An idea of why The Nine are the celestial bodies they are

So through Bungie and Rite of The Nine cutscenes and lore we know that The Nine are the eight recognized planets: Mercury, Venus, Earth, Mars, Saturn, Jupiter Neptune, Uranus. And also our star/sun Sol.

But why them specifically? (The in universe why.) What makes them special above various moons and planetoids like Luna, Europa, Pluto, Nessus, Io, Titan?

Here's my best theory:

The Nine do a lot of things with gravity, to the point it might be an essential part of what they are. Each member of the Nine are related to a celestial body. Even anchored to it, as the lore entry Anamórphōsis: Aítion that talks about Theia and it's consciousness definitively states it died when it smashed to pieces against Earth, partially merging with it and also becoming the moon.

So now maybe it's no coincidence that they are the nine largest ones, because the larger the mass the stronger the gravitational pull. This might explain why anything smaller than Mercury isn't paired up with a Nine like being, their gravity is insufficient to pull in enough dark matter to form the looping strings of dark matter thought that make up the Nine.