A GearPair with 'Orbiting Centres':
•  One gear centre revolves, or 'orbits', around another gear centre. 
•  A gear 'carrier' connects the centres of the two gears. 
The combination of an orbiting gear that engages with a sun gear and also an annular gear is called a planetary gear train.
Planetary Gears have a number of advantages over simple and compound GearTrains.
We can design:
The path of the centre of the revolving Gear  Gear 2 in both of the images  is a circle. This is not very useful, or interesting. However, the path of a different Point on the revolving gear is interesting and sometimes useful.
Points on a GEAR rolling around the outside of a fixed gear. See top image (below)
Epicycloid: A curve given by a point on the pitchcircle of a gear that rolls around the outside of the stationary gear
Epitrochoid: A curve given by a Point that is not on the pitchcircle of the gear that rolls around the outside of a stationary gear
Points on a gear rolling around the inside of a fixed gear. See bottom image (below)
Hypocycloid: A curve given by a point on the pitchcircle of a gear that rolls around the inside of the stationary gear
Hypotrochoid: A curve given by a Point that is not on the pitchcircle of the gear that rolls around the inside of a stationary gear.
Peritrochoid: A curve given by a Point on an orbiting gear, that is bigger than a fixed gear, and its centre is inside the pitch circle of the fixed gear.
External GearPair : Gear 1 : Fixed to the Frame  it does not rotate Gear 2 : Rotates about its own centre AND Orbits around the centre of Gear 1 Lineofcentres : Rotates with the centre of rotation at the centre of Gear 1 

Internal GearPair : Gear 1 : Fixed to the Frame  it does not rotate Gear 2 : Rotates about its own centre AND Orbits around the centre of Gear1 Lineofcentres : Rotates with the centre of rotation at the centre of Gear 1 