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. 
It is important to remember the rule that the Lines in the Gears must join the Points at each end of the Lineof Centres.
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 Mesh 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 Mesh 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 