So, the aliens came up with a plan to move to other planets.  Since all the good ones were already taken, they decided the best thing to do would be to blend in with the populations already living on these planets.  It was easier than declaring war and blowing everybody to smithereens, and besides, the cute happy aliens really didn't want to hurt anybody.

In order to assimilate fully with the creatures on their worlds of choice, they formed a department called the "Aliens of Extraordinary Ability."  These were the smartest aliens on their planet, and these aliens were sent all over the galaxy to learn every teensy weensy detail they could about other civilizations.  They would then go back to their planet, infuse everyone scheduled to move to the new planet with all the information (through a cool mind-meld thingy that's way too complicated to describe here--only the especially nerdy would understand it), and then the aliens could move on in and nobody would be the wiser.