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

Abstract

Context. Hubble Space Telescope (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 in K2-18b’s atmosphere in some conditions. However, such models cannot predict the cloud cover, which is driven by atmospheric dynamics and thermal contrasts, and thus neither can they predict the real impact of clouds on spectra. Aims. 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. Methods. We simulated the atmospheric dynamics, water cloud formation, and spectra of K2-18b for a H2-dominated atmosphere using a 3D general circulation model. We analysed the impact of atmospheric composition (with metallicity from 1× solar to 1000× solar), concentration of cloud condensation nuclei, and planetary rotation rate. Results. 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 with very efficient heat redistribution. This regime preferentially leads to cloud formation at the sub-stellar point or at the terminator. Clouds form at metallicity ≥100× solar with relatively large particles (radius = 30–450 μm). At 100–300× solar metallicity, the cloud fraction at the terminators is small with a limited impact on transit spectra. At 1000× solar metallicity, very thick clouds form at the terminator, greatly flattening the transit spectrum. The cloud distribution appears very sensitive to the concentration of cloud condensation nuclei and to the planetary rotation rate, although the impact on transit spectra is modest in the near-infrared. Fitting HST transit data with our simulated spectra suggests a metallicity of ~100–300× solar, which is consistent with the mass-metallicity trend of giant planets in the Solar System. In addition, we found that the cloud fraction at the terminator can be highly variable in some conditions, leading to a potential variability in transit spectra that is correlated with spectral windows. This effect could be common on cloudy exoplanets and could be detectable with multiple transit observations. Finally, the complex cloud dynamics revealed in this study highlight the inherent 3D nature of clouds shaped by couplings between microphysics, radiation, and atmospheric circulation.

Possibilities for an Aerial Biosphere in Temperate Sub Neptune-Sized Exoplanet Atmospheres

“…Temperate sub Neptune atmospheres can have the right temperatures and pressures to maintain covalent bonds, needed for large, diverse molecules. For example, different models for K2-18b estimate atmospheric temperatures at pressures above 0.1 bar, ranging from 250 to 400 K [12,14,[49][50][51]. Note that pressure will have only a minor role on the chemical stability of metabolism compared to the effects of temperature [52].…”

“…Atmosphere models for the sub Neptune K2-18b, currently the only temperate sub Neptune with atmosphere observations, show that only water ice clouds and not liquid water clouds can form, given the exterior heating from the host star and the internal heating from interior energy [50,51]. This important point is made by [50,51], who show that K2-18 b has to have high water content, with a metallicity of greater than 100× solar, in order for water to condense, and that water condenses to the ice phase and not the liquid phase (as seen in Figure 4). Other than high water content, sub Neptunes must have lower interior energies than assumed for current K2-18 b models in order for liquid water clouds to exist, according to Figure 4.…”

“…Some life particles may re-circulate from the night side back to the planet dayside without reaching destructively hot layers. According to the simulations in [51], atmospheric circulation from the planet night side back to the dayside occurs at a range of altitudes from 0.1 down to 1 bar. At 0.1 bar, the temperatures remain around 320 K and so life particles trapped in this flow zone will survive.…”

“…Sub Neptunes slightly cooler than K2-18 b are more likely to have liquid water clouds (see Figure 4) and may even have cooler lower layers where the atmosphere recirculates. Second, the cloud location and extent, and the amount of condensed water is extremely sensitive to a number of factors [51], including particle size (which controls sedimentation rate) and atmospheric metallicity (which sets up dayside temperatures and the dayside radiative timescale, and also the advective timescale by setting up latitudinal temperature gradients). The particle size distribution itself is controlled by the number population of the underlying cloud condensation nuclei (CCN).…”

“…As a relevant aside, CCN on sub Neptune exoplanets may be externally delivered in the form of micrometeorites, or may be created in the atmosphere itself from photochemical hazes or condensed salts. For more details, see [51]. Third, despite the dominant dayside/nightside overturning pattern, there are more atmospheric dynamics including jet formation and possible sensitivity of equatorial winds to a number of assumptions that need to be explored in more modeling studies.…”