there are three keys with galaxy-related names that unlock treasure chests. all three are used indefinitely, and fairly often

they are:

Earth key - unlocks bronze chests. without this key, don't bother to try to unlock them unless you're prepared to face a mimic. found on juraika, in the burkaqua chief's house.

Star key - unlocks silver chests, which often contain better items than bronze chests. silver chests never have mimics. found on vedan, in angela's bar.(requires earth key)

Sun key - unlocks gold chests, which usually contain items of the highest quality/rareness. gold chests never have mimics. found on rosa, in the old man's house in johannasberg, after a cutscene. (requires star key)