Exploration of the solar system has confirmed that most of its planets have magnetic fields. Although the planets differ in important details, the reason they are magnetic is fundamentally the same: In each planet, a self-sustaining dynamo has operated at some time in history. In such a dynamo, electric currents and magnetic fields are continuously induced by motions of conducting fluid inside the planet.
The evidence for self-sustaining dynamo action throughout the solar system—Venus may be the lone planetary exception—is not surprising because only three basic ingredients are needed: a large volume of electrically conducting fluid, planetary rotation, and an energy source to stir the fluid. Planets generally acquire all three in abundance during their formation. Indeed, dynamo action is not limited to the planets; it produces the 11-year solar cycle,1 and evidence exists for dynamo action in the Moon, in some of the Galilean satellites, and even in...
