The SPACE Program: I. The featureless spectrum of HD 86226 c challenges sub-Neptune atmosphere trends
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 characte...
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| Main Authors: | , , , , , , , , , , , , , , , , , , , , , , , , , , , , |
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| Format: | Article (Journal) |
| Language: | English |
| Published: |
15 September 2025
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| In: |
Astronomy and astrophysics
Year: 2025, Volume: 701, Pages: 1-21 |
| ISSN: | 1432-0746 |
| DOI: | 10.1051/0004-6361/202554916 |
| Online Access: | Verlag, kostenfrei, Volltext: https://doi.org/10.1051/0004-6361/202554916 Verlag, kostenfrei, Volltext: https://www.aanda.org/articles/aa/abs/2025/09/aa54916-25/aa54916-25.html 
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| Author Notes: | K. Angelique Kahle, Jasmina Blecic, Reza Ashtari, Laura Kreidberg, Yui Kawashima, Patricio E. Cubillos, Drake Deming, James S. Jenkins, Paul Mollière, Seth Redfield, Qiushi Chris Tian, Jose I. Vines, David J. Wilson, Lorena Acuña, Bertram Bitsch, Jonathan Brande, Kevin France, Kevin B. Stevenson, Ian J.M. Crossfield, Tansu Daylan, Ian Dobbs-Dixon, Thomas M. Evans-Soma, Cyril Gapp, Antonio García Muñoz, Kevin Heng, Renyu Hu, Evgenya L. Shkolnik, Keivan G. Stassun, and Johanna Teske |
| Summary: | 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. |
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| Item Description: | Gesehen am 20.01.2026 |
| Physical Description: | Online Resource |
| ISSN: | 1432-0746 |
| DOI: | 10.1051/0004-6361/202554916 |