Well, clearly it's supposed to mean a federation consisting of planets. Another way of phrasing it would be "Planetary Federation."
So what we want is the genitive form of planeta
, being used in a partitive
sense. And that would be planetae
. Unless you go with the earlier Greek form planetes
, in which case it would be planetei
So I think I was close. Maybe something like Foedus Planetae Consociatus
or Foedus Planetei Consociatus