The Atmospheric Composition of Sub-Neptune K2-18 b and Implications for its Formation
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arXiv
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| Autori principali: | , , , , , , , , , , |
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| Natura: | Preprint |
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2025
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| author | Fernández-Rodríguez, Gareb Morello, Giuseppe Tan, Jonathan C. Pallé, Enric Swain, Mark R. Poultourtzidis, Efthymios Biagini, Alfredo Changeat, Quentin Jiang, Chengzi Pozuelos, Francisco J. Amado, Pedro J. |
| author_facet | Fernández-Rodríguez, Gareb Morello, Giuseppe Tan, Jonathan C. Pallé, Enric Swain, Mark R. Poultourtzidis, Efthymios Biagini, Alfredo Changeat, Quentin Jiang, Chengzi Pozuelos, Francisco J. Amado, Pedro J. |
| contents | Unlocking the atmospheres of sub-Neptunes is among JWST's major achievements, yet such observations demand complex analyses that strongly affect interpretations. We present an independent reanalysis of the original JWST transmission spectrum of K2-18 b, to assess the robustness of previously claimed detections, explore the parameter space, and implications for its formation. The observations were reduced using a combination of public and customized pipelines producing a total of 12 different versions of the transmission spectrum by varying: spectral binning, limb-darkening, and a novel correction for the occulted stellar spot. We then performed atmospheric retrievals using TauREx 3, comparing models of varying complexity, robustly detecting CH$_4$ (3-4$σ$) across all configurations. The evidence for CO$_2$ is weaker and highly model-dependent. The tentative detection of dimethyl sulphide (DMS) 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 lower mean molecular weight atmosphere. 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 atmosphere with an elevated O and an even more elevated C abundance, leading to a super-solar C/O. We show that the physical properties of the system planets K2-18 c, and K2-18 b are consistent with those expected by the in situ formation theory of Inside-Out Planet Formation (IOPF), interior to the carbon "soot" line, where an elevated C/O ratio of a primordial atmosphere is expected to be inherited from the protoplanetary disk. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2510_18098 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | The Atmospheric Composition of Sub-Neptune K2-18 b and Implications for its Formation Fernández-Rodríguez, Gareb Morello, Giuseppe Tan, Jonathan C. Pallé, Enric Swain, Mark R. Poultourtzidis, Efthymios Biagini, Alfredo Changeat, Quentin Jiang, Chengzi Pozuelos, Francisco J. Amado, Pedro J. Earth and Planetary Astrophysics Unlocking the atmospheres of sub-Neptunes is among JWST's major achievements, yet such observations demand complex analyses that strongly affect interpretations. We present an independent reanalysis of the original JWST transmission spectrum of K2-18 b, to assess the robustness of previously claimed detections, explore the parameter space, and implications for its formation. The observations were reduced using a combination of public and customized pipelines producing a total of 12 different versions of the transmission spectrum by varying: spectral binning, limb-darkening, and a novel correction for the occulted stellar spot. We then performed atmospheric retrievals using TauREx 3, comparing models of varying complexity, robustly detecting CH$_4$ (3-4$σ$) across all configurations. The evidence for CO$_2$ is weaker and highly model-dependent. The tentative detection of dimethyl sulphide (DMS) 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 lower mean molecular weight atmosphere. 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 atmosphere with an elevated O and an even more elevated C abundance, leading to a super-solar C/O. We show that the physical properties of the system planets K2-18 c, and K2-18 b are consistent with those expected by the in situ formation theory of Inside-Out Planet Formation (IOPF), interior to the carbon "soot" line, where an elevated C/O ratio of a primordial atmosphere is expected to be inherited from the protoplanetary disk. |
| title | The Atmospheric Composition of Sub-Neptune K2-18 b and Implications for its Formation |
| topic | Earth and Planetary Astrophysics |
| url | https://arxiv.org/abs/2510.18098 |