We know they're there. In the Stars arc, Seiya said that any planet, regardless of size, that had life, also had a Senshi. I interpret this to mean anything big enough to possibly hold a kingdom, so that excludes microscopic asteroids but includes larger ones. The ones large enough to have names are probably large enough for kingdoms, at least that's what I'm going on. This article will list all of the named asteroids/minor planets and dwarf planets and what I think Senshi from each would be like.
Edit: OK, there could probably be 100 more Senshi for every named asteroid. I'm just going to stop with what I have.
Haumea: Haumea is the Hawaiian goddess of childbirth, so I'd expect the Senshi to be more mature, at least 25, but probably 30 years old. She'd be patient, nurturing, and stern. As for appearance, I'd expect her to look, well, Hawaiian with tan skin, black hair, and brown eyes. Her fuku would be light green, yellow, and brown, and she'd have tropical flowers in her hair.
Makemake: Makemake is the Rapa Nui (Easter Island) god of fertility and creator. I don't know much about him, but I'd expect the Senshi to be strict and serious. She'd have Hispanic heritage a and have gray (for the stone heads) and red (for red dirt common around there, I guess) fuku. She could be any age, really, as long as she isn't a child, but I like 17 like Uranus and Neptune.
Eris: As the Greek goddess of strife and discord, I would expect her to, like Saturn, not be a welcomed Senshi. Because of this, she'd be quick-tempered and distant. She'd have long black hair, gray eyes, and black and gray fuku. I picture her age to be 16, like the Guardians.
Orcus: Hmmm another downer. The Italic and Roman god of the Underworld. Maybe this is why Naoko didn't include Dwarfs (Pluto doesn't count). Because her name sounds so Orca-whaley, I picture her with wavy dark blue hair, dark blue eyes, and blue and black fuku. She'd be 19.
Quaoar: Just gonna be blatantly honest, I know nothing about this god other than he's from Southern California. For that reason only, I expect the Senshi to be adventurous but air-headed (going on stereotypes, sorry Californians) with light brown hair and striking green eyes. Her fuku would be green and orange. Her age would be 16.
Salacia: Like the Roman goddess of saltwater, she'd be sweet and compromising. Her dark blue hair would be decorated with fish nets, and her eyes would be dark green. Her fuku would be dark blue and dark green, and she'd be 16.
Rhadamanthus: He was a Greek king and judge of the dead. I would expect the Senshi to be fair and wise with black eyes and black and periwinkle fuku. Her hair would be periwinkle and kept in a long braid down her back. She'd be 17.
Huya: Named after the Wayuu god of rain, Sailor Huya would be generous and friendly. Her orange hair would be up in pigtails, and her eyes would be gray. Her fuku would be orange and pink, and she'd be 9.
Typhon: The deadliest of all Greek monsters. Sailor Typhoon would be bold and brave, but not evil because she is a Senshi. She would have very short green hair, and her fuku would be orange and green, her eyes red. She'd be 20.
Deucalion: Son of Prometheus in Greek mythology. The Senshi would be intelligent and strong. Her curly red hair would be kept in a ponytail, and she'd have yellow eyes. Her fuku would be orange and yellow, and she'd be 15.
Logos: I couldn't find anything on this Greek god because no matter what I googled, if I had 'logos' and 'Greek' in the same search, I got a Nike ad. So I don't know what this Senshi would be like.
Ceto: The goddess of sea dangers would be reckless, have gold hair, green eyes, gray and green fuku, and be 15.
Borasisi: A god of the sun in a made-up religion. The Senshi would be of African descent (because we need at least one!) with gold eyes and orange and red fuku. She'd be 17.
Sila-Nunam: Named after the Inuit god of sky, weather and life and the Earth goddess, this Senshi would be very motherly. She'd have gray hair, light blue eyes, blue and green fuku, and be 23.
Teharonhiawako: A Mohawk Earth god. The Senshi would be patient and connected with nature, with brown, green, and orange fuku, brown eyes, and be 18.
Chaos: Not the evil being Sailor Moon had to fight, but the actual dwarf planet. Against her namesake, Sailor Chaos is a 5-year-old orphan that is an absolute sweetheart. Her red hair is kept in a ponytail with a pink bow, and her fuku is red and green. Her eyes are pink.
Altjira: Like the Aboriginal god of the sky, Sailor Altjira would be impatient but a good soldier. Her fuku would be gray and blue, blue eyes, and be 16.
Sedna: This is a Senshi I really want to see! She is the Inuit goddess of marine animals. She'd be gentle, adventurous, and quiet. She'd be of Inuit ethnicity obviously with long black hair kept in a traditional up-do and dark brown eyes. Her fuku would be dark blue, teal, and silver. She'd be 17.
Ixion: Ixion is the son Ares and also a god of war. I picture the Senshi being a lot like Mars, but perhaps less sympathetic. Her hair would be ruby red and in a bob-cut. She'd have brown eyes, red and crimson fuku, and be 17 years old.
Varuna: Varuna is the Hindu god of the sky, water, celestial ocean, law, and underworld. Sailor Varuna would be patriotic, serious, and have a strong need for justice, but she'd also have an adventurous side. She'd have olive skin, gray eyes, and dark purple hair in a traditional Indian bun (sorry, don't know what it's called). Her fuku would be aqua and green, and she'd be 17.
Astraea: Astraea is the Greek goddess of innocence and purity and is the celestial virgin. Sailor Astraea would be innocent and pure as her name implies, as well as being shy but kind. Her hair would be very long and dark blond with sparkling star barrettes. She'd have light blue eyes and white fuku with shining silver accents. She would be 16.
