A planetary gearbox is a gearbox with the input shaft and the output shaft aligned. A planetary gearbox can be used to transfer the biggest torque in the most compact form (known as torque density).
The bicycle’s acceleration hub is a superb 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-quickness hub, a one-stage planetary gear system is used, for a 
five-rate hub a 2-stage. Each planet gear system includes a reduction state, a direct coupling and an acceleration setting.
In mathematical terms, the tiniest reduction ratio is 3: 1, the largest is 10: 1. At a ratio of less than 3, sunlight gear becomes too large against the earth gears. At a ratio higher than 10 sunlight wheel becomes too little and the torque will drop. The ratios are often 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 centuries.