Gliese 12 b enters the Codex as one of the nearest temperate Earth-sized worlds yet found — about 0.9 × Earth across and only 40 light-years distant. It circles a quiet M dwarf every 12.8 days near the inner edge of the habitable zone, warm enough that liquid water is a careful maybe rather than a guarantee. TESS and CHEOPS confirmed the 2024 discovery, and its host’s low activity makes the planet a prized target for JWST atmosphere hunts. With mass still loosely bounded, the story is size, light, and proximity: a near-Earth twin next door. It endures in the Galactic Codex as a temperate chapter for the nearest rocky worlds.
