The SPACE Program

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

¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
² Department of Physics and Astronomy, Heidelberg University, Im Neuenheimer Feld 226, 69120 Heidelberg, Germany
³ Department of Physics, New York University Abu Dhabi, Abu Dhabi, UAE
⁴ Center for Astrophysics and Space Science (CASS), New York University Abu Dhabi, Abu Dhabi, UAE
⁵ JHU Applied Physics Laboratory, 11100 Johns Hopkins Rd, Laurel, MD 20723, USA
⁶ Department of Astronomy, Graduate School of Science, Kyoto University, Kyoto, Japan
⁷ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
⁸ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁹ University of Maryland, College Park, MD, USA
¹⁰ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
¹¹ Centro de Astrofísica y Tecnologías Afines (CATA), Casilla 36-D, Santiago, Chile
¹² Astronomy Department and Van Vleck Observatory, Wesleyan University, Middletown, CT, USA
¹³ Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD, USA
¹⁴ Instituto de Astronomía, Universidad Católica del Norte, Angamos 0610, 1270709 Antofagasta, Chile
¹⁵ Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA
¹⁶ Department of Physics, University College Cork, Cork, Ireland
¹⁷ Department of Physics and Astronomy, University of Kansas, 1082 Malott Hall, 1251 Wescoe Hall Dr, Lawrence, KS, USA
¹⁸ Department of Physics, Washington University, St. Louis, MO 63130, USA
¹⁹ McDonnell Center for the Space Sciences, Washington University, St. Louis, MO 63130, USA
²⁰ School of Information and Physical Sciences, University of Newcastle, Callaghan, NSW, Australia
²¹ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
²² Faculty of Physics, Ludwig Maximilian University, Munich, Germany
²³ Department of Physics & Astronomy, University College London, UK
²⁴ Department of Physics, University of Warwick, Coventry, UK
²⁵ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²⁶ Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
²⁷ School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85281, USA
²⁸ Department of Physics & Astronomy, Vanderbilt University, Nashville, TN, USA
²⁹ Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC, USA
³⁰ Observatories, Carnegie Institution for Science, Pasadena, CA, USA

^★ Corresponding author: kahle@mpia.de

Received: 31 March 2025
Accepted: 13 July 2025

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.