Hamish Innes

Rocky exoplanets around M dwarfs have been investigated by many theoretical and observational studies in the recent years. In contrast to close-in, hot rocky exoplanets often termed "exo-Venuses", recent observational campaigns suggest that many of these planets may either not posses an atmosphere, or only a very thin atmosphere either full of hazes or not made of CO2, therefore not resembling Venus. In this study we aim at predicting possible atmospheric evolutionary pathways of such planetary atmospheres starting from compositional and evolutionary constraints of the star-planet system (including redox profiles in the interior set by core formation). We then model the evolution of the atmospheres depending on outgassing from the interior, atmospheric chemical equilibrium, greenhouse effects, water condensation incl. the atmospheric cold trap, solubility of volatiles in the melt, and atmospheric escape. We show that depending on various key factors such as planetary mass, initial bulk redox state, magma ocean evolution and atmospheric escape efficiency, different types of atmospheres are predicted, with CO2-dominated, Venus-like atmospheres being restricted to specific scenarios, only, while in other scenarios for example bare-rock atmospheres or thin N2-O2-dominated, abiotically generated atmospheres will be more likely, suggesting a wide diversity of "exo-Venus" atmospheres.

In the past five years, there has been increasing interest in sub-Neptune exoplanets in the habitable zone of their stars. These planets are abundant and easier to observe via transmission spectroscopy owing to their hydrogen-rich atmospheres. Claimed biosignature detections on the sub-Neptune K2-18 b also motivate further studies into whether their climates are suitable for life. Whether it is possible for sub-Neptunes in the traditional Earth-like habitable zone to have liquid water oceans (and be a “Hycean” planet) is debated. One possible limiting factor is the inhibition of convection in which radiative layers forming in condensing regions (due to mean molecular weight gradients) make planetary interiors too hot for oceans to condense. In this work we aim to determine the conditions under which ocean formation on a sub-Neptune can occur, assuming that the hydrogen-water envelope evolves from a hot, well-mixed initial state. To determine the conditions under which oceans will form on a sub-Neptune, we calculate the atmospheric structure at which an interior adiabat passes through the critical point of water (for a given deep water composition). Temperature-pressure profiles warmer than this “critical profile” will form water clouds in the upper atmosphere; profiles colder than this will bubble out hydrogen from a water rich ocean state. To calculate the critical profile, we use a non-ideal Peng-Robinson equation of state to calculate the moist adiabatic gradient in adiabatic regions, and Rosseland mean opacities to calculate the temperature gradient in the deeper regions where convection is inhibited. We explore the parameter space of sub-Neptunes by varying the temperature at 1 bar, the internal heat flux and the background composition. We find that the critical ocean-forming state must have an extremely cold upper atmosphere if there are convectively inhibited layers. At 1 bar, the temperature in most simulations is

The TRAPPIST-1 system has already been intensely studied by the JWST in the recent years, and the first successful observations hint at that the innermost planets of the system may not even possess an atmosphere [1,2], or if then only in combination with photochemical hazes [3]. To evaluate the range of possible atmospheric pathways for the different rocky planets in the TRAPPIST-1 system and compare them with the most recent observations, we conduct a large parameter study in which we first model the potential interior thermal evolution of the TRAPPIST-1 planets using 2D mantle convection simulations, by employing planetary compositions and interior structures as derived in [4] assuming that all seven planets are rocky planets (i.e. without large fractions of volatiles), which is still in line with the observed masses and radii [4]. We take into account initially super-heated cores from the planet accretion phase, as well as heating by radiogenic decay, tidal heating and induction heating from the star [5]. Melting in the interior leads to volcanic activity, with outgassing depending on the mantle redox state, melting temperature, melt extrusion efficiency, gas speciation in the melt, solubility of volatiles in the melt, and atmospheric chemical evolution (assuming chemical equilibrium in the atmosphere). The evolving atmospheric compositions are modelled under various atmospheric escape scenarios to assess their potential survivability.The absence of an atmosphere for the innermost planets may then be explained most easily if they possess a low redox state in their interior mantles, comparable to Mercury, leading to low-mean-molecular-weight and low-pressure atmospheres [6] that are less stable against atmospheric escape. Here we show the full range of possible secondary outgassed atmospheres in the TRAPPIST-1 predicted by our coupled thermo-chemical interior-atmosphere model. [1] Greene, T.P., Bell, T.J., Ducrot, E. et al. (2023). Thermal emission from the Earth-sized exoplanet TRAPPIST-1 b using JWST. Nature 618, 39–42.[2] Zieba, S., Kreidberg, L., Ducrot, E. et al. (2023). No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c. Nature 620, 746–749.[3] Ducrot, E., Lagage, P.O., Min, M. et al. (2024). Combined analysis of the 12.8 and 15 μm JWST/MIRI eclipse observations of TRAPPIST-1 b. Nature Astronomy, doi: 10.1038/s41550-024-02428-z.[4] Carone, L., Barnes, R., Noack, L., et al. (2025). From CO2-to H2O-dominated atmospheres and back-How mixed outgassing changes the volatile distribution in magma oceans around M dwarf stars. Astronomy & Astrophysics, 693, A303.[5] Kislyakova, K. G., Noack, L., Johnstone, et al. (2017). Magma oceans and enhanced volcanism on TRAPPIST-1 planets due to induction heating. Nature Astronomy, 1(12), 878-885.[6] Brachmann, C., Noack, L., Baumeister, P.A., Sohl, F. (2025). Distinct types of CHON atmospheres and surface pressures depending on melt redox state and outgassing efficiency. Icarus, 429, 116450.

