A Comprehensive Reanalysis of K2-18 b’s JWST NIRISS+NIRSpec Transmission Spectrum

Context. Recent JWST observations of the temperate sub-Neptune K2-18 b have been interpreted as suggestive of a liquid water ocean with possible biological activity. Signatures of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) have been observed in the near-infrared (using the NIRISS and NIRSpec instruments) and mid-infrared (using MIRI). However, the statistical significance of the atmospheric imprints of these potential biomarkers has yet to be quantified from a joint analysis of the entire planet spectrum. Aims. We aim to test the robustness of the proposed DMS and DMDS detections by simultaneously modeling the NIRISS and NIRSpec observations jointly with the MIRI spectrum for the first time, considering different data reductions and modeling choices. Methods. We used three well-tested pipelines to re-reduce the JWST observations and two retrieval codes to analyze the resulting transmission spectra as well as previously published data. Results. The first joint analysis of the panchromatic (0.6–12 µm) spectrum of K2-18 b finds insufficient evidence for the presence of DMS and/or DMDS in the atmosphere of the planet. We find that any marginal preferences are the result of limiting the number of molecules considered in the model and oversensitivity to small changes between data reductions. Conclusions. Our results confirm that there is no statistical significance for DMS or DMDS in K2-18 b’s atmosphere. While previous works have demonstrated this on MIRI or NIRISS/NIRSpec observations alone, our analysis of the full, panchromatic transmission spectrum does not support claims of potential biomarkers. Using the best-fitting model including DMS and/or DMDS on the published data, we estimate that ∼25 more MIRI transits would be needed for a 3σ rejection of a flat line relative to DMS and/or DMDS features in the planet’s mid-infrared transmission spectrum.

Sub-Neptunes (1.8R⊕ ≲ Rp ≲ 3.5R⊕) are the most common class of exoplanets in our galaxy, yet their interior compositions remain elusive. Proposed interior structure models include gaseous ”mini-Neptunes” with thick H2-dominated envelopes, and ”Hycean” worlds with a thin H2 atmosphere overlying a deep liquid water layer (e.g. [1, 2, 3]).The advent of the James Webb Space Telescope (JWST) has revolutionized exoplanet studies by providing high-precision near-infrared (NIR) spectroscopy, allowing us to characterise their atmospheres in unprecedented detail. Among these sub-Neptunes, K2-18b is one that has captured significant attention. Discovered in 2015 during the Kepler K2 mission [4, 5], it orbits within the habitable zone of an M dwarf, making it a prime target for studying the atmospheric composition, interior structure, and habitability of sub-Neptunes.Previous studies using Hubble Space Telescope (HST) and Spitzer data identified an H2-rich atmosphere with significant H2O absorption features, suggesting the possibility of a liquid water ocean and habitable conditions, making K2-18b highly relevant for astrobiology studies [6, 7, 8]. However, similarities between CH4 and H2O absorption features in the HST bandpass (1.1–1.7 µm) led to competing interpretations of K2-18b’s atmospheric chemistry [9, 10].JWST observations from the NIRISS SOSS and NIRSpec G395H instruments revealed strong absorption features between 0.9–5.2 µm. The original study [11] interpreted these as robust detections of CO2 (∼ 1% detected at 5σ) and CH4 (∼ 1% at 3σ) in an H2-rich atmosphere, alongside non-detections of H2O, CO, and NH3. Tentative (∼ 1σ) signs of dimethyl sulfide (DMS), a potential biosignature, were also reported. These abundances could point towards a Hycean-like scenario with a biogenic source of atmospheric CH4 [12]. Multiple studies have also argued in favour of a “mini-Neptune” scenario that is equally compatible with the JWST observations (e.g. [13, 14]). Moreover, recent independent reanalyses of the JWST data [15] reported no reliable evidence for CO2 or DMS, contradicting the original findings.Recently, new JWST observations from the MIRI LRS instrument (∼6–12 µm) were released. The original analysis reported further evidence for DMS and dimethyl disulfide (DMDS) in the atmosphere – another gas proposed as a biosignature [16]. However, emerging evidence of an abiotic pathway to DMS in cometary matter has raised doubts over the reliability of these compounds as definitive biosignatures [17].JWST’s observations have undoubtedly brought us closer to understanding the nature of K2-18b and sub-Neptunes more broadly. Nonetheless, no consensus yet exists on which model best explains K2-18b’s atmospheric composition. Although these studies have significantly expanded the realm of what we currently understand to be sub-Neptunes, the growing number of degenerate solutions highlights the need for more standardised methodologies across studies to ensure robust exoplanetary characterisation.This study aims to refine our understanding of K2-18b by addressing key factors that influence atmospheric retrievals and characterization. First, we consider the effects of uncertainties in stellar mass and radius on derived planetary parameters in retrievals and models. To improve the treatment of K2-18’s UV spectrum, we incorporate previously unused HST STIS measurements in the UV, refining the input stellar flux used in atmospheric modelling. We employ the Iraclis data reduction pipeline [7, 18], which has not yet been applied to the JWST observations of K2-18b, offering an independent method to analyse the existing data and validate the reproducibility of previous studies (e.g. [11, 15]). Up until now, atmospheric retrieval studies of K2-18b have been limited to free chemistry, which assumes no physical or chemical processes in the atmosphere. Our retrieval framework includes both free chemistry retrievals and retrievals coupled with equilibrium chemistry models, allowing us to self-consistently solve for thermochemical equilibrium, fit key parameters such as metallicity and the C/O ratio, and predict the chemical species that could form and condense based on the retrieved elemental abundances and the pressure-temperature profile. Additionally, we perform supplementary forward modelling to account for haze/cloud microphysics, disequilibrium chemistry, and radiative feedbacks, providing a more physically motivated understanding of K2-18b’s atmosphere. Our studies leverage JWST observations from NIRISS, NIRSpec, and MIRI to inform our retrievals and chemical models of constraints to the transmission spectrum. Finally, we discuss new scenarios that could explain the JWST observations of K2-18b.Our study highlights the broader implications of K2-18b as a natural laboratory for testing atmospheric retrieval methodologies and advancing our search for habitable environments beyond Earth.

