moons form what is known as the circum-Jovian disk (CJD), an analog of the circum-stellar disk (CSD) that surrounds the sun, but instead has Jupiter at its center. The other 93+ non-Galilean moons around Jupiter also define the CJD, but their creation might be different due to the size differentials.

According to the paper, there are three main differences between the formation of planets and the formation of moons. Moon formation happens on a much faster time scale—around 10–100 times faster than planet formation. The system itself is also always gaining additional material from the CSD and losing it to whatever is at the center of the disk, which in the CJD's case is Jupiter.

And finally, there aren't nearly as many examples of systems with multiple large moons as there are planetary systems, at least since the discovery of exoplanets 30 years ago. Jupiter and Saturn remain our only examples of large moon systems, and it will be awhile before any multi-exo-moon system will be found.

Fraser discusses the formation of our own moon, which was dramatically different than that of the Galileans.

So what can we tell about the formation of these moon systems from the two we know about? The paper breaks the process down into a three-step process. First is the formation of the CJD, which includes gas and dust as well as moons. This was originally supported by a "minimum mass model" developed in the 1980s that assumed the disk was static and contained approximately the overall mass of the Galilean moons. In 2002, a new theory was developed that modeled the CJD as a "gas-starved disk" where the original CJD was relatively material poor but had plenty of additional material added to it by gravitational capture from the CSD.

That gravitational capture is believed to have played a key role in the formation of the Galilean moons and marks the second phase of their creation. However, Jupiter is a planet, and one of the requirements of a planet is that it clears its orbital path. Since Jupiter is the largest planet, it does so very effectively, which includes what astronomers consider "pebbles" (but on Earth could be considered a decent-sized boulder a few meters across).

One way for moons to accrete given this paucity of small material is by using even smaller material—small dust particles can make their way into the CJD without being disrupted by Jupiter, though there's some debate about how effective this process is. Another method would be "planetesimal capture" where Jupiter's gravity well catches the core of what would have ended up being a planet, but then ends up simply being one of the giant planet's moons. They could have been gravitationally disturbed by Saturn, and then slowed in their orbit by running through the gas cloud surrounding early Jupiter that made up the CJD.

Fraser discusses the missions that will explore Jupiter's moons in more detail.

There are some differences in the Galilean moons themselves that can be used to prove or disprove these different formation theories. For example, Callisto isn't in resonance with Jupiter at all, unlike the rest of its Galilean brethren.

One potential theory for that is that Jupiter's fourth moon was formed under different conditions, or maybe was hit by its own impactor that knocked it from its natural course. Callisto is again an outlier as it's only partially "differentiated" (meaning it has a separate core, mantle, and outer shell), unlike its three compatriots. Some pebble accretion models think that Callisto is still early on in its formation journey and will eventually begin to look more like its peers.

But ultimately those questions, and many more about the formation of large moon systems, will be hard to answer without more data. The Jupiter Icy Moon Explorer (JUICE) mission will help shed some light on those questions, but even then it's still only one, or at most two, data sets that we have available.

Until exoplanet hunting telescopes become powerful enough to start finding exomoons as often as they currently find planets, many of these formation theories will remain untested. That data will eventually come along someday, and when it does it will help us understand some important parts of our own solar system better.