TY - JOUR
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 - García Muñoz, Antonio
AU - Heng, Kevin
AU - Hu, Renyu
AU - Shkolnik, Evgenya L.
AU - Stassun, Keivan G.
AU - Teske, Johanna
N1 - Publisher Copyright:
© The Authors 2025.
PY - 2025/9/1
Y1 - 2025/9/1
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 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 (Teq=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.
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 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 (Teq=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.
KW - planets
KW - planets and satellites: atmospheres
KW - planets and satellites: gaseous planets
KW - satellites: individual: HD 86226 c
KW - techniques: spectroscopic
UR - https://www.scopus.com/pages/publications/105016454704
U2 - 10.1051/0004-6361/202554916
DO - 10.1051/0004-6361/202554916
M3 - Article
AN - SCOPUS:105016454704
SN - 0004-6361
VL - 701
JO - Astronomy and Astrophysics
JF - Astronomy and Astrophysics
M1 - A184
ER -