Hot and cloudy: High temperature clouds in super-Earths and sub-Neptunes

TL;DR

This work investigates cloud formation on hot rocky exoplanets, using a coupled radiative-transfer, equilibrium-chemistry, and microphysical-cloud framework to predict condensates and their spectral imprints across a grid of 144 atmospheric compositions. By contrasting rainout equilibrium with a dynamic cloud model (ExoLyn), the study highlights how the oxygen budget and vertical mixing control cloud mass, extent, and grain sizes, with TiO2-dominated clouds emerging as a robust outcome for O2/CO2-like atmospheres. It finds that oxidation-rich atmospheres readily form oxide clouds that damp transmission features in the UV/optical and can influence temperature inversions, while carbon-rich atmospheres favor graphite and carbide condensates that flatten spectra more broadly. The results underscore the need for improved high-temperature optical data and for incorporating cloud-radiation feedback and magma-ocean outgassing to accurately interpret JWST-era observations of hot, rocky exoplanets such as 55 Cnc e.

Abstract

JWST observations provide for the first time evidence for an atmosphere on a rocky exoplanet - 55 Cnc e. The atmosphere of 55 Cnc e is hot with $\text{T}_{\text{eq}}>2000$K and shows strong variability, for which cloud formation above a molten crust could be one possible explanation. The composition of the atmosphere of 55 Cnc e is still unknown but suggests the presence of volatiles. We have run cloud formation models on a grid of N-dominated, O-dominated, C-dominated and H-dominated atmospheres to investigate which type of cloud we could expect on hot super-Earths and hot sub-Neptunes ($1000$K $<$ T $<$ $3000$K). Our models combine radiative transfer with equilibrium chemistry of the gaseous and condensed phases, vertical mixing of condensable species, sedimentation, nucleation and coagulation. We find that the condensability of species is highly dependent on the oxygen abundance of an atmosphere. Oxygen poor atmospheres can be heated by UV and optical absorbers PS, TiO and CN which create temperature inversions. These inhibit condensation. Oxygen rich atmospheres are colder without temperature inversions, and are therefore more favourable environments for cloud formation. The major expected cloud component in O-dominated atmospheres with solar refractory abundance is TiO$_2$(s). Spectral features of clouds in these worlds are stronger in transmission than in emission, in particular at short wavelengths. We find a lack of optical data of solid species in comparison to the variety of stable cloud components which can form on hot, rocky planets.

Figures (14)

• Figure 1: The sketch illustrates the different components of our pipeline. The main loop consists of self-consistent temperature profile and equilibrium chemistry computations achieved through iterations between HELIOS and FastChem3. Simple rainout is used in these computations to mimic the presence of a cloud in the atmosphere. For some selected cases we run the cloud loop, where we use the output of our main loop as input for the detailed cloud model ExoLyn and recompute the gas phase again with FastChem3 after cloud formation. petitRADTRANS is called last to simulate transmission spectra from the outputs of the cloud loop.
• Figure 2: The pie charts show the percentage of the five volatiles in the atmosphere of the 55 Cnc e-type atmosphere and the four carbon- and oxygen- enriched cases. Orange represents the mole fraction of atomic hydrogen, blue the oxygen fraction and yellow shows the carbon fraction. In all five models, C/O$=0.133$ and the mole fractions of nitrogen, sulphur and phosphorus are kept constant with N$=0.0008012$, S$= 0.002597867$ and P$=0.0013388$. Their sum is indicated in purple in the pie charts.
• Figure 3: Temperature structures of all models from our grid study. They are grouped according to their atmospheric type. The row indicates the dominating volatile in the atmosphere and the column indicates the C/O ratio of the model. The six blue and turquoise profiles correspond to models which have low P+S abundance. The six red and orange profiles have high P+S abundance. In each of these groups, the shade of the colour indicates the H/O ratio. The darker the blue/red shade, the lower the H/O ratio, the greener/yellower, the higher the H/O ratio. It is to note, that these colours do not indicate a numerical value of H/O ratio. The colour bar merely indicates a trend, but the lowest H/O ratio for H-atmospheres is higher than the lowest H/O ratio for O-atmospheres.
• Figure 4: Opacities of some relevant strongly UV and optical absorbing species at $2000$ K and $10^{-5}$ bar. The altitude is chosen according to where the opacity contribution is the strongest.
• Figure 5: Shown are all condensates which appear in at least one model in the full grid and reach volume mixing ratios in the atmosphere $\geq1$ ppb. Each panel represents one out of twelve atmospheric types: The row indicates the dominating volatile in the atmosphere and the column indicates the C/O ratio of the model. Each panel displays condensates as coloured bars, where each condensate has it's own attributed colour and the name is indicated in the bar. The x-axis shows the fraction of models in which each condensate appears for the corresponding atmospheric type.