AU Mic b joins the Codex as a Neptune-sized world racing around a still-forming red dwarf only about 32 light-years away. Roughly 4.8 × Earth across, it transits a violently young M1 star whose dusty debris disk still sparkles with planetesimal collisions. Discovered in 2020, the planet completes an orbit in a little over eight days under flare-star weather that would strip fragile atmospheres. Its size and mass place it among ice-giant cousins rather than rocky siblings, a hot Neptunian laboratory in a newborn system. It stands in the Galactic Codex as proof that giant worlds can settle early while their host star is still thrashing.
