A planetary gear is a type of gear system that consists of a central gear, known as the sun gear, surrounded by one or more gears called planet gears. These planet gears rotate around the sun gear and are often mounted on a movable arm called a carrier. This design allows for compact and efficient power transmission, making planetary gears popular in various applications, including automotive transmissions and industrial machinery. The outermost gear in a planetary gear system is called the ring gear, which encircles the planet gears. The interaction between the sun gear, planet gears, and ring gear enables different gear ratios, allowing for speed and torque adjustments. This versatility makes planetary gears essential in devices like automatic transmissions and robotic systems.