The SPACE Program

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 (MgSiO 3 ), 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.

Authors

Publication Details

Journal
Astronomy and Astrophysics
Published
2025-08-14
DOI
https://doi.org/10.1051/0004-6361/202554916
Citations
1
Primary Topic
Astro and Planetary Science
Type
article
Field-Weighted Citation Impact
1.15

Funders

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The SPACE Program

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1 citations
Astronomy and Astrophysics
Astro and Planetary Science
1.15
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The SPACE Program

Reza Ashtari, Tansu Daylan, K. Angelique Kahle, Kevin Heng, Jasmina Blecic, Bertram Bitsch, Kevin B. Stevenson, Laura Kreidberg, Kevin France, Ian J. M. Crossfield, Renyu Hu, J. S. Jenkins, Jonathan Brande, Seth Redfield, Johanna Teske, Drake Deming, T. M. Evans, Evgenya L. Shkolnik, Ian Dobbs‐Dixon, L. Acuña, Yui Kawashima, José I Vines, Patricio E. Cubillos, P. Mollière, Cyril Gapp, Keivan G. Stassun, David J. Wilson, Qiushi Tian
article en
1 citations

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 (MgSiO 3 ), 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.

Astronomy and AstrophysicsVol. 701
National Aeronautics and Space Administration, Washington University in St. Louis, New York University Abu Dhabi, Space Telescope Science Institute, York University, Austrian Science Fund, Deutsches Zentrum für Luft- und Raumfahrt, Agencia Nacional de Investigación y Desarrollo, Japan Society for the Promotion of Science
Openalex Percentile: Top 17%
Astro and Planetary Science
1.15
DOIOpen Access PDFOpenAlex
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