LHS 1140 b is a small planet orbiting an M4.5V dwarf star within its habitable zone. Recent measurements suggest that the planet has a low density, indicating the presence of a substantial amount of water by mass or a thick H2-rich atmosphere. Using the NIRSpec instrument on the James Webb Space Telescope (JWST), the authors obtained a transmission spectrum of LHS 1140 b between 1.7 and 5.2 μm. By combining spectral retrievals and self-consistent atmospheric models, they found that the transmission spectrum is inconsistent with a H2-rich atmosphere, leaving a water world as the most plausible explanation for the planet's low density. The data favor a high-mean-molecular-weight atmosphere, possibly dominated by N2 with significant amounts of H2O and CO2. This suggests that the planet could have liquid-water oceans, making it a potential habitable world. The authors also discuss the implications of these findings for future observations and the possibility of detecting and characterizing such planets.LHS 1140 b is a small planet orbiting an M4.5V dwarf star within its habitable zone. Recent measurements suggest that the planet has a low density, indicating the presence of a substantial amount of water by mass or a thick H2-rich atmosphere. Using the NIRSpec instrument on the James Webb Space Telescope (JWST), the authors obtained a transmission spectrum of LHS 1140 b between 1.7 and 5.2 μm. By combining spectral retrievals and self-consistent atmospheric models, they found that the transmission spectrum is inconsistent with a H2-rich atmosphere, leaving a water world as the most plausible explanation for the planet's low density. The data favor a high-mean-molecular-weight atmosphere, possibly dominated by N2 with significant amounts of H2O and CO2. This suggests that the planet could have liquid-water oceans, making it a potential habitable world. The authors also discuss the implications of these findings for future observations and the possibility of detecting and characterizing such planets.