What is a planetary gear system?

• A. A system where a sun gear drives planet gears that orbit around a central carrier
• B. A set of gears arranged in a straight line along a shaft
• C. A gear that only exists in service manuals
• D. A method to count teeth on gears

Answer: (A)

A planetary gear system is defined by a sun gear at the center that drives planet gears, which orbit around a central carrier. The planets are mounted on that carrier and mesh with both the sun gear and, in many designs, a ring gear, forming a compact, high-torque reduction arrangement. This description—sun gear driving planet gears that orbit around a central carrier—captures the unique setup of planetary gear trains: multiple gears working together in a compact space to provide large gear reductions.

The other descriptions don’t fit: a straight-line arrangement along a shaft describes a simple spur gear train, not a planetary layout; a gear that only exists in service manuals is not a real gear system; and counting teeth on gears is just a measurement task, not a description of a planetary arrangement.