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arXiv · 2609.35607

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

Yiwei Chai·Néstor Espinoza·Chloe Fisher·Erik Meier Valdes·Matthew Hooton·Brett Morris·Natalie H. Allen·Adam J. Burgasser·Brice-Olivier Demory·Amélie Gressier·Kevin Heng·Mercedes López-Morales·Meng Tian
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Abstract

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, $R_p=2.7~R_\oplus$, $M_p=10~M_\oplus$, $T_\mathrm{eq}=1100$ K) and TOI-125 c ($P=9.15$ d, $R_p=2.85~R_\oplus$, $M_p=6.8~M_\oplus$, $T_\mathrm{eq}=825$ 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.

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Yiwei Chai, Néstor Espinoza, Chloe Fisher, Erik Meier Valdes, Matthew Hooton, Brett Morris, Natalie H. Allen, Adam J. Burgasser, Brice-Olivier Demory, Amélie Gressier, Kevin Heng, Mercedes López-Morales, Meng Tian. 2026-09-28. JWST NIRSpec G395H Transmission Spectra of Two Similar Sub-Neptunes in the TOI-125 System. https://arxiv.org/abs/2609.35607

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