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 most compact form (referred to as torque density).
The bicycle's acceleration hub is a superb 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-rate hub, a one-stage planetary equipment system is used, for a five-speed hub a 2-stage. Each planet gear system includes a reduction state, a direct coupling and 
an acceleration mode.
In mathematical terms, the smallest reduction ratio is 3: 1, the largest is 10: 1. At a ratio of significantly less than 3, the sun gear becomes too large against the planet gears. At a ratio greater than 10 sunlight wheel becomes too small and the torque will drop. The ratios are usually absolute i.electronic. 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 years and years.