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Register a new userFormation and dynamics of water clouds on temperate sub-Neptunes: The example of K2-18b
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Hubble (HST) spectroscopic transit observations of the temperate sub-Neptune K2-18b were interpreted as the presence of water vapour with potential water clouds. 1D modelling studies also predict the formation of water clouds at some conditions. However, such models cannot predict the cloud cover, driven by atmospheric dynamics and thermal contrasts, and thus their real impact on spectra. The main goal of this study is to understand the formation, distribution and observational consequences of water clouds on K2-18b and other temperate sub-Neptunes. We simulated the atmospheric dynamics, water cloud formation and spectra of K2-18b for H2-dominated atmosphere using a 3D GCM. We analysed the impact of atmospheric composition (with metallicity from 1*solar to 1000*solar), concentration of cloud condensation nuclei and planetary rotation rate. Assuming that K2-18b has a synchronous rotation, we show that the atmospheric circulation in the upper atmosphere essentially corresponds to a symmetric day-to-night circulation. This regime preferentially leads to cloud formation at the substellar point or at the terminator. Clouds form for metallicity >100*solar with relatively large particles. For 100-300*solar metallicity, the cloud fraction at the terminators is small with a limited impact on transit spectra. For 1000*solar metallicity, very thick clouds form at the terminator. The cloud distribution appears very sensitive to the concentration of CCN and to the planetary rotation rate. Fitting HST transit data with our simulated spectra suggests a metallicity of ~100-300*solar. In addition, we found that the cloud fraction at the terminator can be highly variable, leading to a potential variability in transit spectra. This effect could be common on cloudy exoplanets and could be detectable with multiple transit observations.
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Results from the Kepler mission indicate that the occurrence rate of small planets ($<3$ $R_oplus$) in the habitable zone of nearby low-mass stars may be as high as 80%. Despite this abundance, probing the conditions and atmospheric properties on any habitable-zone planet is extremely difficult and has remained elusive to date. Here, we report the detection of water vapor and the likely presence of liquid and icy water clouds in the atmosphere of the $2.6$ $R_oplus$ habitable-zone planet K2-18b. The simultaneous detection of water vapor and clouds in the mid-atmosphere of K2-18b is particularly intriguing because K2-18b receives virtually the same amount of total insolation from its host star ($1368_{-107}^{+114}$ W m$^{-2}$) as the Earth receives from the Sun (1361 W m$^{-2}$), resulting in the right conditions for water vapor to condense and explain the detected clouds. In this study, we observed nine transits of K2-18b using HST/WFC3 in order to achieve the necessary sensitivity to detect the water vapor, and we supplement this data set with Spitzer and K2 observations to obtain a broader wavelength coverage. While the thick hydrogen-dominated envelope we detect on K2-18b means that the planet is not a true Earth analog, our observations demonstrate that low-mass habitable-zone planets with the right conditions for liquid water are accessible with state-of-the-art telescopes.
The atmospheric composition of exoplanets with masses between 2 and 10 M$_oplus$ is poorly understood. In that regard, the sub-Neptune K2-18b, which is subject to Earth-like stellar irradiation, offers a valuable opportunity for the characterisation of such atmospheres. Previous analyses of its transmission spectrum from the Kepler, Hubble (HST), and Spitzer space telescopes data using both retrieval algorithms and forward-modelling suggest the presence of H$_2$O and an H$_2$--He atmosphere, but have not detected other gases, such as CH$_4$. We present simulations of the atmosphere of K2-18 b using Exo-REM, our self-consistent 1D radiative-equilibrium model, using a large grid of atmospheric parameters to infer constraints on its chemical composition. We show that our simulations favour atmospheric metallicities between 40 and 500 times solar and indicate, in some cases, the formation of H$_2$O-ice clouds, but not liquid H$_2$O clouds. We also confirm the findings of our previous study, which showed that CH$_4$ absorption features nominally dominate the transmission spectrum in the HST spectral range. We compare our results with results from retrieval algorithms and find that the H$_2$O-dominated spectrum interpretation is either due to the omission of CH$_4$ absorptions or a strong overfitting of the data. Finally, we investigated different scenarios that would allow for a CH$_4$-depleted atmosphere. We were able to fit the data to those scenarios, finding, however, that it is very unlikely for K2-18b to have a high internal temperature. A low C/O ratio ($approx$ 0.01--0.1) allows for H$_2$O to dominate the transmission spectrum and can fit the data but so far, this set-up lacks a physical explanation. Simulations with a C/O ratio $<$ 0.01 are not able to fit the data satisfactorily.
