r = 50; //radius (in pixels)
f = 1; //orbits per second
angle = time*Math.PI*2*f;
value + [Math.sin(angle)*r, -Math.cos(angle)*r]
I haven't played with using Particular yet, however you could use 3d Stroke.
Just enable the taper and adjust the Start Thickness to 0 and End to 100. Perhaps adjust the end to a bit less then 100 so it doesn't 'connect' as much. Check Loop, then animate the offset. You could also enable the Comp Camera and parent the camera to the Null.. Then you have complete control.
I'll play with Particular tonight and see. This is me off the top of my head..
Also, here is nifty Orbiting Expression
