Replace the question mark with the two letters to complete the puzzle?
NE US RN ER RS TH US ?
RY
The list is formed by using last two letter of planet's names in the order from Neptune To Mercury
NEPTU:NE
URAN:US
SATU:RN
JUPIT:ER
MA:RS
EAR:TH
VEN:US
MERCU:RY
