Planetary Gears :
A gearset in which all of the gears are in one plane, grouped around each other like the planets around the sun. The central gear is called the "sun gear". In mesh with it is a circular grouping of gears, called "planet gears", mounted on a rotating carrier. The planet gears also engage teeth on the inner periphery of the "ring gear". By holding any one of the three gear elements motionless, different ratios can be produced between the other two. Planetary gearsets are common in automatic transmissions.