LHS 1140 b is a potentially habitable water world orbiting in the habitable zone of its M4.5V dwarf host. Recent mass and radius constraints suggest it has either a thick H2-rich atmosphere or substantial water by mass. Using the NIRSpec instrument on JWST, researchers obtained a transmission spectrum between 1.7 and 5.2 micrometers. By combining spectral retrievals and self-consistent atmospheric models, they found the spectrum inconsistent with H2-rich atmospheres, leaving a water world as the remaining scenario to explain the planet's low density. The data favor a high-mean-molecular-weight atmosphere, possibly N2-dominated with H2O and CO2, with modest confidence. Forming the planet by accreting C- and N-bearing ices could naturally give rise to a CO2- or N2-dominated atmosphere. If the planet evolves to or has the climate-stabilizing mechanism to maintain a moderate-size CO2/N2-dominated atmosphere, it could have liquid-water oceans. The models suggest CO2 absorption features with an expected signal of 20 ppm at 4.2 micrometers. As the existence of an atmosphere on TRAPPIST-1 planets is uncertain, LHS 1140 b may well present the best current opportunity to detect and characterize a habitable world. Keywords: Exoplanet atmospheric composition – JWST data analysis – Transmission spectroscopy.LHS 1140 b is a potentially habitable water world orbiting in the habitable zone of its M4.5V dwarf host. Recent mass and radius constraints suggest it has either a thick H2-rich atmosphere or substantial water by mass. Using the NIRSpec instrument on JWST, researchers obtained a transmission spectrum between 1.7 and 5.2 micrometers. By combining spectral retrievals and self-consistent atmospheric models, they found the spectrum inconsistent with H2-rich atmospheres, leaving a water world as the remaining scenario to explain the planet's low density. The data favor a high-mean-molecular-weight atmosphere, possibly N2-dominated with H2O and CO2, with modest confidence. Forming the planet by accreting C- and N-bearing ices could naturally give rise to a CO2- or N2-dominated atmosphere. If the planet evolves to or has the climate-stabilizing mechanism to maintain a moderate-size CO2/N2-dominated atmosphere, it could have liquid-water oceans. The models suggest CO2 absorption features with an expected signal of 20 ppm at 4.2 micrometers. As the existence of an atmosphere on TRAPPIST-1 planets is uncertain, LHS 1140 b may well present the best current opportunity to detect and characterize a habitable world. Keywords: Exoplanet atmospheric composition – JWST data analysis – Transmission spectroscopy.