A planetary gearbox is a gearbox with the input shaft and the output shaft aligned. A planetary gearbox is utilized to transfer the largest torque in the the majority of compact form (known as torque density).
The bicycle’s acceleration hub is an excellent exemplory case of a planet-wheel mechanism: Perhaps you have ever wondered ways to get so much power and capabilities in such a small hub? For a three-swiftness hub, a one-stage planetary gear system can be used, for a five-speed hub a 2-stage. Each planet gear system 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 less than 3, the sun gear becomes too big 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 is not known, but was functionally described by Leonardo da Vinci in 1490 and has been used for years and years.