They're the original "planets" of astrology, alchemy and hermetic magic, from back when others weren't known about. Basically, important things in the sky.
"Planet" literally means "wanderer," as I recall. The "planets" were 'stars' that Greek astrologer/astronomers that didn't sit still in relation to the other stars; instead of slowly cruising across the heavens year by year, they would wander and wobble through the seasons. Thus they were dubbed 'wandering stars,' or planets.
The sun and moon were not in this category (again, if my recollection is accurate), but they, along with the planets, were grouped as celestial spheres, believed to radiate certain harmonics or vibrations, sometimes called "the music of the spheres," which would influence lives and minds on earth.
So the important things in the sky were the spheres, only some of which were planets, unless my memory is wobbling badly.
But back on topic, a campaign i'm constructing is using feat trees with their own category names for ease of reference. At the moment I've just got a handful. Psionic and Chi are new categories; in both cases a Supernatural feat is needed as a prerequisite to gain access to the tree. The intent is that a mixed-role hero, or a hero whose background makes one of the supernatural feats (Psion and Chi Focus, respectively) a favored skill, has access to the follow-up feats; but i don't add 'em all to the General list, 'cos that would clutter it up pretty badly.
One other category I've got is Exotic, which is never a favored category for anyone. Instead, certain backgrounds give access to certain feats - there's a canine species that has Scent and Bite as favored feats, both of which are exotic. That sort of thing.
I expect more feat trees to develop as i continue to expand the wackiness. For example, some backgrounds I'll soon be designing should necessitate the creation of biomechanical feats.
JBowtie, your setting sounds intriguing to me. Looks like you've got a pretty specific magical system in mind, and you're using True 20 in sort of a toolkit manner to construct it. Looks good to me. I'd certainly enjoy reading more about the spheres you mentioned, and the kinds of powers they grant, as well as the interactions you mentioned. Will heroes be able to draw from multiple spheres as per multi-role rules, or does the setting dictate a more specialized approach?
No kitten is going to tell me what to do. I'll fall off this branch to my death if I feel like it!