= Planetary system dynamics
<Planetary system dynamics> explains the changing <orbits> of planets and smaller bodies under gravity and dissipative forces. A <Kepler orbit> supplies a useful two-body reference; interactions with other bodies produce <secular perturbations>, <orbital resonances> and close encounters.
Conservative interactions exchange energy and angular momentum within the system, while tides, gas drag and radiation forces can dissipate or transfer them to other reservoirs. Comparing orbital times, migration times and collision times identifies which approximation applies. The resulting theory connects present configurations with possible formation histories without assuming that every observed orbit has stayed fixed since birth.
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