High Mean Molecular Weight Atmosphere
A high mean molecular weight (high-μ) atmosphere is one in which the average molecular mass of constituents (H₂O, CO₂, N₂, etc.) exceeds the ~2.3 amu of a primordial H₂/He envelope. High-μ atmospheres are diagnostic of volatile-rich formation and/or significant atmospheric loss of light gases, and they produce smaller transit-depth features (smaller scale heights) for transmission spectroscopy.
toi-1130b is characterized by jwst at μ = 5.5 (+1.3/−0.8) amu — far above hydrogen-dominated baselines — supporting a volatile-rich, ex-situ formation history beyond the water-ice-line (src-jwst-toi-1130b-atmosphere-2026-05). High-μ atmospheres are also relevant to the hycean vs mini-Neptune retrieval debate around k2-18b, where hydrogen-dominated and volatile-rich models compete.