A planetary gearbox is a gearbox with the input shaft and the result 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 great example of a planet-wheel mechanism: Perhaps you have ever wondered how you can get so much power and features in such a little hub? For a three-acceleration hub, a one-stage planetary gear system can be used, for a five-swiftness hub a 2-stage. Each planet gear program includes a reduction state, a primary coupling and an acceleration mode.
In mathematical terms, the tiniest reduction ratio is 3: 1, the largest 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 small and the torque will drop. The ratios are usually absolute i.electronic. 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 centuries.