Kevin Heng

The atmospheres of sub-Neptunes provide a window into their internal structure and history, shedding light on the origin of this common, but enigmatic, class of exoplanets. However, the physical and chemical processes that shape sub-Neptunes’ transmission spectra, in particular cloud and haze formation, are not well understood. To identify possible correlations between transmission spectra and UV irradiation, the Sub-Neptune Planetary Atmosphere Characterization Experiment (SPACE) Program observed an array of sub-Neptunes and their host stars using the Hubble Space Telescope (HST), measuring the planets’ transmission spectra between 1.1 and 1.7 μ m with the Wide Field Camera 3 (WFC3) and the stars’ UV spectra with the Space Telescope Imaging Spectrograph (STIS). Here, we present the observations of HD 191939 b carried out as part of the SPACE Program, which reveal no significant spectral features in the transmission spectrum. The data deliver moderate evidence at significance levels between 2.0 σ and 3.2 σ against a cloud-free atmosphere with solar metallicity, rendering this scenario unlikely, but still possible. A supersolar metallicity of HD 191939 b might be consistent with the known trend of increasing atmospheric metallicity with decreasing planet mass. Both hydrocarbon haze formation and cloud condensation can be efficient at HD 191939 b’s zero-albedo equilibrium temperature of (880 ± 20) K, particularly in atmospheres with supersolar metallicity, possibly additionally muting absorption features.

Previous studies report apparent chemical abundance discrepancies between M dwarfs and FGK stars in the solar neighborhood, raising questions about the origin of these differences. Using a statistical pipeline, we revisit whether M dwarf abundance ratios are consistent with non–local thermodynamic equilibrium (non-LTE) measurements of FGK stars, and independently validate these results with predictions from state-of-the-art Galactic chemical evolution (GCE) models of the Milky Way’s thin disk. Abundance trends for [C, O, Na, Mg, Al, Si, Ca, Ti, Ni/H] versus [Fe/H] are derived for a benchmark sample of 47 M dwarfs from the APOGEE, CARMENES, and Subaru/InfraRed Doppler (IRD) surveys. These are compared to non-LTE measurements of FGK stars and to GCE predictions calculated with the open-source code OMEGA+ . Our predictions take into account the intrinsic metallicity scatter of the solar neighbourhood and different stellar yield sets. We find that the M dwarf trends align with FGK stars within uncertainties for most elements (C, O, Na, Al, Si, Ni), resolving prior discrepancies. The observational trends also agree well with GCE predictions. However, the Ti trends are not consistent between spectroscopic samples, likely due to systematic differences in abundance determination techniques. We also show that M dwarf abundances differ significantly between different surveys in the literature. The abundance ratios of M dwarfs and FGK stars are mostly chemically consistent in the solar neighborhood and can be effectively modeled. GCE slopes provide an independent benchmark of these compositional trends.

We investigate the geometric albedos of hot Jupiters by comparing observational data from space telescopes TESS, Kepler, CoRoT, and CHEOPS against theoretical models. The study aims to understand the distribution of observed geometric albedos across different bandpasses and how these observations align with or deviate from model predictions. We curated a comprehensive sample of observed geometric albedos, using either existing Spitzer secondary eclipse measurements or a scaling law between the equilibrium and dayside temperature to remove any contaminating thermal planetary emission. We then utilised hierarchical Bayesian modelling to identify trends with planetary properties such as equilibrium temperature, gravity, and stellar metallicity. On a population level, we found no statistical difference in the distributions of geometric albedos measured by TESS compared to those measured by Kepler, CoRoT, and CHEOPS. We confront the geometric albedo sample with a simple but first-principles model that includes Rayleigh scattering by molecular hydrogen and absorption by sodium, water, titanium oxide, and vanadium oxide. We find that the abundance of sodium and water are the key absorbers that influence the geometric albedos of hot Jupiters, whilst the addition of titanium oxide and vanadium oxide (in the absence of condensation) results in vanishing geometric albedos that are inconsistent with the observed distributions.

