A planetary gearbox is a gearbox with the insight shaft and the output shaft aligned. A planetary gearbox is used to transfer the biggest torque in the many compact form (known as torque density).
The bicycle’s acceleration hub is a good example of a planet-wheel mechanism: Perhaps you have ever wondered ways to get so much power and features in such a little hub? For a three-rate hub, a one-stage planetary gear system is used, for a five-acceleration hub a 2-stage. Each planet gear program 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 less than 3, sunlight gear becomes too large against the planet gears. At a ratio higher than 10 sunlight wheel becomes too little and the torque will drop. The ratios are usually 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 years and years.