There are exactly 100 sidereals at all times. When one Sidereal dies, their "exaltation" passes on to someone else; people tend to have some memories from their past life.
Sidereals are always destined to exalt Sidereal. The other Sidereals track these destinies and usually manage to scoop up the future sidereals as babies and raise them themselves, but they do sometimes miss them until they're older. The actual exaltation happens somewhere in the teens-early 20s.
When they have one of each caste available, Sidereals can perform a magic ritual that literally lets them have visions of the future. (I do not know if they will still need to have all types in 3E or not, but they'll almost certainly find some way to make it so you can't just spam futurevision.)
Unlike literally everyone else in the setting, Sidereals actually know EXACTLY what the fuck is going on in Creation. They know all about how the First Age of Solar rule ended in the "Usurpation" by the Dragon Blooded.... because they were the ones who organized it. They swear it seemed like a good idea at the time (...or at least the least bad option). After having all the Solars killed, they locked the exaltations away. The fact that the jade baby jail Jade Prison got busted open is very "oh shit" for them.
They are likewise responsible for rewriting history, inventing the Immaculate Order (a religion revering the Dragon Blooded), and spreading the lie that the Lunars (and eventually the returned Solars) are demonic Anathema.
NO ONE CAN REMEMBER SIDEREALS EXIST.
They literally fucking deleted themselves from fate. They can interact with people, but mortals will literally always forget they exist as soon as they leave. Other exalted have a shot at shaking off this effect, but it's tough even for them.
In order to counteract this, they make themselves fake identities to wear like costumes, called "resplendent destinies" because they're literally an artificially created fate.
Like every other exalt type, Sidereals were unbeknownst to even them, CURSED when their first incarnations helped kill the tyrannical Primordial titans who created the world. The Sidereal "Great Curse" is that, the more Sidereals are together, the more they suffer from groupthink, and the more they tend to think that all their ideas are great ideas. (It's implied that this was another factor in them deciding the Usurpation was necessary.) This is what I meant when I said that Sidereals literally become stupider the more of them are together at once.
There are two sidereal factions:
(1) the Bronze Faction, who support the Dragon Blooded, agree that the Usurpation was the right thing to do, and continue to work against the reborn Solars; and
(2) the Gold Faction, who want to help restore Solar rule and are playing "spy vs. spy" with the Bronze Faction.
There are 5 Sidereal Castes, each associated with one of the divine MAIDENS, who are the "planets" Mercury, Venus, Mars, Jupiter, and Saturn.
NOTABLY, in the Exalted setting, the word "planet" only ever refers to the Maidens and there is no concept of a planet as anything else but a type of goddess.
The castes are......
Chosen of Journeys: Associated with Mercury; always have starry yellow eyes and anima banners (i.e. magic auras from doing magic stuff). Chosen of Journeys tend to be wanderers, explorers, and guides; their special abilities let everyone GO FAST and speed travel.
Chosen of Serenity: Associated with Venus; always have starry blue eyes and etc. Chosen of Serenity are sometimes literally called "Joybringers"; they're all about cherishing what makes life wonderful. Their special abilities help everyone PERFORM REAL GOOD and improve their own social stuff and artistic ability.
Chosen of Battles: Associated with Mars; always have starry red eyes and etc. Chosen of Battles are not always For Fite, but they always have an appreciation for the role of conflict in society. Their special abilities help everyone TAKE LESS DAMAGE and improve their own fighting/athletic skill.
Chosen of Secrets: Associated with Jupiter; always have starry green eyes and etc. Chosen of Secrets are mysterious fuckos even by the "we are all mysterious fuckos" standards of Sidereals. Their special abilities let them and their allies RESIST MIND READING/MENTAL EFFECTS and improve their information gathering.
Chosen of Endings: Associated with Saturn; always have starry purple eyes and etc. Mysterious Fucko: Goth Edition. Chosen of Endings are the kind of people who are ~assassins for justice~ and tend to have a kind of chuuni fascination with death. Their special abilities let everyone DEAL MORE DAMAGE and improve their skill as martial artists, but also with medicine.