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JWST-SUPER I: New insights into irradiated warm Neptunes atmospheres from MIRI observations of HD 106315 c

Year: 2026
Type: preprint
Abstract: 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 $μ$m 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$_2$O and find tentative evidence for NH$_3$, retrieving abundances of $\log_{10}$(H$_2$O)$=-1.40^{+0.40}_{-0.74}$ 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.
DOI: 10.48550/arxiv.2609.09893
Authors: Dyrek, A., Waters, R., Tsai, S. -M., Holmberg, M., Edwards, B., Pye, J., Güdel, M., Crouzet, N., van Gompel, R., Polman, J., Martinez, F. Ardevol, Barrado, D., De Gruijter, W., Henning, Th., Kendrew, S., Khorshid, N., Miguel, Y., Min, M., Morales-Calderon, M., Topinka, M.
Institutions: Space Telescope Science Institute, Institute of Astronomy and Astrophysics, Academia Sinica, Academia Sinica, Space Research Organisation Netherlands, University of Leicester, Leiden University, Centre National de la Recherche Scientifique, Université Paris Cité, Commissariat à l'Énergie Atomique et aux Énergies Alternatives, Université Paris-Saclay, Astrophysique, Instrumentation et Modélisation, Leiden Observatory, Radboud University Nijmegen, University of Bern, American Museum of Natural History, Centro de Astrobiología, University of Amsterdam, European Space Agency, Chalmers University of Technology, Charles University
Language: English
Open Access status: green
Funders: National Aeronautics and Space Administration, Space Telescope Science Institute, University of Leicester, Academia Sinica, National Science and Technology Council, Science and Technology Facilities Council, Agencia Estatal de Investigación, Eurostars
Awards: G103270039, G1175115578, G2262748287, G256731684, G2650114919, G300979063, G34346518, G3452655474, G3480869486, G425668148, G433322167, G4526231891, G464105488, G5080475149, G5180915604, G58316358, G608736306, G6273510985, G7084143925, G7312380752
OpenAlex ID: W7212162962
Cites: 44
Cited by: 1
Related to: 10