Free Astronomy Magazine July-August 2025

8 JULY-AUGUST 2025 ASTRO PUBLISHING T his graph shows the spectrum, from 0.6 to 5.3 microns, of the exoplanet K2-18 b, taken by the NIRISS and NIRSpec instru- ments on board JWST. The signal is shown with data points and error bars; the interpretation of the signal by the discov- ering group is shown alongside it. Previous Hubble data only extended from 1.1 to 1.7 microns, where the signatures of methane and water vapor would have been virtually indistinguishable. [N. Madhusudhan et al., 2023] T he colored curves in this plot represent transmission spectra of K2-18 b calculated for different atmospheric compositions. The the- oretical data are compared here with real data provided by Hubble. All absorbers are included in the spectral calculation shown in red. Spectra that include absorption by only H ₂ O (blue) and only CH 4 (green) are also shown. The error bars (ver- tical black segments) highlight how much uncertainty characterized those initial spectroscopic observa- tions. [B. Bézard et al., 2019] the infrared spectrum, in the range of 1.1-1.7 microns. Because of this limitation, the potential signatures of water vapor and methane (CH 4 ) could not be distinguished from one another in the transmission spec- trum of the K2-18 system. Nevertheless, a possible signature of water, combined with the certainty of a hydrogen-dominated atmos- phere, helped launch the idea of hycean worlds and prompted Mad- husudhan and colleagues to investi- gate further by requesting observ- ing time with the Webb Space Tele- scope, which was entering into op- eration at the time. The team was able to point the telescope at K2-18 in January and June 2023, collecting new spectra with the NIRISS (Near Infrared Imager and Slitless Spec- trograph, wavelength range 0.8-5.0 microns) and NIRSpec (Near Infra- Red Spectrograph, wavelength range 0.6-5.3 microns) instruments. The high sensitivity of the instru- mentation allowed them to resolve the uncertainty about the water/ methane ratio in the atmosphere of K2-18 b, revealing a strong presence of methane and a complete absence of any water signal. The non-detec- tion of the latter was consistent with the low temperatures expected for the upper atmosphere, which imply the condensation of water and its precipitation. In addition to methane, the spectra taken with Webb revealed an abun-