TRAPPIST-1 d is one of seven roughly Earth-sized planets orbiting an ultra-cool red dwarf M-type star 40 light-years from Earth. Measuring 0.8 × Earth in radius, it orbits every 4.05 days at 0.022 AU on the habitable zone’s inner edge. It receives about 1.12 times Earth’s sunlight, enough that liquid water remains possible, yet the world may run hotter than its outer siblings. Resonant orbits lock it into a compact chain, a laboratory for rocky climates around diminutive stars. It stands eternally in the Galactic Codex as the hinge world of this iconic system, where habitability may begin.