The recent discovery and initial characterization of sub-Neptune-sized exoplanets that receive stellar irradiance of approximately Earths 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_Earth 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 vs. thermochemistry of carbon and nitrogen gases results in observable discriminators between small H2 atmospheres vs. 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 planets transmission spectrum, and a moderate number of repeated transit observations with the James Webb Space Telescope should tell apart a small atmosphere vs. a massive one on planets like K2-18 b. This method thus provides a way to use near-term facilities to constrain the atmospheric mass and habitability of temperate sub-Neptune exoplanets.
M-dwarf stars are promising targets for identifying and characterizing potentially habitable planets. K2-3 is a nearby (45 pc), early-type M dwarf hosting three small transiting planets, the outermost of which orbits close to the inner edge of the stellar (optimistic) habitable zone. The K2-3 system is well suited for follow-up characterization studies aimed at determining accurate masses and bulk densities of the three planets. Using a total of 329 radial velocity measurements collected over 2.5 years with the HARPS-N and HARPS spectrographs and a proper treatment of the stellar activity signal, we aim to improve measurements of the masses and bulk densities of the K2-3 planets. We use our results to investigate the physical structure of the planets. We analyse radial velocity time series extracted with two independent pipelines by using Gaussian process regression. We adopt a quasi-periodic kernel to model the stellar magnetic activity jointly with the planetary signals. We use Monte Carlo simulations to investigate the robustness of our mass measurements of K2-3,c and K2-3,d, and to explore how additional high-cadence radial velocity observations might improve them. Despite the stellar activity component being the strongest signal present in the radial velocity time series, we are able to derive masses for both planet b ($M_{rm b}=6.6pm1.1$ $M_{rm oplus}$) and planet c ($M_{rm c}=3.1^{+1.3}_{-1.2}$ $M_{rm oplus}$). The Doppler signal due to K2-3,d remains undetected, likely because of its low amplitude compared to the radial velocity signal induced by the stellar activity. The closeness of the orbital period of K2-3,d to the stellar rotation period could also make the detection of the planetary signal complicated. [...]
WASP-18b is an utra-hot Jupiter with a temperature difference of upto 2500K between day and night. Such giant planets begin to emerge as planetary laboratory for understanding cloud formation and gas chemistry in well-tested parameter regimes in order to better understand planetary mass loss and for linking observed element ratios to planet formation and evolution. We aim to understand where clouds form, their interaction with the gas phase chemistry through depletion and enrichment, the ionisation of the atmospheric gas and the possible emergence of an ionosphere on ultra-hot Jupiters. We utilize 1D profiles from a 3D atmosphere simulations for WASP-18b as input for kinetic cloud formation and gas-phase chemical equilibrium calculations. We solve our kinetic cloud formation model for these 1D profiles that sample the atmosphere of WASP-18b at 16 different locations along the equator and in the mid-latitudes and derive consistently the gas-phase composition. The dayside of WASP-18b emerges as completely cloud-free due to the very high atmospheric temperatures. In contrast, the nightside is covered in geometrically extended and chemically heterogeneous clouds with disperse particle size distributions. The atmospheric C/O increases to $>0.7$ and the enrichment of the atmospheric gas with cloud particles is $rho_{rm d}/rho_{rm gas}>10^{-3}$. The clouds that form at the limbs appear located farther inside the atmosphere and they are the least extended. Not all day-night terminator regions form clouds. The gas-phase is dominated by H$_2$, CO, SiO, H$_2$O, H$_2$S, CH$_4$, SiS. In addition, the dayside has a substantial degree of ionisation due to ions like Na$^+$, K$^+$, Ca$^+$, Fe$^+$. Al$^+$ and Ti$^+$ are the most abundant of their element classes. We find that WASP-18b, as one example for ultra-hot Jupiters, develops an ionosphere on the dayside.
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