Natural Satellite - Origin and Orbital Characteristics

Origin and Orbital Characteristics

The natural satellites orbiting relatively close to the planet on prograde, uninclined circular orbits (regular satellites) are generally believed to have been formed out of the same collapsing region of the protoplanetary disk that created its primary. In contrast, irregular satellites (generally orbiting on distant, inclined, eccentric and/or retrograde orbits) are thought to be captured asteroids possibly further fragmented by collisions. Most of the major natural satellites of the Solar System have regular orbits, while most of the small natural satellites have irregular orbits. The Earth's Moon and possibly Charon are exceptions among large bodies in that they are believed to have originated by the collision of two large proto-planetary objects (see the giant impact hypothesis). The material that would have been placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting natural satellites. As opposed to planetary-sized bodies, asteroid moons are thought to commonly form by this process. Triton is another exception; although large and in a close, circular orbit, its motion is retrograde and it is thought to be a captured dwarf planet.