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.