A planetary gearbox is a gearbox with the input shaft and the output shaft aligned. A planetary gearbox can be used to transfer the biggest torque in the most compact form (referred to as torque density).
The bicycle’s acceleration hub is a superb 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 equipment system can be used, for a five-acceleration hub a 2-stage. Each planet gear system has a 
reduction state, a primary coupling and an acceleration mode.
In mathematical terms, the smallest reduction ratio is 3: 1, the biggest is 10: 1. At a ratio of less than 3, the sun gear becomes too big against the earth gears. At a ratio higher than 10 the sun wheel becomes too little and the torque will drop. The ratios are often absolute i.e. an integer number.
Whoever invented the planetary gearbox isn’t known, but was functionally described by Leonardo da Vinci in 1490 and has been used for years and years.