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 many compact form (known as torque density).
The bicycle’s acceleration hub is an excellent exemplory case of a planet-wheel mechanism: Have you ever wondered how you can get so much power and features in such 
a small hub? For a three-velocity hub, a one-stage planetary equipment system is used, for a five-speed hub a 2-stage. Each planet gear program has a reduction state, a primary coupling and an acceleration setting.
In mathematical terms, the smallest reduction ratio is 3: 1, the biggest is 10: 1. At a ratio of less than 3, the sun gear becomes too large against the planet gears. At a ratio higher than 10 the sun 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 years and years.