JWST-SUPER I: New insights into irradiated warm Neptunes atmospheres from MIRI observations of HD 106315 c
Abstract
Sulphur-bearing molecules have recently emerged as powerful tracers of atmospheric photochemistry in exoplanets observed with JWST. In several warm giant planets, SO2 has been detected as a product of UV-driven chemical processing, suggesting a close connection between stellar irradiation, atmospheric metallicity, and sulphur chemistry. Whether these trends extend to Neptune-mass planets orbiting hotter stars remains largely unexplored. We investigate the atmospheric composition of the warm Neptune HD 106315 c, a Neptune-mass planet orbiting an F-type host star and subjected to a strong ultraviolet (UV) irradiation environment. Here, we report the low-resolution transmission spectrum between 5 and 12 μm of HD 106315 c obtained with the MIRI Low-Resolution Spectrometer on-board JWST. Our work also includes re-analysis of archival data from HST WFC3 G141 mode of HD 106315 c, as well as contemporaneous XMM and Swift monitoring of the star in the UV and X-ray wavelength ranges. Together with the archival HST WFC3 data, we detect H2O and find tentative evidence for NH3, retrieving abundances of log10(H2O)=−1.40+0.40−0.74 and log10(NH3)=−2.47+0.57−0.89, together with an isothermal terminator temperature of 747+150−155 K. We place stringent upper limits on CH4, SO2, and CS2, finding no robust evidence for any sulphur-bearing species despite the intense irradiation received by the planet. The inferred water abundance implies a strongly metal-enriched atmosphere. Elevated intrinsic temperatures can reconcile the non-detections of CH4 and CS2 through carbon--sulphur coupling, while the absence of SO2 points toward a reduced atmospheric sulphur inventory or a near-solar to mildly enhanced C/O ratio, and places HD 106315 c near the transition between sulphur-rich and sulphur-poor chemical regimes.
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BibTeX
@misc{dyrek2026jwstsuper,
title = {{JWST-SUPER I: New insights into irradiated warm Neptunes atmospheres from MIRI observations of HD 106315 c}},
author = {A. Dyrek and R. Waters and S. -M. Tsai and M. Holmberg and B. Edwards and J. Pye and M. Güdel and N. Crouzet and R. van Gompel and J. Polman and F. Ardevol Martinez and D. Barrado and W. De Gruijter and Th. Henning and S. Kendrew and N. Khorshid and Y. Miguel and M. Min and M. Morales-Calderon and M. Topinka},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.09893},
eprint = {2609.09893},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.09893},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - JWST-SUPER I: New insights into irradiated warm Neptunes atmospheres from MIRI observations of HD 106315 c
AU - A. Dyrek
AU - R. Waters
AU - S. -M. Tsai
AU - M. Holmberg
AU - B. Edwards
AU - J. Pye
AU - M. Güdel
AU - N. Crouzet
AU - R. van Gompel
AU - J. Polman
AU - F. Ardevol Martinez
AU - D. Barrado
AU - W. De Gruijter
AU - Th. Henning
AU - S. Kendrew
AU - N. Khorshid
AU - Y. Miguel
AU - M. Min
AU - M. Morales-Calderon
AU - M. Topinka
PY - 2026
DA - 2026/09/09
PB - arXiv
DO - 10.48550/arxiv.2609.09893
UR - https://arxiv.org/abs/2609.09893
AB - Sulphur-bearing molecules have recently emerged as powerful tracers of atmospheric photochemistry in exoplanets observed with JWST. In several warm giant planets, SO_2 has been detected as a product of UV-driven chemical processing, suggesting a close connection between stellar irradiation, atmospheric metallicity, and sulphur chemistry. Whether these trends extend to Neptune-mass planets orbiting hotter stars remains largely unexplored. We investigate the atmospheric composition of the warm Neptune HD 106315 c, a Neptune-mass planet orbiting an F-type host star and subjected to a strong ultraviolet (UV) irradiation environment. Here, we report the low-resolution transmission spectrum between 5 and 12 μm of HD 106315 c obtained with the MIRI Low-Resolution Spectrometer on-board JWST. Our work also includes re-analysis of archival data from HST WFC3 G141 mode of HD 106315 c, as well as contemporaneous XMM and Swift monitoring of the star in the UV and X-ray wavelength ranges. Together with the archival HST WFC3 data, we detect H_2O and find tentative evidence for NH_3, retrieving abundances of log_10(H_2O)=−1.40^+0.40_−0.74 and log_10(NH_3)=−2.47^+0.57_−0.89, together with an isothermal terminator temperature of 747^+150_−155 K. We place stringent upper limits on CH_4, SO_2, and CS_2, finding no robust evidence for any sulphur-bearing species despite the intense irradiation received by the planet. The inferred water abundance implies a strongly metal-enriched atmosphere. Elevated intrinsic temperatures can reconcile the non-detections of CH_4 and CS_2 through carbon--sulphur coupling, while the absence of SO_2 points toward a reduced atmospheric sulphur inventory or a near-solar to mildly enhanced C/O ratio, and places HD 106315 c near the transition between sulphur-rich and sulphur-poor chemical regimes.
ER -