Sub-Neptunes, the most common planet type, remain poorly understood. Their atmospheres are expected to be diverse, but their compositions are challenging to determine, even with JWST. Here, we present the first JWST spectroscopic study of the warm sub-Neptune GJ 3090 b (2.13 R _⊕, T _{eq,A = 0.3} ∼ 700 K), which orbits an M2V star, making it a favorable target for atmosphere characterization. We observed four transits of GJ 3090 b: two each using JWST NIRISS/SOSS and NIRSpec/G395H, yielding wavelength coverage from 0.6 to 5.2 μm. We detect the signature of the 10833 Å metastable helium triplet at a statistical significance of 5.5σ with an amplitude of 434 ± 79 ppm, marking the first such detection in a sub-Neptune with JWST. This amplitude is significantly smaller than predicted by solar-metallicity forward models, suggesting a metal-enriched atmosphere that decreases the mass-loss rate and attenuates the helium feature amplitude. Moreover, we find that stellar contamination, in the form of the transit light source effect, dominates the NIRISS transmission spectra, with unocculted spot and faculae properties varying across the two visits separated in time by approximately 6 months. Free retrieval analyses on the NIRSpec/G395H spectrum find tentative evidence for highly muted features and a lack of CH₄. These findings are best explained by a high-metallicity atmosphere (>100× solar at 3σ confidence for clouds at ∼μbar pressures) using chemically consistent retrievals and self-consistent model grids. Further observations of GJ 3090 b are needed for tighter constraints on the atmospheric abundances and to gain a deeper understanding of the processes that led to its potential metal enrichment.

Temperate exoplanets between the sizes of Earth and Neptune, known as “sub-Neptunes,” have emerged as intriguing targets for astrobiology. It is unknown whether these planets resemble Earth-like terrestrial worlds with a habitable surface, Neptune-like giant planets with deep atmospheres and no habitable surface, or something exotic in between. Recent JWST transmission spectroscopy observations of the canonical sub-Neptune, K2-18 b, revealed ~1% CH₄, ~1% CO₂, and a nondetection of CO in the atmosphere. While previous studies proposed that the observed atmospheric composition could help constrain the lower atmosphere's conditions and determine the interior structure of sub-Neptunes like K2-18 b, the possible interactions between the atmosphere and a hot, supercritical water ocean at its base remain unexplored. In this work, we investigate whether a global supercritical water ocean, resembling a planetary-scale hydrothermal system, can explain these observations on K2-18 b–like sub-Neptunes through equilibrium aqueous geochemical calculations. We find that the observed atmospheric CH₄/CO₂ ratio implies a minimum ocean temperature of ~710 K, whereas the corresponding CO/CO₂ ratio allows ocean temperatures up to ~1070 K. These results indicate that a global supercritical water ocean on K2-18 b is plausible. While life cannot survive in such an ocean, this work represents the first step toward understanding how a global supercritical water ocean may influence observable atmospheric characteristics on volatile-rich sub-Neptunes. Future observations with better-constrained CO and NH₃ mixing ratios could further help distinguish between possible interior compositions of K2-18 b.

With sizable volatile envelopes but smaller radii than the solar system ice giants, sub-Neptunes have been revealed as one of the most common types of planet in the galaxy. While the spectroscopic characterization of larger sub-Neptunes (2.5–4 R _⊕) has revealed hydrogen-dominated atmospheres, smaller sub-Neptunes (1.6–2.5 R _⊕) could either host thin, rapidly evaporating, hydrogen-rich atmospheres or be stable, metal-rich “water worlds” with high mean molecular weight atmospheres and a fundamentally different formation and evolutionary history. Here, we present the 0.6–2.8 μm JWST/NIRISS/SOSS transmission spectrum of GJ 9827 d, the smallest (1.98 R _⊕) warm (T _eq,A=0.3 ∼ 620 K) sub-Neptune where atmospheric absorbers have been detected to date. Our two transit observations with NIRISS/SOSS, combined with the existing HST/WFC3 spectrum, enable us to break the clouds–metallicity degeneracy. We detect water in a highly metal-enriched “steam world” atmosphere (O/H of ∼4 by mass and H₂O found to be the background gas with a volume mixing ratio of >31%). We further show that these results are robust to stellar contamination through the transit light source effect. We do not detect escaping metastable He, which, combined with previous nondetections of escaping He and H, supports the steam atmosphere scenario. In water-rich atmospheres, hydrogen loss driven by water photolysis happens predominantly in the ionized form, which eludes observational constraints. We also detect several flares in the NIRISS/SOSS light curves with far-UV energies of the order of 10³⁰ erg, highlighting the active nature of the star. Further atmospheric characterization of GJ 9827 d probing carbon or sulfur species could reveal the origin of its high metal enrichment.

