A planetary gearbox is a gearbox with the input shaft and the result shaft aligned. A planetary gearbox is used to transfer the largest torque in the most compact form (known as torque density).
The bicycle’s acceleration hub is a good example of a planet-wheel mechanism: Have you ever wondered how you can get so much power and capabilities in such a small hub? For a three-swiftness hub, 
a one-stage planetary gear system is used, for a five-quickness 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 significantly less than 3, sunlight gear becomes too large against the earth gears. At a ratio higher than 10 sunlight wheel becomes too small 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 years and years.