The SPACE Program. I. The featureless spectrum of HD 86226 c challenges sub-Neptune atmosphere trends
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| Format: | Preprint |
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2025
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| author | Kahle, K. Angelique Blecic, Jasmina Ashtari, Reza Kreidberg, Laura Kawashima, Yui Cubillos, Patricio E. Deming, Drake Jenkins, James S. Mollière, Paul Redfield, Seth Tian, Qiushi Chris Vines, Jose I. Wilson, David J. Acuña, Lorena Bitsch, Bertram Brande, Jonathan France, Kevin Stevenson, Kevin B. Crossfield, Ian J. M. Daylan, Tansu Dobbs-Dixon, Ian Evans-Soma, Thomas M. Gapp, Cyril Muñoz, Antonio García Heng, Kevin Hu, Renyu Shkolnik, Evgenya L. Stassun, Keivan G. Teske, Johanna |
| author_facet | Kahle, K. Angelique Blecic, Jasmina Ashtari, Reza Kreidberg, Laura Kawashima, Yui Cubillos, Patricio E. Deming, Drake Jenkins, James S. Mollière, Paul Redfield, Seth Tian, Qiushi Chris Vines, Jose I. Wilson, David J. Acuña, Lorena Bitsch, Bertram Brande, Jonathan France, Kevin Stevenson, Kevin B. Crossfield, Ian J. M. Daylan, Tansu Dobbs-Dixon, Ian Evans-Soma, Thomas M. Gapp, Cyril Muñoz, Antonio García Heng, Kevin Hu, Renyu Shkolnik, Evgenya L. Stassun, Keivan G. Teske, Johanna |
| contents | 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 rule 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 (Teq=1311K) sub-Neptune HD 86226 c, which orbits its G-type host star. Its high equilibrium temperature prohibits methane-based haze formation, increasing the chances for a clear atmosphere on this planet. We use HST data taken with WFC3 and STIS from the Sub-neptune Planetary Atmosphere Characterization Experiment (SPACE) Program to perform near-infrared 1.1-1.7micrometer transmission spectroscopy and UV characterization of the host star. We report a featureless transmission spectrum that is consistent within 0.4 sigma with a constant transit depth of 418+-14ppm. 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 sigma. Based on an atmospheric retrieval analysis and forward models of cloud and haze formation, we find that the featureless spectrum could be due to a metal enrichment [M/H] above 2.3 (3 sigma confidence lower limit) of a cloudless atmosphere, or silicate (MgSiO3), iron (Fe), or manganese sulfide (MnS) clouds. For these species, we perform an investigation of cloud formation in high-metallicity, high-temperature atmospheres. Our results highlight that HD 86226c does not follow the aerosol trend of sub-Neptunes found by previous studies. Follow-up observations with the JWST could determine whether this planet aligns with the recent detections of metal-enriched atmospheres or if it harbors a cloud species otherwise atypical for sub-Neptunes. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2507_13439 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | The SPACE Program. I. The featureless spectrum of HD 86226 c challenges sub-Neptune atmosphere trends Kahle, K. Angelique Blecic, Jasmina Ashtari, Reza Kreidberg, Laura Kawashima, Yui Cubillos, Patricio E. Deming, Drake Jenkins, James S. Mollière, Paul Redfield, Seth Tian, Qiushi Chris Vines, Jose I. Wilson, David J. Acuña, Lorena Bitsch, Bertram Brande, Jonathan France, Kevin Stevenson, Kevin B. Crossfield, Ian J. M. Daylan, Tansu Dobbs-Dixon, Ian Evans-Soma, Thomas M. Gapp, Cyril Muñoz, Antonio García Heng, Kevin Hu, Renyu Shkolnik, Evgenya L. Stassun, Keivan G. Teske, Johanna Earth and Planetary Astrophysics 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 rule 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 (Teq=1311K) sub-Neptune HD 86226 c, which orbits its G-type host star. Its high equilibrium temperature prohibits methane-based haze formation, increasing the chances for a clear atmosphere on this planet. We use HST data taken with WFC3 and STIS from the Sub-neptune Planetary Atmosphere Characterization Experiment (SPACE) Program to perform near-infrared 1.1-1.7micrometer transmission spectroscopy and UV characterization of the host star. We report a featureless transmission spectrum that is consistent within 0.4 sigma with a constant transit depth of 418+-14ppm. 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 sigma. Based on an atmospheric retrieval analysis and forward models of cloud and haze formation, we find that the featureless spectrum could be due to a metal enrichment [M/H] above 2.3 (3 sigma confidence lower limit) of a cloudless atmosphere, or silicate (MgSiO3), iron (Fe), or manganese sulfide (MnS) clouds. For these species, we perform an investigation of cloud formation in high-metallicity, high-temperature atmospheres. Our results highlight that HD 86226c does not follow the aerosol trend of sub-Neptunes found by previous studies. Follow-up observations with the JWST could determine whether this planet aligns with the recent detections of metal-enriched atmospheres or if it harbors a cloud species otherwise atypical for sub-Neptunes. |
| title | The SPACE Program. I. The featureless spectrum of HD 86226 c challenges sub-Neptune atmosphere trends |
| topic | Earth and Planetary Astrophysics |
| url | https://arxiv.org/abs/2507.13439 |