The Stars

"Our star was tiny and poverty-stricken..."

We aren't given overmuch information on Lethe and Mnemosyne as celestial bodies whatsoever, though we can make a few educated guesses. Looking at the panel on page 35 of Volume 10 both the original Japanese Kanzenban and the English release Eternal Edition (or page 121 of Volume 17 of the original tankoubon that ran from '92 to '97), it appears that Lethe and Mnemosyne may well be binary stars, or less likely, double stars. In laymans' terms, a binary star system is a system of two stars that are gravitationally bound to and in orbit around each other, and a double star (also known as an optical double) are a pair of stars that appear close to each other when viewed from Earth -- they may not necessarily be geographically so. For example, in its 100,000th exposure taken in 1996, the Hubble space telescope delivered a picture of two stars that appear to be doubles -- the fact of the matter is that they are nine billion light years away from each other! A mere trick of luminosity makes them appear like twins. Some double stars can indeed be binary stars (meaning they're gravitationally bound in a mutual orbit), but some are entirely separate objects that merely appear "twinned" from our Earthian viewpoint. Given that we're probably not looking at Lethe and Mnemosyne from Earth in said panel, I would put my money on them being part of an actual binary star system.

Whether these stars are habitable, like our Earth, or simply celestial objects that the twins draw their Sailor powers from (as the remainder of the planet and planetoids in our Solar system, or natural satellites or asteroids), we can't say exactly. The kanji given at the end of the names of their respective power sources, 星 -- usually pronounced ほし hoshi on its own or せい -sei as the suffix for a planet -- think Kinmokusei, or Kinsei, Kasei, Suisei (Venus, Mars, Mercury) etc -- is meant to be read as 'star', as indicated by the furigana of スーター sûtâ above it, which may be Takeuchi-sensei nudging us towards the possibility that Lethe and Mnemosyne are stars like our Sun, not planets.

Another hint that Lethe and Mnemosyne are likely binary stars and not separate planets, is that when Lethe speaks of their homeworld, she uses "hoshi", which despite translating in its most literal sense simply to "star", is also commonly used to mean planet -- compare when in Volume Nine's Act 54, Yaten explains the origins of star seeds and Sailor crystals and she uses the phrase 「僕らの星も」, boku-ra no hoshi mo, "our planet, as well" -- referring to planet Kinmoku. We know from previous information given to us by Kakyuu and the Sailor Starlights that Kinmoku is indeed a planet, and the carrier of its Sailor crystal is Princess Kakyuu herself. (Why this doesn't make her Sailor Kinmoku, and what celestial bodies the Starlights' Sailor crystals are paired with, is all speculation for a different site! ^~)

The manga panel immediately to the left of the panel that shows the binary stars of Lethe and Mnemosyne could well be an image of this planet -- Lethe says that their planet was full of poverty and highly chaotic before Galaxia arrived, and brought death and silence to the place. It appears that ordinary civilians are fighting with each other, perhaps caught in a war (or several), while a helpless Lethe and Mnemosyne can only watch. And yes, binary star systems can and often do contain planets. If Lethe and Mnemosyne are indeed binary stars, then this poor planet will be orbiting either one of the them, in what's described as a S-type orbit, or circling around the two of them as they circle each other -- a P-type or circumbinary orbit.

Wouldn't the existence of the one shared planet between the twins raise the question of "shouldn't there be a Sailor That-Actual-Planet, as well as Sailor Lethe and Sailor Mnemosyne?" Well, not precisely, if we follow the lore given in our source material itself: once again in Act 54, Usagi states that there must be many star systems in the galaxy like the Solar system. Taiki corrects her and notes that our Solar system is a unique place in all the galaxy -- no other star system has "grown to maturity while remaining so well-balanced", and that many star seeds stop growing properly and die out altogether, before they get the chance to mature. It may be that the planet Lethe and Mnenosyne inhabit is one such unfortunate place, and given that it doesn't seem like a very pleasant place to be, it may have suffered from malformed growth of its star seed, which is why it was never given a complimentary Sailor crystal and thus guardian soldier. The closest things it might have in this particular lifetime is indeed the twin guardians of remembrance and oblivion.