Miniature solar systems?

Galileo considered that the satellites of Jupiter formed a miniature solar system. Indeed their rather uniform size and equatorial orbits, coupled with a regular decrease in density from Io to Callisto, encouraged subsequent views that a study of the Galilean system would provide insights into the formation of the solar system, serving as a scale model. It seems reasonable to have expected that the satellite systems of the giant planets might exhibit some systematic regularities as a sort of byproduct of planetary formation. Furthermore, while we have only one solar system, three of the giant planets possess regular satellite systems that mimic miniature solar systems, while Neptune has the ruins of one lying nearby, within about five radii of that planet.

Accordingly, the satellite systems of the giant planets might be expected to provide some general insights into the origins of planetary systems. The most interesting observation concerning them, however, is that they are all different and “the four giant planets exhibit a startling diversity of satellite systems” [1] (Fig. 9.1). The satellites of Saturn and Uranus differ from those of Jupiter in many ways (Neptune presents a special case).

One could ignore the satellites in significantly inclined, eccentric, or retrograde orbits as probable captured objects and hence not necessarily related to the planet to which they have become attached [2].