Hmmm. Not sure what more you want, but here is a picture of the pertinent part of the event sheet from Jailson's example (the second one Kyatric linked to above.)
<img src="http://dl.dropbox.com/u/57899112/RotateAroundObjectEvents.PNG" border="0" />
The first action in the Every Tick event is the one that makes the orbit.
The two global variables, radius and speed, are required since they are used in the first action. Radius of course is the distance of the orbiting object from the center point, and speed is the speed at which it travels in its orbit.
Centerpoint is object about which other objects orbit (in your case it would be the sun.) Object is the object in orbit. (In your case it would be a planet.)
The second action angles the object to follow the orbit path as it moves around the centerpoint. For a round object like a planet, you may not need. Instead you might put a rotate behavior on your planet to make it rotate on its axis as it travels about the sun.