A planetary gearbox is a gearbox with the insight 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 exemplory case of a planet-wheel mechanism: Perhaps you have ever wondered how you can get so much power and capabilities in such a little hub? For a three-acceleration hub, a one-stage planetary gear system is used, for a five-rate hub a 2-stage. Each planet gear program has a reduction state, a direct coupling and an acceleration mode.
In mathematical terms, the tiniest reduction ratio is 3: 1, the largest is 10: 1. At a ratio of significantly less than 3, sunlight gear becomes too big against the earth gears. At a ratio greater than 10 sunlight wheel becomes too small and the torque will drop. The ratios are often absolute i.e. 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 centuries.