Two new studies report JWST transmission-spectroscopy results for temperate sub-Neptunes. One paper presents the first mid-infrared transmission spectrum of TOI-270 d using JWST’s MIRI Low Resolution Spectrometer covering 5–12 microns. Using archival MIRI LRS data, the authors build an empirical relation between detector settling time and flux to better model spectral light curves and improve transmission-spectrum precision. They report notable molecular-feature evidence in the MIRI spectrum at ln B values of 2.8–5.3 when comparing models with physically plausible molecular line absorption versus models without such absorption. The authors find excess absorption beyond previously detected methane (CH4) and carbon dioxide (CO2) and note that this aligns with recent near-infrared results. An agnostic search across 203 species identifies candidate trace molecules, mostly complex molecules, but the study concludes that further observations are needed for definitive identification, and argues that random or systematic noise is unlikely to explain the additional signals.
A second paper delivers the first temperate exo-Neptune benchmark spectrum for TOI-1231 b in the near-infrared (0.65–5.2 microns) using NIRISS and NIRSpec. It reports a strong detection of CH4 and moderate-to-strong evidence for CO2, with no significant detections of other prominent molecules and high upper limits on NH3 and CO, consistent with a deep H2-rich atmosphere.
