The SPACE Program. I. The featureless spectrum of HD 86226 c challenges sub-Neptune atmosphere trends

Creators: Kahle, K. Angelique^{1, 2}; Blecic, Jasmina³; Ashtari, Reza⁴; Kreidberg, Laura¹; Kawashima, Yui⁵; Cubillos, Patricio E.^{6, 7}; Deming, Drake⁸; Jenkins, James S.⁹; Mollière, Paul¹; Redfield, Seth¹⁰; Tian, Qiushi Chris^{10, 11}; Vines, Jose I.¹²; Wilson, David J.¹³; Acuña, Lorena¹; Bitsch, Bertram¹⁴; Brande, Jonathan¹⁵; France, Kevin¹³; Stevenson, Kevin B.⁴; Crossfield, Ian J. M.^{15, 1}; Daylan, Tansu¹⁶; Dobbs-Dixon, Ian³; Evans-Soma, Thomas M.¹⁷; Gapp, Cyril^{1, 2}; García Muñoz, Antonio¹⁸; Heng, Kevin^{19, 20, 21}; Hu, Renyu^{22, 23}; Shkolnik, Evgenya L.²⁴; Stassun, Keivan G.²⁵; Teske, Johanna^{26, 27}

1. Max Planck Institute for Astronomy
2. Heidelberg University
3. New York University Abu Dhabi
4. Johns Hopkins University Applied Physics Laboratory
5. Kyoto University
6. Space Research Institute
7. Osservatorio Astrofisico di Torino
8. University of Maryland, College Park
9. Diego Portales University
10. Wesleyan University
11. Johns Hopkins University
12. Catholic University of the North
13. Laboratory for Atmospheric and Space Physics
14. University College Cork
15. University of Kansas
16. Washington University in St. Louis
17. University of Newcastle Australia
18. University of Paris
19. Ludwig-Maximilians-Universität München
20. University College London
21. University of Warwick
22. Jet Propulsion Lab
23. California Institute of Technology
24. Arizona State University
25. Vanderbilt University
26. Carnegie Institution for Science
27. Carnegie Observatories

Abstract

Sub-Neptune exoplanets are the most abundant type of planet known today. As they do not have a Solar System counterpart, many open questions exist about their composition and formation. Previous spectroscopic studies have ruled out aerosol-free hydrogen-helium dominated atmospheres for many characterized sub-Neptunes but are inconclusive about their exact atmospheric compositions. Here we characterize the hot (T_eq=1311 K) sub-Neptune HD 86226 c (R=2.2 R_⊕, M=7.25 M_⊕), which orbits its G-type host star on a four-day orbit. The planet is located in a special part of the sub-Neptune parameter space: Its high equilibrium temperature prohibits methane-based haze formation, increasing the chances for a clear atmosphere on this planet. We used Hubble Space Telescope data taken with WFC3 and STIS from the Sub-Neptune Planetary Atmosphere Characterization Experiment (SPACE) Program to perform near-infrared (1.1–1.7 μm) transmission spectroscopy and ultraviolet characterization of the host star. We report a featureless transmission spectrum that is consistent within 0.4 σ with a constant transit depth of 418 ± 14 ppm. The amplitude of this spectrum is only 0.01 scale heights for a H/He-dominated atmosphere, excluding a cloud-free solar-metallicity atmosphere on HD 86226 c with a confidence of 6.5 σ. Based on an atmospheric retrieval analysis and forward models of cloud and haze formation, we find that the featureless spectrum could be due to metal enrichment [M/H] > 2.3 (3 σ confidence lower limit) of a cloudless atmosphere, or silicate (MgSiO₃), iron (Fe), or manganese sulfide (MnS) clouds. For these species, we performed a detailed investigation of cloud formation in high metallicity, high-temperature atmospheres. Our results highlight that HD 86226 c does not follow the aerosol trend of sub-Neptunes found by previous studies. Follow-up observations with the James Webb Space Telescope could determine whether this planet aligns with the recent detections of metal-enriched atmospheres or if it harbors a cloud species that is otherwise atypical for sub-Neptunes.

Copyright and License

© The Authors 2025. Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Open Access funding provided by Max Planck Society.

Acknowledgement

This research is based on observations made with the NASA/ESA Hubble Space Telescope obtained from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5–26555. These observations are associated with program 17192. We wish to thank Peter Gao for sharing the aerosol model spectra with us. Furthermore, we thank the anonymous referee for the detailed report that led to the investigation of many important details and a better overall clarity of the text. Q.C.T. and S.R. wish to thank Roy Kilgard for his assistance in the operation of the Wesleyan 24-inch telescope and the data storage system. K.A.K. gratefully acknowledges support from the DLR via project P.S.ASTR1508. Y.K. acknowledges support from JSPS KAKENHI Grant Numbers 21K13984, 22H05150, and 23H01224. J.S.J. gratefully acknowledges support by FONDECYT grant 1240738 and from the ANID BASAL project FB210003. P.E.C. acknowledges financial support by the Austrian Science Fund (FWF) Erwin Schroedinger Fellowship, program J4595-N. Portions of this research were carried out on the High Performance Computing resources at New York University Abu Dhabi. T.D. acknowledges support from the McDonnell Center for the Space Sciences at Washington University in St. Louis.

Data Availability

The HST data used in this paper are associated with the program GO 17192 (P.I., L. Kreidberg) and are publicly available via the Mikulski Archive for Space Telescope at https://mast.stsci.edu. Additional data products derived in this work are available via Zenodo at https://zenodo.org/records/16035649.

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