Context . The global circulation regime of the coolest class of brown dwarfs, known as the Y dwarfs, remains largely unexplored. Aims . We aim to investigate the interplay between convection, rotation, and cloud thermal feedback through a selected sample of Y dwarf atmospheric models. We explore a range of effective temperatures 400 K ≤ T _eff ≤ 600 K and rotation rates P _rot = 2.5–20 h. In this temperature range, salt and sulfide condensates are expected to form. We include KCl, Na ₂ S, and MnS clouds in our simulations to study their effect on the atmosphere. Our goal is to identify circulation regimes and emergent trends across this space, providing insights into the dynamical processes governing Y dwarf atmospheres. Methods . We ran a suite of twelve general circulation models (GCMs) across the outlined parameter grid. For this purpose, we developed additional physics modules for the THOR GCM to model brown dwarf atmospheres. The THOR dynamical core was coupled to modules for interior thermal perturbations near the radiative–convective boundary, a mixing-length convection scheme, a gray two-stream radiative transfer module using Rosseland-mean opacities, and simple cloud tracers with thermal feedback and scattering. Results . Across all simulations, the circulation resides in a radiative-forcing-dominated regime with weak winds, minimal horizontal temperature contrasts, and no persistent jets. Convection controls vertical mixing and sets the extent of the salt and sulfide cloud layers that form below the photosphere. Thermal structures equilibrate quickly and cloud radiative feedback remains insignificant, with limited variability. Conclusions . Y dwarf atmospheres in this parameter range, within the gray radiative transfer framework adopted in this work, remain controlled by thermal radiation from the interior, with small variability primarily set by rotation and clouds playing a secondary role. As our single-band approach does not capture spectral windows that could probe deeper into the cloud-rich layers, our constraints on cloud radiative feedback are likely conservative, and we outline possible causes and pathways toward more active regimes.

JWST is transforming our ability to characterize small exoplanets, from sub-Neptunes to rocky worlds. A key open question is whether highly irradiated rocky planets can retain atmospheres or are stripped bare by stellar irradiation—a boundary that remains to be mapped observationally. Here we present the first JWST secondary eclipse observations of the rocky exoplanet GJ 3473 b, obtained with MIRI F1500W photometry. Using four visits, we confidently detect the eclipse at an average depth of 186 ± 45 ppm, somewhat lower than expected for a blackbody. We test a wide range of data reduction and analysis assumptions and provide new insights into MIRI detector settling behavior that will benefit future observations. We model a suite of airless surfaces with varied compositions, textures, and degrees of space weathering, as well as idealized atmospheric scenarios including the possibility of atmospheric collapse. Both atmospheric and bare-rock interpretations remain consistent with the data, but we exclude thick CO ₂ atmospheres, placing a 95% credible upper limit of 1.2–6.5 bar on the surface pressure. We also find tentative evidence for visit-to-visit variability in eclipse depth (33–371 ppm), though additional data are required to confirm this. Our results highlight the challenges and intrinsic degeneracies in interpreting MIRI F1500W eclipse measurements of rocky exoplanets, indicating that such observations alone may not uniquely distinguish between bare-rock and atmospheric scenarios. Future spectroscopic or phase-curve observations will be required to determine whether or not GJ 3473 b hosts a substantial atmosphere.

The James Webb Space Telescope offers exceptional spectral resolution and wavelength coverage, which are essential for studying the coldest brown dwarfs, particularly Y dwarfs. These objects are at the cold end of the substellar sequence and exhibit atmospheric phenomena such as cloud formation, chemical disequilibrium, and radiative-convective coupling. We examined a curated sample of 22 late-T to Y dwarfs through Bayesian atmospheric retrieval (nested sampling) and supervised machine learning (random forests). Our Bayesian model comparison indicates that cloud-free models are generally favored for the hottest objects in the sample (T6-T8). Conversely, later-type dwarfs exhibit varying preferences, with both gray cloud and cloud-free models providing comparable fits. The atmospheric parameters retrieved are consistent across the applied methodologies. Evidence of vertical mixing and disequilibrium chemistry is found in several objects; notably, the Y1 dwarf WISEPAJ1541-22 favors a gray cloud model and shows elevated abundances of both CO and CO ₂ compared to equilibrium chemistry calculations. As anticipated, the abundances of H ₂ O, CH ₄ , and NH ₃ increase with decreasing effective temperature over the T-Y sequence.

