Well, going from this entry
, I'd say the best word for "united" in this sense would be consociatus
. "Federation" is from the Latin word foedus
, a league or covenant. And "planets" would be planetae
. My Latin's rusty, but maybe something like Foedus Consociatus de Planetae
. I'm too rusty to be sure what's right in that context, but de
does not belong.