Exoplanet atmospheres

Abstract:

With its expanded wavelength coverage and increased precision compared to previous space-based observatories, JWST provides the opportunity to revisit benchmark planets and view them in a new light. Here, we conduct an in-depth study of the atmosphere of the hot-Saturn WASP-96 b combining a new JWST NIRSpec/G395H transit with archival NIRISS/SOSS and Very Large Telescope/FORS2 transmission spectra. The combined spectrum shows clearly visible features from H2O, CO2, and Na. CO, though, remains unconstrained, precluding a firm metallicity derivation from free retrievals alone. However, self-consistent grids yield a broadly superstellar atmospheric metallicity of 2–6× stellar. When combined with a roughly stellar C/O ratio ( 0.41−0.09+0.10 from self-consistent grids), we find that WASP-96 b potentially formed via core-accretion beyond the H2O snowline and subsequently accreted volatile-rich material. Free retrievals also find a moderate preference ( lnB = 2.69) for models with SO2 versus without. WASP-96 b falls directly on the proposed “SO2 shoreline” and the retrieved SO2 abundance is well-matched to predictions from photochemical models. Our combined spectrum displays an optical slope, which our models fit with opacity from scattering aerosols—either small-particle condensate clouds or photochemical hazes—though we cannot completely rule out the broad wings of Na or the effects of stellar contamination. Future observations are necessary to disentangle these effects. Finally, we explore the possibility for limb asymmetry in WASP-96 b’s transmission spectrum and provide several tests to identify asymmetries in our data. We encourage the community to prioritize the development of a robust pathway to quantify the presence of limb asymmetry—particularly for low signal-to-noise cases.

Abstract:

Previous analysis of Neptune’s atmosphere using near-infrared Gemini/NIFS observations found the strongest spectral signature of hydrogen sulphide (HS) to be at the planet’s south pole. Conversely, analysis of microwave observations with the Atacama Large Millimeter/submillimeter Array in 2019 suggested a distribution of HS that peaks in the mid-latitudes and is strongly depleted towards the pole. We analyse near-infrared observations from VLT-SINFONI in 2018 and fit a parametrized cloud model to the data using nested sampling. By prescribing a latitudinally varying methane (CH) profile previously derived from visible light observations, we find general agreement with the microwave analysis, with an enhancement of HS by a factor of 4 at the southern mid-latitudes compared to polar latitudes. The stronger spectral signature at the pole is explained with a deeper cloud top, resulting in a higher cloud-top HS column abundance in this region. Our results are indicative of deep upwelling at the mid-latitudes, with downwelling at the pole and possibly near the equator.

Abstract:

The extreme conditions in Venus’s lower atmosphere make robust calibration of in situ observations challenging. Consequently, measurements from past entry probes provided mixed evidence regarding the existence of a near-surface particulate layer (NSPL). Although the Venera 11 (1978) and Venera 13 and 14 (1982) landers performed in situ spectrophotometric observations during descent, the original datasets were later lost. However, a subset has been reconstructed by digitising graphical outputs produced during the missions’ initial data-processing phase [1]. Following careful analysis to identify and mitigate errors and other artefacts, the reconstructed dataset retains the reliable downward-looking spectra acquired by the three landers from ~62 km altitude to the surface.Previous retrievals from the reconstructed Venera 13 indicated an NSPL centred at ~3.5–5 km, with particulate optical properties consistent with a basaltic composition [2]. Following the methodology of [2], we use NEMESIS, a radiative transfer and retrieval code [3], to perform near-surface retrievals from the reconstructed Venera 11 and Venera 14 datasets. The results from Venera 11, 13, and 14 retrievals are compared with reported detections and non-detections from other instruments on earlier in situ missions, to explore potential formation pathways for the NSPL in light of the combined observational record.References:[1] Ignatiev, N. I., Moroz, V. I., Moshkin, B. E., Ekonomov, A. P., Gnedykh, V. I., Grigor’ev, A. V., and Khatuntsev, I. V. Cosmic Research 35(1), 1–14 (1997).[2] Kulkarni, S. V., Irwin, P. G. J., Wilson, C. F., & Ignatiev, N. I. Journal of Geophysical Research: Planets, 130, e2024JE008728, (2025).[3] Irwin, P. G., Teanby, N. A., de Kok, R., Fletcher, L. N., Howett, C. J., Tsang, C. C., Wilson, C. F., Calcutt, S. B., Nixon, C. A., and Parrish, P. D. Journal of Quantitative Spectroscopy and Radiative Transfer 109(6), 1136–1150 (2008). 