The Kepler space telescope has provided exquisite data with which to perform asteroseismic analysis on evolved star ensembles. Studying star clusters offers significant insight into stellar evolution and structure, due to having a large number of stars with essentially the same age, distance, and chemical composition. This study analysed eight solar-like oscillating evolved stars that are members of the open cluster NGC 6811 and modelled them for the first time. The fundamental stellar parameters are obtained from the interior model using observational asteroseismic and non-asteroseismic constraints. The stellar interior models are constructed using the mesa evolution code. The mass-loss method is included in the interior models of the stars. The stellar masses and radius ranges of the stars are 2.23–2.40 M⊙ and 8.47–12.38 R⊙, respectively. Typical uncertainties for the mass and radius are ∼0.11 M⊙ and ∼0.09 R⊙, respectively. The model masses and radii are compared with masses and radii obtained from asteroseismic and non-asteroseismic methods (scaling relations and classic methods). The stellar ages fell in the range between 0.71 and 0.82 Gyr, with a typical uncertainty of ${\sim}18$ per cent. The model ages of the stars calculated in this study are compatible with those reported in the literature for NGC 6811.

Solid tidal friction above a liquid water reservoir as the origin of the south pole hotspot on Enceladus

The upcoming generation of extremely large ground-based telescopes and space observatories promises to transform our understanding of rocky exoplanets within the habitable zones of their stars. Observations from the James Webb Space Telescope are already challenging current models of exoplanetary atmospheres and interiors (e.g., Foley 2024). Conventionally, a habitable zone exoplanet orbits within a circumstellar region where liquid water could potentially exist on its surface (e.g., Hart 1979; Kasting et al. 1993). It is generally assumed that silicate weathering regulates atmospheric carbon dioxide (CO2) to levels supporting liquid water, providing a stabilizing feedback on climate (Walker et al. 1981); otherwise, planetary habitability would be a matter of luck. This negative feedback underpins the concept of the circumstellar habitable zone (CHZ) and may play a critical role in climate regulation on water-bearing, tectonically active rocky exoplanets. Identifying evidence for a carbon cycle on exoplanets and verifying the validity of the habitable zone concept are key objectives for future research (e.g., Bean et al. 2017)—efforts that would benefit from a deeper understanding of the carbonate–silicate cycle. In particular, the interplay between global climate, atmospheric CO2, and silicate weathering rates is not fully understood. While the role of continental weathering in this negative feedback has been extensively studied, seafloor weathering has received comparatively less attention despite its potential to be equally significant. For instance, during the Late Mesozoic—known for its hothouse climate state—seafloor weathering fluxes were comparable in magnitude to those of continental weathering (e.g., Coogan & Gillis 2013). Here, I explore the factors controlling basalt dissolution by applying the more mechanistic weathering model of Maher and Chamberlain (2014) to both the modern and Late Mesozoic upper oceanic crust.The oceanic crust, through its formation, alteration, and subduction, is a key component of this geochemical cycle. Carbon is transferred from Earth’s mantle to surface reservoirs (e.g., the atmosphere and oceans) via volcanic outgassing. Concurrently, basalt dissolution and carbonate precipitation recapture dissolved carbon from seawater, storing it within the oceanic crust. Over geological timescales, subducted tectonic plates carry these carbonates back into the mantle reservoir, ultimately supplying carbon to volcanoes. Nonetheless, the primary controls on seafloor weathering rates remain debated. Most models have historically focused on the dependence of mineral dissolution kinetics on temperature and CO2 concentrations, yet observational data suggest that both kinetic and thermodynamic factors govern global weathering