I
wish I had images of Galaxia's planet to show more clearly what it looks
like, but since I don't, I can only go by what she says.
Her planet was barren -- empty -- alone. And she hated it. She grew up there alone for most of her life before she was finally able to leave, but her whole time there she considered a living "hell."
The way I picture how it might have appeared is rocky, but still flat, maybe a few plants here and there, but not really any life. Just emptiness, a reflection of Galaxia's emotions. The planet was small, probably on the outskirts of the galaxy, unnoticed and all alone. Galaxia didn't care enough about it to give it a name, in fact she didn't even consider it her "true" planet, of course not. Her powers were too great to be the guardian of a tiny world. She had to belong to something much greater. This of course is all purely speculation as we are never shown any image of it in either the anime or manga.
In her own words:
I was born alone as trash, on a planet of trash, I could do nothing but wander in solitude, living my days like I was in hell. But I have power as a sailor soldier. One day, it suddenly awakened. I was a chosen one. But being a mere sailor soldier would be the same as being trash. I had to have the strongest planet, carrying the strongest power! I came searching for a planet of my own, suitable for me! And then I found it. A magnificent planet. And to get that planet, I need the galaxy's ultimate power!
Nothing is said about her home planet in the anime, so we are left to guess exactly what it might have looked like. We can only hope it was a lot better than what her manga self had to live through.
