TY - UNPB
T1 - The SPACE Program. I. The featureless spectrum of HD 86226 c challenges sub-Neptune atmosphere trends
AU - Kahle, K. Angelique
AU - Blecic, Jasmina
AU - Ashtari, Reza
AU - Kreidberg, Laura
AU - Kawashima, Yui
AU - Cubillos, Patricio E.
AU - Deming, Drake
AU - Jenkins, James S.
AU - Mollière, Paul
AU - Redfield, Seth
AU - Tian, Qiushi Chris
AU - Vines, Jose I.
AU - Wilson, David J.
AU - Acuña, Lorena
AU - Bitsch, Bertram
AU - Brande, Jonathan
AU - France, Kevin
AU - Stevenson, Kevin B.
AU - Crossfield, Ian J.M.
AU - Daylan, Tansu
AU - Dobbs-Dixon, Ian
AU - Evans-Soma, Thomas M.
AU - Gapp, Cyril
AU - Garćıa Muñoz, Antonio
AU - Heng, Kevin
AU - Hu, Renyu
AU - Shkolnik, Evgenya L.
AU - Stassun, Keivan G.
AU - Teske, Johanna
PY - 2025/7/17
Y1 - 2025/7/17
N2 - 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.
AB - 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.
U2 - 10.48550/arXiv.2507.13439
DO - 10.48550/arXiv.2507.13439
M3 - Preprint
T3 - arXiv e-prints
BT - The SPACE Program. I. The featureless spectrum of HD 86226 c challenges sub-Neptune atmosphere trends
ER -