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Also thinking along the lines of Galaxy Score XP:
XP = (Galaxy Score / Average Galaxy Score) x Current Planet XP Formula

Which would mean, if the top galaxy was 100mil, Average Galaxy score was 40mil, and the bottom galaxy was 10mil..
Hitting the top galaxy would give you: 2.5 x Planet XP
Hitting a galaxy the average galaxy would give you: 1 x Planet XP
Hitting the worst galaxy in the game would give you: 0.25 x Planet XP
(Planet XP = the current XP formula).

or, to give the players in worse galaxys a better XP system.. we could have the formula take the attackers galaxy score as the bench mark instead of the average galaxy score. Thus meaning the crappy galaxys get much more XP on there attacks than the good galaxys and the good galaxys have to attack other good galaxys in order to get valuable XP.