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 the majority of compact form (referred to as torque density).
The bicycle’s acceleration hub is a great exemplory case of a planet-wheel mechanism: Perhaps you have ever wondered how you can get so much power and features in such a small hub? For a three-velocity hub, a one-stage planetary equipment system is used, for a five-quickness 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 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 usually 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.