Astropedia Textbook

The theory of the origin of the Solar System can also explain the origins of the planets' satellite systems. Satellites as a group appear to be a chaotic mix of different kinds of planetary bodies, with no pattern: Giant planets have families of diverse moons that are all small compared to the planet; some of the moons move in prograde directions, some retrograde. Other planets have no moons.

The most important process that formed satellites was similar to the accretion process that formed the planets themselves. Each of the four giant planets attracted enough material to form its own miniature "solar nebula." In this disk-shaped cloud of gas and dust, the accretion growth process repeated on a smaller scale. Each giant planet became an analog of the Sun, and satellites grew around it, analogous to planets. The most abundant building materials were ice and the black carbonaceous dirt common to the outer Solar System, so giant planets developed systems of dirty-ice satellites. Just like the Solar System on a smaller scale, these satellites ended up in prograde, circular orbits, with the plane of the satellites' orbits aligned with the equatorial plane of the planet.

Retrograde orbit

Planets also gained satellites by capturing them. Some planetesimals went into irregular orbits due to close encounters with giant planets, and a few of them were captured into orbits around the planets. When a planetesimal approaches a planet, it has a 50/50 chance of passing on either the "prograde side" or the "retrograde side,” so orbits of captured objects can be either prograde or retrograde. If it passes close enough to be slowed by the extended primordial atmosphere of the planet, or if it suffers a collision that reduces its relative velocity to the planet, then it has a chance of being captured into an orbit around the planet. The change in velocity needed to capture an approaching planetesimal is lowest at large distances from the planet, which may explain why the outermost satellites of some planets appear to be captured objects. For instance, eight of Jupiter's outermost moons are black, carbonaceous objects, similar to asteroids and comet nuclei in the region. Half of the eight are in prograde orbits, and half are in retrograde orbits. This is exactly what we would expect if these satellites were captured planetesimals.

The Pluto system, including all three satellites, Charon, Nix, and Hydra.

An artist's depiction of a collision between two planetary bodies. Such an impact between the Earth and a Mars-sized object likely formed the Moon and the large objects that hit the Earth around this time were part of a random process.

A third process of satellite formation is the accretion of debris ejected by a large impact. This is the favored theory of the formation of the Earth's Moon. If a large planetesimal hit a planet late in the planet-forming process, it could partially disrupt the planet and blow material into orbit. This debris would form a large satellite in orbit around the planet. Pluto's satellite Charon is also large relative to Pluto itself. It may have formed as the result of a similar large impact, although many scientists think it is more likely to be a captured object.