A planetary gearbox is a gearbox with the input shaft and the result shaft aligned. A planetary gearbox can be used to transfer the largest torque in the the majority of compact form (known as torque density).
The bicycle’s acceleration hub is a good example of a planet-wheel mechanism: Have you ever wondered ways to get so much power and capabilities in such a small hub? For a three-acceleration hub, a one-stage planetary equipment system can be used, for a five-speed hub a 2-stage. Each planet gear program includes a reduction state, a direct coupling and an acceleration setting.
In mathematical terms, the smallest reduction ratio is 3: 1, the largest is 10: 1. At a ratio of significantly less than 3, sunlight gear becomes too large against the planet 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.