The prisoner of gaseous giant
Rocky moon that is most likely a fragment of a massive celestial that crashed into Herco in the past, and de-orbited it closer to PE31 star. Many of these fragments, including Rikkit, got imprisoned by Herco's gravity and travelled with the planet to the centre of the system. Drifting at sprinter's pace around its' parent, Rikkit gathered enough of Herco's rings material to create a visible gap in them. Loose silicate regolith on Rikkit's surface most likely comes from there.