fluxes, underscoring the need for models that can incorporate this dual control. Kinetic weathering models (e.g., Walker et al. 1981) fail to account for the changes in Earth’s weatherability through time (e.g., West et al. 2005). To address this, Maher & Chamberlain (2014) developed a solute transport model that integrates hydrological and tectonic influences and imposes a thermodynamic limit on weathering rates. However, this model has only occasionally been applied to continental weathering in exoplanet climate studies (e.g., Graham & Pierrehumbert 2020, 2024), and its relevance to seafloor weathering remains largely unexplored (Hakim et al. 2021).In this work, I assessed the model’s sensitivity to key parameters by comparing its predictions with observed age-dependent carbon content in the upper oceanic crust (Gillis & Coogan 2011). I then extended the model into two dimensions to represent the evolving age distribution of Earth’s seafloor and examine its impact on global weathering fluxes. Simulations using this supply-limited seafloor weathering model successfully reproduce observed age-related trends in carbon content within Earth’s upper oceanic crust and provide further evidence that over 80% of carbonate formed within 20 Myr of crust formation (Gillis & Coogan, 2011; Albers et al. 2023). This model can also capture the higher CO2 concentrations in Late Mesozoic crust compared to Cenozoic crust. Our results indicate that crustal age, permeability, porosity and fluid flow strongly influence weathering rates. These findings challenge the prevailing view that elevated temperatures primarily drove enhanced carbon uptake during the Late Mesozoic, emphasizing further the importance of incorporating geologic and hydrologic processes into climate models. Consequently, seafloor weathering emerges as a necessary process not only for understanding Earth’s past climate but also for interpreting future observations of potentially habitable rocky exoplanets.

Since the James Webb Space Telescope (JWST) became available in 2022, its observations of exoplanet transit spectra have revolutionized the field of exoplanet science. Its observations have enabled significant new discoveries, in particular, contributing to the ongoing effort of characterizing a wide array of exoplanet atmospheres [1][2][3]. Distinct data reduction pipelines have been produced to process JWST observational spectra [4][5] – which for some cases have resulted in slightly distinct transmission spectra, which may consequentially lead to distinct interpretations when characterizing said planetary atmospheres.The goal of this study is to have a grasp of the extent of how distinct data reduction pipelines affect the extraction of transmission spectra and the consequent characterization of exoplanet atmospheres. Here we present an analysis of a small array of hot Jupiter transit spectra by the JWST/NIRISS instrument [6], whose 2 major spectral orders cover the wavelength range 0.7 μm to 2.5 μm. The same dataset of transit observations for this small exoplanet population were processed through multiple pipelines [4][5].Here we explore how distinct pipelines extract distinct exoplanet transmission spectra from the same observational dataset – across a small sample of 5 hot Jupiters. We then apply the exoplanet atmospheric retrieval code TauREx 3 [7] to the distinct transmission spectra extracted by the several distinct data reduction pipelines. This allows to retrieve the distinct set of parameters that characterize these planetary atmospheres – and to compare how the distinct pipelines affect the retrieved parameters that characterize the planetary atmospheres across this small sample of hot Jupiters. This may provide a useful cross-validation between distinct data reduction approaches for this JWST instrument – increasingly relevant as new dedicated missions to the study of exoplanet atmospheres through transit spectroscopy – such as the ESA Ariel mission [8] - are expected to come online on the coming years.Figure 1: WASP-39b JWST/NIRISS transmission spectrum extracted by 6 distinct data reduction pipelines, including our group’s IRACLIS pipeline. Following the data release from Feinstein et al, 2022 [4].

