If you are attaching planets to animation paths you could create a predefined ruler. Although I might be confusing you.
Suppose you want planet B to be x distance from planet A. From top view Create an object such as a cube (to act as a ruler) that has a length equal to (x +(radius of B). Place one end of the cube even with the outer edge of planet A. Next place your animation path so that it's even with the other end of the cube. When you attach planet B to the animation path, the planets will be the correct distance from each other.
In otherwords, just create an object that represents the correct distance and use it as a guide. You can snap to the verts on the object for more precision.
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Exceed Expectations!