Formation and dynamics of water clouds on temperate sub-Neptunes: The example of K2-18b

Abstract

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.

Accepted for publication in A&A. 16 pages, 14 figures

References in corpus (22)

• Habitable Zones Around Main-Sequence Stars: Dependence on Planetary Mass
• Growth Model Interpretation of Planet Size Distribution
• A Precise Water Abundance Measurement for the Hot Jupiter WASP-43b
• The habitability of Proxima Centauri b II. Possible climates and Observability
• ExoMol molecular line lists XXXV: a rotation-vibration line list for hot ammonia
• A Sub-Neptune Exoplanet with a Low-Metallicity Methane-Depleted Atmosphere and Mie-Scattering Clouds
• HAT-P-26b: A Neptune-Mass Exoplanet with a Well Constrained Heavy Element Abundance
• ExoMol line lists VII: The rotation-vibration spectrum of phosphine up to 1500 K
• Condensation-inhibited convection in hydrogen-rich atmospheres: Stability against double-diffusive processes and thermal profiles for Jupiter, Saturn, Uranus, and Neptune
• Impact of Clouds and Hazes on the Simulated JWST Transmission Spectra of Habitable Zone Planets in the TRAPPIST-1 System
• The interior and atmosphere of the habitable-zone exoplanet K2-18b
• Effects of a fully 3D atmospheric structure on exoplanet transmission spectra: retrieval biases due to day-night temperature gradients
• Spitzer Observations Confirm and Rescue the Habitable-Zone Super-Earth K2-18b for Future Characterization
• 1D atmospheric study of the temperate sub-Neptune K2-18b
• Toward the analysis of JWST exoplanet spectra: Identifying troublesome model parameters
• Photochemical hazes in sub-Neptunian atmospheres with focus on GJ 1214 b
• Seeing above the Clouds with High Resolution Spectroscopy
• A theoretical room-temperature line list for 15NH3
• The high-energy environment and atmospheric escape of the mini-Neptune K2-18 b
• Consistently Simulating a Wide Range of Atmospheric Scenarios for K2-18b with a Flexible Radiative Transfer Module
• The First Habitable Zone Earth-sized Planet from TESS. III: Climate States and Characterization Prospects for TOI-700 d
• Formation of secondary atmospheres on terrestrial planets by late disk accretion

Cited by in corpus (34)

• Day-night cloud asymmetry prevents early oceans on Venus but not on Earth
• How to identify exoplanet surfaces using atmospheric trace species in hydrogen-dominated atmospheres
• Unveiling shrouded oceans on temperate sub-Neptunes via transit signatures of solubility equilibria vs. gas thermochemistry
• The runaway greenhouse on subNeptune waterworlds
• Peering into the Young Planetary System AB Pic. Atmosphere, Orbit, Obliquity & Second Planetary Candidate
• A 3D picture of moist-convection inhibition in hydrogen-rich atmospheres: Implications for K2-18 b
• Methane as a dominant absorber in the habitable-zone sub-Neptune K2-18 b
• Water Condensation Zones around Main Sequence Stars
• Water Ice Cloud Variability & Multi-Epoch Transmission Spectra of TRAPPIST-1e
• 3D convection-resolving model of temperate, tidally-locked exoplanets
• Data availability and requirements relevant for the Ariel space mission and other exoplanet atmosphere applications
• Possibilities for an Aerial Biosphere in Temperate Sub Neptune-Sized Exoplanet Atmospheres
• Toward a multidimensional analysis of transmission spectroscopy. Part I: Computation of transmission spectra using a 1D, 2D, or 3D atmosphere structure
• A survey of exoplanet phase curves with Ariel
• Atmospheric properties of AF Lep b with forward modeling
• Microphysics of Water Clouds in the Atmospheres of Y Dwarfs and Temperate Giant Planets
• Planetary evolution with atmospheric photoevaporation II: Fitting the slope of the radius valley by combining boil-off and XUV-driven escape
• The transmission spectrum of WASP-17 b from the optical to the near-infrared wavelengths: combining STIS, WFC3 and IRAC datasets
• GJ 3090 b: one of the most favourable mini-Neptune for atmospheric characterisation
• Atmospheric dynamics of temperate sub-Neptunes. Part I: dry dynamics
• YunMa: Enabling Spectral Retrievals of Exoplanetary Clouds
• Greater climate sensitivity and variability on TRAPPIST-1e than Earth
• The Possibility of Hydrogen-Water Demixing in Uranus, Neptune, K2-18b and TOI-270d
• A Population Analysis of 20 Exoplanets Observed from the Optical to the Near-infrared Wavelengths with HST: Evidence for Widespread Stellar Contamination
• Deciphering Sub-Neptune Atmospheres: New Insights from Geochemical Models of TOI-270 d
• Unraveling the non-equilibrium chemistry of the temperate sub-Neptune K2-18 b
• Tighter constraints on the atmosphere of GJ 436 b from combined high-resolution CARMENES and CRIRES${}^{+}$ observations
• Investigating aerosols as a way to reconcile K2-18 b JWST MIRI and NIRISS/NIRSpec observations
• Three-dimensional Transport-induced Chemistry on Temperate sub-Neptune K2-18b, Part I: the Effects of Atmospheric Dynamics
• The Diversity of Exoplanetary Environments and the Search for Signs of Life Beyond Earth
• Photochemistry and Spectral Characterization of Temperate and Gas-Rich Exoplanets
• Possible favored Great Oxidation Event scenario on exoplanets around M-Stars with the example of TRAPPIST-1e
• Three-dimensional transport-induced chemistry on temperate sub-Neptune K2-18b, Part II: the combined effects of atmospheric dynamics and chemical reactions
• Impact of Clouds on the Atmosphere-Mantle Interface of Sub-Neptunes