JWST NIRSpec G395H Transmission Spectra of Two Similar Sub-Neptunes in the TOI-125 System

The James Webb Space Telescope (JWST) has enabled the first in-depth characterisations of sub-Neptune atmospheres, revealing a diversity of chemical compositions and cloud properties that challenge current models. Multi-planet systems provide an opportunity to compare planetary atmospheres while controlling for formation conditions within the same protoplanetary disk. We present JWST NIRSpec G395H 3-5 um observations of two similarly-sized sub-Neptunes orbiting a sun-like star within the TOI-125 system: TOI-125 b (P=4.65 d, R p=2.7 R, M p=10 M, T eq=1100 K) and TOI-125 c (P=9.15 d, R p=2.85 R, M p=6.8 M, T eq=825 K), comprising of one full transit of TOI-125 b but only 70% of TOI-125 c's transit due to larger than predicted TTVs. We obtain precise R 100 transmission spectra for both planets, with median errorbars of 50 ppm for TOI-125 b and 85 ppm for TOI-125 c. Atmospheric retrievals are compatible with a 150x solar metallicity atmosphere or clouds above 3 mbar for TOI-125 b, and a 100x solar metallicity atmosphere or clouds above 10 mbar for TOI-125 c. Our results may support recent evidence for metal enrichment in sub-Neptunes, while also further complicating the proposed trend in atmospheric feature size with planet temperature. However, shorter wavelength follow-up observations of both planets are needed to establish whether their spectra are truly muted or feature-rich, with implications for the nature of sub-Neptunes around sun-like stars.