The atmospheric composition of the sub-Neptune K2-18b and insights into its formation
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Le résumé fourni par la source
Unlocking the atmospheres of sub-Neptune planets is among the revolutionary accomplishments of JWST. However, such observations require complex data analysis methodologies, which have a strong impact on the derived conclusions. Here, we present an independent re-analysis of the JWST transmission spectrum of the temperate sub-Neptune K2-18 b, aimed at assessing the robustness of previously claimed atmospheric detections, exploring the planet’s possible parameter space, and determining the implications behind its formation. The NIRISS/SOSS and NIRSpec/G395H observations were reduced using a combination of public and customised pipelines. We produced a total of 12 different versions of the transmission spectrum by varying key steps: spectral binning, limb-darkening treatment, and the application of a novel correction for an occulted stellar spot, as well as error inflation and instrumental offsets. We performed atmospheric retrievals using TauREx 3, comparing models of varying complexity. We robustly detected CH 4 at a >3σ significance across the majority of reduction and retrieval setups. Hints of CO 2 appear in most configurations, but the evidence is weak and strongly model-dependent. The tentative detection of dimethyl sulphide, reported in previous studies, vanishes in our most comprehensive retrieval models. We find that correcting the stellar spot in the NIRISS transit is a critical step, introducing a uniform offset that primarily drives the inference of a lighter atmosphere characterised by a lower mean molecular weight. Furthermore, the assumed complexity of the retrieval model itself introduces significant biases; including more molecules systematically increases the retrieved CH 4 abundance and atmospheric mean molecular weight, even for species without spectral features. The data are consistent with a hydrogen-rich (i.e. primordial) atmosphere with an elevated O abundance and an even more elevated C abundance, leading to a C/O ratio that is significantly greater than solar. We show that the physical properties of the K2-18 system planets (i.e. the innermost planet, K2-18 c, and K2-18 b) are consistent with those expected by the in situ formation theory of inside-out planet formation. Furthermore, these properties predict a temperature of K2-18 b at the time of formation of ≳500 K. This is much warmer than the current equilibrium temperature and just interior to the carbon ‘soot’ line, where an elevated C/O ratio and a correspondingly volatile-poor (dry) atmospheric composition is expected to be inherited from the protoplanetary disk, arguing against an Hycean world scenario.
Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.
Le contrôle bibliographique ouvert
DOI retrouvé dans Crossref DOI retrouvé ; titre concordant.
- Titre Crossref
- The atmospheric composition of the sub-Neptune K2-18b and insights into its formation
- Date Crossref
- 01/06/2026
- Éditeur
- EDP Sciences
- Type
- journal-article
Ce recoupement confirme des métadonnées liées au DOI. Il ne confirme ni la méthode ni les conclusions de l’étude, et il ne compte pas comme une seconde source scientifique indépendante.
Où se fait cette recherche
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Universidad de La Laguna pays non établi dans la noticeUniversité ou école supérieure
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Instituto de Astrofísica de Canarias pays non établi dans la noticeStructure de recherche
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Chalmers University of Technology Earth & Environment pays non établi dans la noticeUniversité ou école supérieure
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Osservatorio Astronomico di Palermo pays non établi dans la noticeStructure de recherche
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Astronomical Observatory pays non établi dans la noticeStructure de recherche
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Instituto de Astrofísica de Andalucía pays non établi dans la noticeStructure de recherche
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Institute of Astronomy pays non établi dans la noticeStructure de recherche
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Jet Propulsion Laboratory pays non établi dans la noticeStructure de recherche
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Aristotle University of Thessaloniki Department of Physics pays non établi dans la noticeUniversité ou école supérieure
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University of Groningen Kapteyn Institute pays non établi dans la noticeUniversité ou école supérieure
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Departamento de Astrofísica pays non établi dans la noticeInstitution
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University of Virginia Dept. of Astronomy & Virginia Institute for Theoretical Astronomy pays non établi dans la noticeUniversité ou école supérieure
Universidad de La Laguna, Instituto de Astrofísica de Canarias et Earth & Environment — Chalmers University of Technology, avec 9 autres affiliations.
Une affiliation ne permet pas de déduire la nationalité d’un auteur.