The 
Orbit POP makes particles orbit a center or centers.
You can optionally specify the leaders to follow using a bunch of
“center particles”: either a 
group within the input
stream (set with the Center group parameter) or the particles from a
second input stream (connect a particle stream to the node’s second
input).
You can specify the orbit axis, radius, and revolution speed for all particles, or control each with per-particle attributes.
Using the Orbit tool
Click the
Orbit button on the Drive Particles tab of the shelf.
Click
the particle system you want to be affected and press Enter to confirm your selection.
By default, the node looks for axis, radius, and speed attributes on the input particles. To specify values for these parameters manually, turn on the corresponding override checkbox on the Orbit tab and set the parameter (The orbit speed is set in RPM).
On the Center tab, use the Center type parameter to choose the center point the particles should orbit.
Tip
Reduce the Scale parameter on the Orbit tab below 1 to get smoother transitions into orbit. When the scale is high, the node pushes the input particles from their current location into orbit almost instantly. Reducing the scale reduces the effect of the orbit node on particles, reducing the speed at which the particles move into their orbits, creating smoother motion.
However, scale is relative to the spatial size of the orbit - a larger orbit probably means you want a higher transition speed. If you set the scale too low, the node may not be able to apply enough force to move the particles into proper orbits at all.
Tip
Animate or control the radius, axis, and/or speed parameters with expressions to create interesting effects beyond a simple ring of orbiting particles.
Tip
To have each particle orbit in a different direction, creating a “swarming globe” effect, use expressions to set the axis components to random values with the particle ID as the seed:
rand($ID + 1.0001)
For specific parameter help see the Orbit node help.