A planetary gearbox is a gearbox with the input shaft and the result shaft aligned. A planetary gearbox can be used to transfer the largest torque in the the majority of compact form (known as torque density).
The bicycle’s acceleration hub is a great example of a planet-wheel mechanism: Have you ever wondered how you can get so much power and features in such a little hub? For a three-speed hub, a one-stage planetary gear system can be used, for a five-rate hub a 2-stage. 
Each planet gear program has a reduction state, a primary coupling and an acceleration mode.
In mathematical terms, the smallest reduction ratio is 3: 1, the largest is 10: 1. At a ratio of less than 3, sunlight gear becomes too large against the earth gears. At a ratio higher than 10 sunlight wheel becomes too little and the torque will drop. The ratios are often absolute i.electronic. an integer number.
Whoever invented the planetary gearbox is not known, but was functionally described simply by Leonardo da Vinci in 1490 and has been used for centuries.