I would create a global variable called something like "orient-planets" and set it to be 0 in your setup method.
globals [ orient-planet ]
let orient-planets 0
Then in this part of your code you can make an if statement to check if this number is 0.
if orient-planets = 0 [ ]
Inside of the brackets you would then just have to set orient-planets to be a number other than 0, and they will only change their direction one time.
ask planets [
if orient-planets = 0 [
facexyz random-xcor random-ycor random-zcor
set orient-planets 1