JWST NIRSpec G395H Transmission Spectra of Two Similar Sub-Neptunes in the TOI-125 System
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, Rp=2.7 R⊕, Mp=10 M⊕, Teq=1100 K) and TOI-125 c (P=9.15 d, Rp=2.85 R⊕, Mp=6.8 M⊕, Teq=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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BibTeX
@misc{chai2026jwst,
title = {{JWST NIRSpec G395H Transmission Spectra of Two Similar Sub-Neptunes in the TOI-125 System}},
author = {Yiwei Chai and Néstor Espinoza and Chloe Fisher and Erik Meier Valdes and Matthew Hooton and Brett Morris and Natalie H. Allen and Adam J. Burgasser and Brice-Olivier Demory and Amélie Gressier and Kevin Heng and Mercedes López-Morales and Meng Tian},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.35607},
eprint = {2609.35607},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.35607},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - JWST NIRSpec G395H Transmission Spectra of Two Similar Sub-Neptunes in the TOI-125 System
AU - Yiwei Chai
AU - Néstor Espinoza
AU - Chloe Fisher
AU - Erik Meier Valdes
AU - Matthew Hooton
AU - Brett Morris
AU - Natalie H. Allen
AU - Adam J. Burgasser
AU - Brice-Olivier Demory
AU - Amélie Gressier
AU - Kevin Heng
AU - Mercedes López-Morales
AU - Meng Tian
PY - 2026
DA - 2026/09/28
PB - arXiv
DO - 10.48550/arxiv.2609.35607
UR - https://arxiv.org/abs/2609.35607
AB - 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.
ER -