A planetary gearbox is a gearbox with the input shaft and the output shaft aligned. A planetary gearbox is utilized to transfer the biggest torque in the the majority of compact form (known as torque density).
The bicycle’s acceleration hub is an excellent example of a planet-wheel mechanism: Have you ever wondered ways to get so much power and features in such a little hub? For a three-speed hub, a one-stage planetary equipment system can be used, for a five-quickness hub a 2-stage. Each planet gear system includes 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, sunlight gear becomes too large 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 simply by Leonardo da Vinci in 1490 and has been used for centuries.