Hebe: Hebe was the Greek goddess of youth. As such, her Senshi would be fun-loving, giggly, and immature, but also a pushover. She would have crazy curly orange hair, blue eyes, and yellow and light blue fuku. She would be only 7 years old.
Iris: Because she symbolizes the goddess of rainbows, her hair and fuku would be colored just that. The canon Asteroid Senshi have crazy hairstyles you can't put into words, and Iris would be one, too, but I'll try to explain. Pretty much, she'd have 7 ponytails, one each color of the rainbow, and 7 braids over the top of her head. She'd have light blue eyes and be 13 years old. She'd be bubbly and spontaneous.
Flora: The goddess of flowers and spring. Flora would be energetic, happy, and optimistic. Her light green hair would be in a waterfall braid with blue flowers. She'd have brown eyes and yellow and hot pink fuku. She'd be 14.
Metis: Metis was the Greek Titaness of wisdom. Sailor Metis is very smart, rivaling Mercury, but she's a bit of a know-it-all. She has black hair that she keeps in a braid-y bunch and emerald green eyes. Her fuku is green and dark purple, and she's the same age as the Guardian Senshi.
Hygieia: Hygieia is the Greek goddess of health and sanitation. Her Senshi would be fairly athletic, determined, and sometimes a little obsessive. Like Mercury, she wants to be a doctor, so she's also smart. Her eyes and hair would be mint green. It would be split into three sections, braided individually, then those three braids would be braided together. Her fuku would be mint green and light blue. She would be 15.
Parthenope: Like her Greek Siren counterpart, Sailor Parthenope is an excellent singer and has a seductive and manipulative personality. Her blond hair is kept in a ponytail with a gold cuff, and her fuku is gold and orange. She has orange eyes and is 21 years old.
Victoria: Victoria is the Roman goddess of victory. Her Senshi is very athletic and doesn't accept loss without a fight. She's very determined. Her brown hair is kept in a messy bun, and she has purple eyes. Her fuku is purple and gold, and she's 18 years old.
Egeria: Like the Roman nymph, Egeria is shy, loyal, devoted to nature, and determined. Her black hair is in two pigtails that are tied together, then put in a rope braid. The bottom of the braid is tied off with a green bow. She has yellow eyes, green fuku, and is 15 years old.
Irene: Irene was the goddess of peace. Sailor Irene wouldn't accept her soldier duties willingly at first because of her deep hatred of conflict. Her silver hair would be kept in two cones on the side of her head with gold ornaments, and she'd have dark brown, almost black eyes. Her fuku would be olive green and silver. She would be 15.
Eunomia: Like the Greek goddess of good order, she would be patient but strict. She would have dark blond hair she keeps in a top braid and bun. Her eyes are red, and her fuku is red and yellow. She's 17.
Proserpina: The goddess of spring and the Underworld is kind but gullible. Her crimson hair is kept in a braided bun, and her fuku is pomegranate and lime green. Her eyes are pomegranate, and she's 18.
Bellona: Bellona is a Roman goddess of war, so she'd be blunt, harsh, and cold. Her long wavy brown hair is kept loose, and her eyes are gold. Her fuku is black and gold.
Amphitrite: A Greek sea goddess with blond hair decorated with shells and light blue and cream fuku. Her eyes can be light blue. Like the canon Asteroid Senshi, she's 15.
Leucothea: Oh, whadya know, another water deity! Her hair can be light blue and in fishtail braid. Her eyes are dark blue, and her fuku is dark blue and dark blue-green. Just to keep it simple, she can be 15, too.
Fides: Her patron goddess is of trust, so as such, she'd be very reliable and honest. Her gold hair is kept in one of those braids that wraps around your head, her eyes are gold, and her fuku is light blue and pink. She'd be 15.
Earth: We all want this. Even if Mamoru is technically the Senshi of Earth, I'd love it if Naoko found some dramatic twist for there to be an Earth Senshi, too, that no one knew about. I think her main color would be blue because most of Earth is water, and brown would be secondary because of, well, dirt. I don't think green would be part of her fuku because not every place has pretty grass, but everyone has dirt. I promise. Maybe her hair could be green.
Nemesis: This planet has to have a Senshi, an evil Senshi, but a Senshi no less. Other places villains came from, like the Dead Moon, could be counted as planets, but they're dead planets, as in no more Senshi. Nemesis, on the other hand, never actually died. Bad people still live there, so technically, it's alive yet. I kind of picture her looking a lot like Saturn. Sorry, Hotaru!
Comet: Stars get guardians. Why wouldn't comets? I'm thinking a silver-based Senshi.
Parallel Saturn/Uranus/Neptune/Pluto: Because why should just the Guardians have kids? Yeah, I know Hotaru is underage, and Michiru and Haruka are lesbians, but it's implied in the manga second generations aren't born naturally anyway. We're never shown dads in "Parallel Sailor Moon", so maybe that's why. Maybe, like with Princess Serenity, only a mom is needed. It's kind of creepy to think about, but I want mini Outers, so I'm good with it.
Someone to represent Artemis: Every other Main 12 god and a lot of other major gods have Senshi, but not Artemis (unless you count a cat that's a dude). This is my favorite goddess, too! I suppose the reason is because Artemis guards the moon, and that could conflict with Usagi/Selene, who is like the Moon guardian. Well, maybe Usagi had a sister in her past life. I'd go with that. I picture her having star-shaped odango ^_^