The formation of regular satellites

Most models of planet formation envisage that early protoplanets were encompassed by a dusty gaseous disc. A simulation of planet formation by the capture-theory model has shown the formation of such a disc and suggests that its mass was close to that of the protoplanet and that it was a few au in extent. This investigation proposes that satellites were formed in the way proposed for planet formation in the solar-nebula theory. First, dust in the disc settled into the mean plane, a process that was accelerated by accumulation of the dust into larger particles. The dust carpet was gravitationally unstable and broke up into regions that formed satellitesimals, solid bodies of from tens of metres to kilometre size. These finally collected together to form satellites. Numerical studies show that disc formation and the formation of satellitesimals took place on a very short time-scale, a few thousand years and a few years, respectively, but the final stage of satellite formation was much slower, taking up to 10(5) yr. Satellitesimals formed within the gaseous disc and, as they accumulated to form satellites, so their orbits steadily decayed. In the final stages, when the orbits of the fully formed satellites were decaying, coupling between the satellites led to commensurabilities in the orbits, as is observed for the inner three Galileans.