The recent discovery and initial characterization of sub-Neptune-sized exoplanets that receive stellar irradiance of approximately Earth’s raised the prospect of finding habitable planets in the coming decade, because some of these temperate planets may support liquid-water oceans if they do not have massive H2/He envelopes and are thus not too hot at the bottom of the envelopes. For planets larger than Earth, and especially planets in the 1.7–3.5 R ⊕ population, the mass of the H2/He envelope is typically not sufficiently constrained to assess the potential habitability. Here we show that the solubility equilibria versus thermochemistry of carbon and nitrogen gases typically results in observable discriminators between small H2 atmospheres versus massive ones, because the condition to form a liquid-water ocean and that to achieve the thermochemical equilibrium are mutually exclusive. The dominant carbon and nitrogen gases are typically CH4 and NH3 due to thermochemical recycling in a massive atmosphere of a temperate planet, and those in a small atmosphere overlying a liquid-water ocean are most likely CO2 and N2, followed by CO and CH4 produced photochemically. NH3 is depleted in the small atmosphere by dissolution into the liquid-water ocean. These gases lead to distinctive features in the planet’s transmission spectrum, and a moderate number of transit observations with the James Webb Space Telescope should tell apart a small atmosphere versus a massive one on planets like K2-18 b. This framework thus points to a way to use near-term facilities to constrain the atmospheric mass and habitability of temperate sub-Neptune exoplanets.

A comparison of the interiors of Jupiter and Saturn

The Wide Field Camera 3 (WFC3) instrument on the Hubble Space Telescope has provided an abundance of exoplanet spectra over the years. These spectra have enabled analysis studies using atmospheric retrievals to constrain the properties of these objects. However, follow-up observations from the JWST have called into question some of the results from these older datasets, and highlighted the need to properly understand the degeneracies associated with retrievals of WFC3 spectra. In this study, we perform atmospheric retrievals of 38 transmission spectra from WFC3 and use model comparison to determine the complexity required to fit the data. We explore the effect of retrieving system parameters such as the stellar radius and planet’s surface gravity, and thoroughly investigate the degeneracies between individual model parameters – specifically the temperature, abundance of water, and cloud-top level. We focus on three case studies (HD 209458b, WASP-12b, and WASP-39b) in an attempt to diagnose some of the issues with these retrievals, in particular the low retrieved temperatures when compared to the equilibrium values. Our study advocates for the careful consideration of parameter degeneracies when interpreting retrieval results, as well as the importance of wider wavelength coverage to break these degeneracies, in agreement with previous studies. The combination of data from multiple instruments, as well as analysis from multiple data reductions and retrieval codes, will allow us to robustly characterize the atmosphere of these exoplanets.

