Planet modules can be used to control the motion of a planet (or any other celestial body, such as a moon, the sun, or an asteroid) within the solar system. This allows to implement sophisticated analytic ephemerides solutions which take into account perturbations from other celestial objects.
Planets which are not controlled via a DLL module are updated directly by Orbiter. Depending on the settings in the definition file, Orbiter either uses an unperturbed 2-body approximation, resulting in a conic section trajectory (e.g. an ellipse), or uses a dynamic update procedure based on the gravitational forces acting on the planet. Both methods have limitations: the 2-body approach ignores perturbations and is only valid if no massive bodies other than the orbit reference object are nearby. The dynamic update accumulates numerical errors over time, causing the orbits slowly to diverge from the correct trajectories.
By using a planet module, analytic perturbation solutions can be used which avoid the shortcomings of the methods described above. Perturbation solutions typically describe the perturbed orbit of a planet by expressing the state vectors as a trigonometric series. These series are valid over a limited period of time, after which they start to diverge. Examples of perturba tion solutions used in Orbiter are the VSOP87 solution for the 8 major planets and the sun, or the ELP2000 solution for the moon.