“Water worlds” - planets with rocky cores overlain by water-rich envelopes - may make up a significant proportion of the sub-Neptune population and their atmospheric characterisation with JWST can offer insights into planetary formation and migration (e.g., Benneke et al. 2024). Whereas colder water worlds may be able to form liquid water oceans and have hydrogen-rich upper atmospheres, warmer planets would contain water in a supercritical state. In this case, hydrogen in the envelope mixes with the supercritical water and the observable upper atmosphere would have a high mean molecular weight with a smaller scale height. In intermediate regimes, a supercritical water layer will lie below a condensing cloud deck and hydrogen-dominated upper atmosphere. In this work, we aim to define the boundary between these three regimes using a 1D radiative-convective model. Our model uses the SOCRATES radiative transfer code coupled to a radiative-convective driver to solve for equilibrium temperature-pressure profiles in a water world atmosphere. We account for the effects of convective inhibition in the condensing layers and the non-idealness of supercritical water at high temperatures and pressures, both of which can dramatically affect the structure of the atmosphere. The resulting framework allows us to demarcate the transition between ocean worlds and sub-Neptunes with a mixed envelope as a function of instellation, interior flux and envelope composition. In the case where water is supercritical, the level at which condensation occurs (and the cold-trapping of water vapour) determines whether the observable atmosphere is hydrogen or water-dominated. We discuss whether the change in composition of the upper atmosphere accompanied by this transition would be observable.

Theoretical predictions and observational data indicate a class of sub-Neptune exoplanets may have water-rich interiors covered by hydrogen-dominated atmospheres. Provided suitable climate conditions, such planets could host surface liquid oceans. Motivated by recent JWST observations of K2-18 b, we self-consistently model the photochemistry and potential detectability of biogenic sulfur gases in the atmospheres of temperate sub-Neptune waterworlds for the first time. On Earth today, organic sulfur compounds produced by marine biota are rapidly destroyed by photochemical processes before they can accumulate to significant levels. Domagal-Goldman et al. suggest that detectable biogenic sulfur signatures could emerge in Archean-like atmospheres with higher biological production or low UV flux. In this study, we explore biogenic sulfur across a wide range of biological fluxes and stellar UV environments. Critically, the main photochemical sinks are absent on the nightside of tidally locked planets. To address this, we further perform experiments with a 3D general circulation model and a 2D photochemical model (VULCAN 2D) to simulate the global distribution of biogenic gases to investigate their terminator concentrations as seen via transmission spectroscopy. Our models indicate that biogenic sulfur gases can rise to potentially detectable levels on hydrogen-rich water worlds, but only for enhanced global biosulfur flux (≳20 times modern Earth’s flux). We find that it is challenging to identify DMS at 3.4 μm where it strongly overlaps with CH₄, whereas it is more plausible to detect DMS and companion byproducts, ethylene (C₂H₄) and ethane (C₂H₆), in the mid-infrared between 9 and 13 μm.

Hycean worlds are a proposed subset of sub-Neptune exoplanets with substantial water inventories, liquid surface oceans, and extended hydrogen-dominated atmospheres favorable for habitability. We aim to quantitatively define the inner edge of the Hycean habitable zone (HZ) using a 1D radiative-convective model. As a limiting case, we model a dry hydrogen–helium envelope above a surface ocean. For a 1 bar (10,100 bar) atmosphere, the hydrogen greenhouse effect alone sets the inner edge of the HZ at 0.216 au (0.58, 3.71 au) for a Sun-like G star and at 0.0364 au (0.110, 0.774 au) for an 3500 K M star. Introducing water vapor into the atmosphere, the runaway greenhouse instellation limit is greatly reduced due to the presence of superadiabatic layers where convection is inhibited. This moves the inner edge of the HZ from ≈1 au for a G star to 1.6 au (3.85 au) for a Hycean world with a H₂–He inventory of 1 bar (10 bar). For an M star, the inner edge is equivalently moved from 0.17–0.28 au (0.54 au). Our results suggest that most of the current Hycean world observational targets are not likely to sustain a liquid water ocean. We present an analytical framework for interpreting our results, finding that the maximum possible outgoing longwave radiation scales approximately inversely with the dry mass inventory of the atmosphere. We discuss the possible limitations of our 1D modeling and recommend the use of 3D convection-resolving models to explore the robustness of superadiabatic layers.