Using reanalysis data from National Centers for Environmental Prediction/National Center for Atmospheric Research, ERA‐interim, and Hadley Centre Sea Ice and Sea Surface Temperature for the period of 1979–2012, the variations in atmospheric mass (AM) over liquid water oceans, continents, and sea‐ice‐covered Arctic regions during boreal winter are investigated. It is found that AM may migrate in a compensatory manner among these three types of surfaces on interannual time scales. There are two pairs of strong antiphase relations. One lies in a zonal orientation between the Eurasian continent and the midlatitude Pacific (referred to as Eurasian continent/Pacific antiphase relation) and exhibits a teleconnection pattern characterized by two strong correlation centers, one over Eurasia and one over the North Pacific. The other antiphase AM relation, referred to as ocean/ice‐covered Arctic antiphase relation (OIAR), exhibits a meridional orientation between the ice‐covered Arctic and liquid water oceans, including the Atlantic and Pacific. In the context of the OIAR, two teleconnection patterns are observed. One features three strong correlation centers, one each over the Mediterranean, Arctic, and North Pacific, and corresponds to AM fluctuations over liquid water oceans. The other is characterized by three strong correlation centers over the Mediterranean, the Arctic, and East Asia, and corresponds to AM fluctuations over the ice‐covered Arctic. These teleconnections are the results of thermal contrasts among the three types of surfaces. Rossby waves and vertical circulations play important roles in the formation of these teleconnections. Interestingly, these teleconnections may have significant and widespread influences on the winter climate in the Northern Hemisphere, especially in regions near the Mediterranean, the northern Eurasia, parts of North America, and East Asia.

Mildly irradiated mini-Neptunes have densities potentially consistent with them hosting substantial liquid-water oceans (“Hycean” planets). The presence of CO2 and simultaneous absence of ammonia (NH3) in their atmospheres has been proposed as a fingerprint of such worlds. JWST observations of K2-18b, the archetypal Hycean, have found the presence of CO2 and the depletion of NH3 to <100 ppm; hence, it has been inferred that this planet may host liquid-water oceans. In contrast, climate modeling suggests that many of these mini-Neptunes, including K2-18b, may likely be too hot to host liquid water. We propose a solution to this discrepancy between observation and climate modeling by investigating the effect of a magma ocean on the atmospheric chemistry of mini-Neptunes. We demonstrate that atmospheric NH3 depletion is a natural consequence of the high solubility of nitrogen species in magma at reducing conditions; precisely the conditions prevailing where a thick hydrogen envelope is in communication with a molten planetary surface. The magma ocean model reproduces the present JWST spectrum of K2-18b to ≲3σ, suggesting this is as credible an explanation for current observations as the planet hosting a liquid-water ocean. Spectral areas that could be used to rule out the magma ocean model include the >4 μm region, where CO2 and CO features dominate: magma ocean models suggest a systematically lower CO2/CO ratio than estimated from free-chemistry retrieval, indicating that deeper observations of this spectral region may be able to distinguish between oceans of liquid water and magma on mini-Neptunes.

Dimethyl sulfide (DMS) is a biogenic compound produced in sea-surface water and outgased to the atmosphere. Once in the atmosphere, DMS is a significant source of cloud condensation nuclei in the unpolluted marine atmosphere. It has been postulated that climate may be partly modulated by variations in DMS production through a DMS-cloud condensation nuclei-albedo feedback. We present here a modelled estimation of the response of DMS sea-water concentrations and DMS fluxes to climate change, following previous work on marine DMS modeling (Aumont et al., 2002) and on the global warming impact on marine biology (Bopp et al., 2001). An atmosphere—ocean general circulation model (GCM) was coupled to a marine biogeochemical scheme and used without flux correction to simulate climate response to increased greenhouse gases (a 1% increase per year in atmospheric CO2 until it has doubled). The predicted global distribution of DMS at 1 × CO2 compares reasonably well with observations; however, in the high latitudes, very elevated concentrations of DMS due to spring and summer blooms of Phaeocystis can not be reproduced. At 2 × CO2, the model estimates a small increase of global DMS flux to the atmosphere (+2%) but with large spatial heterogeneities (from −15% to +30% for the zonal mean). Mechanisms affecting DMS fluxes are changes in (1) marine biological productivity, (2) relative abundance of phytoplankton species and (3) wind intensity. The mean DMS flux perturbation we simulate represents a small negative feedback on global warming; however, the large regional changes may significantly impact regional temperature and precipitation patterns.

