A planetary gearbox is a gearbox with the input shaft and the output shaft aligned. A planetary gearbox is used to transfer the 
largest torque in the most compact form (referred to as torque density).
The bicycle’s acceleration hub is an excellent exemplory case of a planet-wheel mechanism: Have you ever wondered how you can get so much power and capabilities in such a little hub? For a three-swiftness hub, a one-stage planetary gear system can be used, for a five-acceleration hub a 2-stage. Each planet gear program has a reduction state, a primary coupling and an acceleration setting.
In mathematical terms, the smallest reduction ratio is 3: 1, the biggest is 10: 1. At a ratio of significantly less than 3, the sun gear becomes too big against the planet gears. At a ratio higher than 10 the sun wheel becomes too little and the torque will drop. The ratios are usually absolute i.e. an integer number.
Whoever invented the planetary gearbox is not known, but was functionally described by Leonardo da Vinci in 1490 and has been used for centuries.