While the four giant planets were forming, they were not doing it alone. As each of the giant protoplanets stole gas from the Solar Nebula, the material had swirled around the icy kernels to form gas discs like the Solar Nebula on a much smaller scale. Exactly as in the Solar Nebula itself, the particles in these discs had begun to lump together into larger building blocks – and new, independent worlds had started to appear in orbit around the planets. These would become the giant planets' satellite systems – their moons. Because these moons formed from discs, like the planets, they now tend to orbit their planetary hosts in a thin plane, each in the same direction as the others and in fairly circular paths. Moons with these orbital characteristics also tend to be large. They are known as regular satellites.

It is probable that the regular satellites grew to maturity very quickly, even before their planets did. Why? Simply a question of scale. The discs that surrounded the newly emerging giant planets were much smaller than the Solar Nebula, so they had correspondingly shorter orbital timescales. Their rich cargoes of icy volatiles grew to protoplanet dimensions much more quickly than the planets did. But not all of the moons formed at the same time. The Jovian disc, right on the snow line, would have been the richest. So Jupiter's regular satellites – Io, Europa, Ganymede and Callisto – no doubt formed first, alongside their planet, at T-plus 2–3 million years. These are known today as the Galilean moons, after their discoverer.