To determine where to search for life in our solar system or in other extrasolar systems, the concept of habitability has been developed, based on the only sample we have of a biological planet—the Earth. Habitability can be defined as the set of the necessary conditions for an active life to exist, even if it does not exist. In astronomy, a habitable zone (HZ) is the zone defined around a sun/star, where the temperature conditions allow liquid water to exist on its surface. This habitability concept can be considered from different scientific perspectives and on different spatial and time scales. Characterizing habitability at these various scales requires interdisciplinary research. In this article, we have chosen to develop the geophysical, geological, and biological aspects and to insist on the need to integrate them, with a particular focus on our neighboring planets, Mars and Venus. Important geodynamic processes may affect the habitability conditions of a planet. The dynamic processes, e.g., internal dynamo, magnetic field, atmosphere, plate tectonics, mantle convection, volcanism, thermo-tectonic evolution, meteorite impacts, and erosion, modify the planetary surface, the possibility to have liquid water, the thermal state, the energy budget, and the availability of nutrients. They thus play a role in the persistence of life on a planet. Earth had a liquid water ocean and some continental crust in the Hadean between 4.4 and 4.0 Ga (Ga: billions years ago), and may have been habitable very early on. The origin of life is not understood yet; but the oldest putative traces of life are early Archean (~3.5 Ga). Studies of early Earth habitats documented in the rock record hosting fossil life traces provide information about possible habitats suitable for life beyond Earth. The extreme values of environmental conditions in which life thrives today can also be used to characterize the “envelope” of the existence of life and the range of potential extraterrestrial habitats. The requirement of nutrients by life for biosynthesis of cellular constituents and for growth, reproduction, transport, and motility may suggest that a dynamic and rocky planet with hydrothermal activity and formation of relief, liquid water alteration, erosion, and runoff is required to replenish nutrients and to sustain life (as we know it). The concept of habitability is very Earth-centric, as we have only one biological planet to study. However, life elsewhere would most probably be based on organic chemistry and leave traces of its past or recent presence and metabolism by modifying microscopically or macroscopically the physico-chemical characteristics of its environment. The extent to which these modifications occur will determine our ability to detect them in astrobiological exploration. Looking at major steps in the evolution of life may help determining the probability of detecting life (as we know it) beyond Earth and the technology needed to detect its traces, be they morphological, chemical, isotopic, or spectral.

We present a model for the thermal evolution of Magma Ocean (MO) in interaction with a degassing atmosphere of H2O and CO2. The interior model is based on parameterized convection and is coupled to the atmospheric Model of Marcq et al. (2017, https://doi.org/10.1002/2016JE005224) through heat and volatiles. A new equation for the mass balance of volatiles is implemented, correcting Salvador et al. (2017, https://doi.org/10.1002/2017je005286). We found that the domain for water condensation is extended: for instance, depending on the cloud cover and resulting albedo, 0.13 Earth's ocean mass might be sufficient to form a water ocean on early Venus (instead of 0.3 MEO in Salvador et al. (2017, https://doi.org/10.1002/2017je005286)). Comparing our results with other recent models, we discuss the relative influence of the model hypotheses, such as mantle melting curves (which depend on mantle composition), the treatment of the atmosphere (e.g., gray or convective‐radiative) and the treatment of the last stages of the MO solidification (e.g., episodic resurfacing, stagnant lid…). We also apply our results to exoplanets. They suggest that liquid water might be present at the surface of Trappist‐1e and 1f, provided that those planets' volatile primitive contents were dominated by H2O and CO2.

The stomatal density and index of fossil Ginkgo leaves (Early Jurassic to Early Cretaceous) have been investigated to test whether these plant fossils provide evidence for CO(2)-rich atmosphere in the Mesozoic. We first assessed five sources of natural variation in the stomatal density and index of extant Gingko biloba leaves: (1) timing of leaf maturation, (2) young vs. fully developed leaves, (3) short shoots vs. long shoots, (4) position in the canopy, and (5) male vs. female trees. Our analysis indicated that some significant differences in leaf stomatal density and index were evident arising from these considerations. However, this variability was considerably less than the difference in leaf stomatal density and index between modern and fossil samples, with the stomatal index of four species of Mesozoic Ginkgo (G. coriacea, G. huttoni, G. yimaensis, and G. obrutschewii) 60-40% lower than the modern values recorded in this study for extant G. biloba. Calculated as stomatal ratios (the stomatal index of the fossil leaves relative to the modern value), the values generally tracked the CO(2) variations predicted by a long-term carbon cycle model confirming the utility of this plant group to provide a reasonable measure of ancient atmospheric CO(2) change.