Observations of transiting hot Jupiters have revealed a mismatch between the values of the Bond versus geometric albedos. In the planetary science literature, the ratio of these quantities is known as the phase integral. It has been extensively measured for the solar system planets and shown to generally be nonunity in value. We use existing Cassini data on Jupiter to derive bandpass-integrated geometric albedos and phase integrals in the CHEOPS, TESS, and Ariel bandpasses, demonstrating that these quantities vary markedly across these different wavelength ranges. By performing a population study of geometric albedos and phase integrals, we demonstrate that atmospheres with partial cloud cover may be identified using measurements of the phase integral if its measured uncertainty is ∼0.1, which corresponds to an uncertainty of ∼3% on the optical/visible secondary eclipse depth. The upcoming Ariel space mission will conduct an unprecedented statistical survey of cloud cover on hot Jupiters via the simultaneous measurement of ∼100 infrared phase curves and optical secondary eclipses. Whenever available, the shape of optical phase curves of reflected light will directly constrain the phase integral, spherical albedo, degree of cloud cover, and scattering asymmetry factor.

Photoevaporation shapes the observed radii of small exoplanets and constrains the underlying distributions of atmospheric and core masses. However, the diversity of atmospheric chemistries corresponding to these distributions remains unelucidated. We develop a first-principles carbon–hydrogen–oxygen–sulfur–silicon (CHOSSi) outgassing model that accounts for nonideal gas behavior (via fugacities) at high pressures, as well as the tendency for water and hydrogen to dissolve in melt (via solubility laws). We use data-driven radius valley constraints to establish the relationship between the atmospheric surface pressures and melt temperatures of sub-Neptunes. Sub-Neptunes with less massive rocky cores retain less of their primordial hydrogen envelopes, which leads to less heat retention and diminished melt temperatures at the surfaces of these cores. Lower melt temperatures lead thermodynamically to the dominance of carbon-, oxygen-, sulfur-, and silicon-bearing molecules over molecular hydrogen, which naturally produce a diversity of mean molecular weights. Our geochemical outgassing calculations robustly predict a gradient of mean molecular weight across the radius valley, where the strength of this gradient is primarily driven by the oxygen fugacity of the molten cores and not by the carbon enrichment (or “metallicity”) of the atmosphere. Smaller sub-Neptunes are predicted to have less hydrogen-dominated atmospheres. Establishing the precise relationship between the observed and outgassed chemistries requires an understanding of how convection near the core interacts with large-scale atmospheric circulation (driven by stellar heating) near the photosphere, as well as the influence of photochemistry.

It is an open question whether small planets around M dwarfs are able to maintain atmospheres. The Hot Rocks Survey aims to address this question by observing nine rocky exoplanets orbiting M dwarfs with MIRI emission photometry to constrain the onset of atmospheres. In this paper, we present two MIRI F1500W (15 μm) eclipses of LTT 3780 b, an ultrashort-period super-Earth (P = 0.768 days, R = 1.325 R _⊕, M = 2.46 M _⊕) that receives 111× Earth’s instellation, the highest in the survey. We find a combined eclipse depth of 312 ± 38 ppm, which is consistent between different data reduction and analysis assumptions, bolstering our confidence in the eclipse detection. This eclipse depth is consistent with the thermal emission from a bare rock surface, with a dayside temperature of T day = 114 3 − 99 + 104 K, 98% ± 9% of the maximum temperature predicted for a zero-albedo, zero heat redistribution blackbody. We are able to confidently rule out CO₂-based atmospheres down to a 0.01 bar surface pressure to greater than 3σ (ruling out an approximately Mars-like atmosphere). We are unable to rule out a pure H₂O 1 bar atmosphere, though we argue that this composition is unlikely on such a highly irradiated planet, nor O₂ atmospheres due to the lack of features in the bandpass, though we can put constraints on CO₂-mixture atmospheres. As a potential bare rock, we consider a variety of surface composition models, but are unable to distinguish between them. However, LTT 3780 b is an excellent target for follow-up JWST observations to determine its surface composition and rule out additional atmospheric compositions.