So quick summary, using the brilliant deductive skills of @Keegan and what @Gryphonic added to it, we have:
M (Neptune), ER (Jupiter), C (Venus), UR (Earth), Y (Saturn)
V (Earth), E (Saturn), N (Neptune), U (Jupiter), S (Mercury)
E (Mercury), A (Jupiter), R (Neptune), T (Venus), H (Saturn)
J (Mercury), UP (Neptune), I (Venus), TE (Saturn), R (Earth)
S (Venus), A (Neptune), TU (Jupiter), R (Mercury), N (Earth)
NE (Saturn), P (Venus), T (Jupiter), UN (Mercury), E (Earth)
You use every letter or letter combo only once from the outer ring (Largest Letters) from each planet, and you never use any letters from the planet you are spelling. I hope this makes sense.