A planetary gearbox is a gearbox with the insight 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 good example of a planet-wheel mechanism: Perhaps you have ever wondered how you can get so much power and features in such a small hub? For a three-rate hub, a one-stage planetary equipment system is used, for a five-speed hub a 2-stage. Each planet gear system has a reduction state, a primary coupling and an acceleration mode.
In mathematical terms, the smallest reduction ratio is 3: 1, the biggest is 10: 1. At a ratio of less than 3, sunlight gear becomes too large against the planet gears. At a ratio greater than 10 the sun wheel becomes too little 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 simply by Leonardo da Vinci in 1490 and has been used for centuries.