We have the Sun and the Moon Pokemon in Generation 7, obviously, and Ultra Beasts come from different places than the Earth and there's a right number of them so it's possible, right? Planets have variety of associations like the Greek/Roman gods, or in alchemy. Solgaleo is clearly the "sun-devouring lion" that represents an alchemical process and the Sun is assigned to gold element in alchemy. Buzzwole and Pheromosa immidiately suggest masculine Mars and feminine Venus.

I know that people also tried to pigeonhole them into 7 deadly sins, mostly because Guzzlord is called "Gluttony" and they all are generally evil, but I think planets make more sense.

>Nihilego (UB-01 Symbiont) - Mercury/Hermes/Mercury: it's the first encounter (first from the Sun), steals bodies (god of thieves), mineral-based but fluid-like, is a Poison-type and makes you insane (Mercury as an element)
>Buzzwole (UB-02 Absorption) - Mars/Ares/Iron: red, masculine, has a spear-like proboscis, Fighting-type, Mars in alchemy is Iron which is prominent in blood (Buzzwole is a mosquito)
>Pheromosa (UB-02 Beauty) - Venus/Aphrodite/Copper: feminine, all about beauty, antique mirrors were made from copper (mirror is Venus's symbol)
>Xurkitree (UB-03 Lighting) - Jupiter/Zeus/Tin: god of lightning (Electric-type), it is a tree (oak is Zeus's symbol), Tin is used in cables
>Celesteela (UB-04 Blaster) - Uranos/Caelum: god of the sky (it's a rocket)
>Kartana (UB-04 Blade) - Saturn/Kronos/Lead: sickle-wielding god of the harvest (Grass-type, it's a sword)
>Guzzlord (UB-05 Guzzlord Glutton) - Neptune/Poseidon: it's crab-like (water creature), has trident-like arms
>Necrozma (UB Black) - Pluto/Hades: not exactly Ultra Beast (not exactly a planet), has a dark coloration and death-based name (ruler of the underworld)