Water Ice Line
The water ice line (or snow line) is the radial distance in a protoplanetary disk beyond which water condenses as solid ice grains. Planets that accrete primarily beyond this boundary acquire volatile-rich, water-dominated solids; planets accreted inside it are drier and rockier. The ice line is therefore a pivotal compositional boundary for inferring ex-situ vs in-situ formation when atmospheric retrievals reveal the bulk volatile inventory.
src-jwst-toi-1130b-atmosphere-2026-05 uses toi-1130b’s high mean molecular weight (μ = 5.5 amu), high metallicity, and low He I to argue the planet formed beyond the water ice line and migrated inward — the canonical ex-situ formation signature.