The James Webb Space Telescope (JWST) has enabled new insights into the atmospheres of temperate low-mass exoplanets, including the Hycean candidate TOI-270 d. Using JWST's NIRSpec G395H instrument and combining it with previous HST WFC3 data, researchers analyzed the transmission spectrum of TOI-270 d, a sub-Neptune with a potential habitable ocean under a hydrogen-rich atmosphere. The spectrum shows strong signatures of methane (CH₄) and carbon dioxide (CO₂) at 3.8-4.9σ and 2.9-3.9σ confidence levels, respectively, and no evidence of ammonia (NH₃). These findings are consistent with predictions for a Hycean world. Additionally, the spectrum provides tentative evidence for carbon disulfide (CS₂) at 2.3-3.0σ and potential hints of water vapor (H₂O) at 1.6-4.4σ, depending on data analysis. The results do not strongly indicate the presence of clouds or hazes, nor do they show effects of stellar heterogeneities, consistent with previous studies. For the smaller inner planet TOI-270 b, the spectrum is inconsistent with a featureless spectrum at ~3σ, suggesting a hydrogen-rich atmosphere. The findings support TOI-270 d as a Hycean world, with a potential ocean under a hydrogen-rich atmosphere. The study highlights the importance of JWST in characterizing exoplanet atmospheres and provides insights into the potential habitability of such worlds. Further observations are needed to confirm the presence and abundance of water vapor and other molecules.The James Webb Space Telescope (JWST) has enabled new insights into the atmospheres of temperate low-mass exoplanets, including the Hycean candidate TOI-270 d. Using JWST's NIRSpec G395H instrument and combining it with previous HST WFC3 data, researchers analyzed the transmission spectrum of TOI-270 d, a sub-Neptune with a potential habitable ocean under a hydrogen-rich atmosphere. The spectrum shows strong signatures of methane (CH₄) and carbon dioxide (CO₂) at 3.8-4.9σ and 2.9-3.9σ confidence levels, respectively, and no evidence of ammonia (NH₃). These findings are consistent with predictions for a Hycean world. Additionally, the spectrum provides tentative evidence for carbon disulfide (CS₂) at 2.3-3.0σ and potential hints of water vapor (H₂O) at 1.6-4.4σ, depending on data analysis. The results do not strongly indicate the presence of clouds or hazes, nor do they show effects of stellar heterogeneities, consistent with previous studies. For the smaller inner planet TOI-270 b, the spectrum is inconsistent with a featureless spectrum at ~3σ, suggesting a hydrogen-rich atmosphere. The findings support TOI-270 d as a Hycean world, with a potential ocean under a hydrogen-rich atmosphere. The study highlights the importance of JWST in characterizing exoplanet atmospheres and provides insights into the potential habitability of such worlds. Further observations are needed to confirm the presence and abundance of water vapor and other molecules.