Abstract:

Rocky planets hosted by M-dwarf stars represent the most abundant and accessible class of potentially habitable targets for atmospheric characterisation with current and planned observatories. Owing to the close proximity of the habitable zone around these cooler stars, such planets are expected to be tidally locked, giving rise to a set of atmospheric circulation regimes determined primarily by planetary rotation rate and incident stellar flux. Previous climate modelling studies have commonly identified a characteristic 'eyeball' habitable climate for these worlds, illustrative of the approximately circular area surrounding the sub-stellar point where surface temperatures rise above freezing. Using an ensemble of three general circulation models (ExoCAM, LFRic, and ROCKE-3D), we examine the influence of circulation regime on surface habitability across the inner edge of the M-dwarf habitable zone, simulating Earth-like aquaplanets with rotation periods spanning the ‘fast’, ‘Rhines’, and ‘slow’ regimes (4.25–44.33 days). We make use of a new metric of surface habitability which has been previously validated against past and present habitability on Earth, and extends beyond the traditionally-used 'liquid water' temperature condition to define two habitable temperature ranges for each microbial and complex life, as well as using surface water fluxes as a proxy for water and nutrient availability. This additional constraint produces spatial patterns of habitability that differ from those defined by temperature alone, whereby large areas surrounding the substellar point with habitable temperatures but negative net precipitation (P - E < 0) are now designated as ‘limited’ habitability. Furthermore, distinct spatial patterns of habitability emerge across the ensemble for each regime, indicating a dependence on the atmospheric circulation and associated transport of heat and moisture. For slower-rotators, habitable area is substantially reduced as surface moisture is largely confined to the day-side, while faster rotators show a more extensive habitable area but greater variation between the models in global habitable fraction.

Abstract:

Whether a planet's volcanic gas is oxidising or reducing is inherited from redox conditions in the planet's mantle. It is often presumed that reactions between iron species control mantle oxygen fugacity. However, iron alone need not be the sole dictator of how oxidising the interior of a planet is. Carbon is a powerful redox element, with great potential to feed back upon the mantle redox state as it melts. Despite Earth being carbon-poor, it has been proposed that the oxygen fugacity of Earth's upper mantle is in part controlled by carbon (Holloway et al., 1992; Stagno et al., 2013); a slightly-higher volatile endowment could make carbon-powered geochemistry inescapable. Indeed, a number of known rocky exoplanets are predicted to have formed with carbon contents greater than Earth (Bergin et al., 2023). We offer a framework for how carbon is transported from solid planetary interior to atmosphere, tracking redox couplings between carbon and iron. We also incorporate a coupled 1D energy- and mass-balance model to provide first-order predictions of the rate of volcanism. We show that carbon-iron redox coupling would maintain interior oxygen fugacity in a narrow range: more reducing than Earth magma, but not reducing enough to prevent CO2 outgassing entirely.Bergin, E. A., Kempton, E. M.-R., Hirschmann, M., Bastelberger, S. T., Teal, D. J., Blake, G. A., Ciesla, F. J., & Li, J. (2023). Exoplanet Volatile Carbon Content as a Natural Pathway for Haze Formation. The Astrophysical Journal, 949, L17. Holloway, J. R., Pan, V., & Gudmundsson, G. (1992). High-pressure fluid-absent melting experiments in the presence of graphite: Oxygen fugacity, ferric/ferrous ratio and dissolved CO2. European Journal of Mineralogy, 4(1), 105–114. Stagno, V., Ojwang, D. O., McCammon, C. A., & Frost, D. J. (2013). The oxidation state of the mantle and the extraction of carbon from Earth’s interior. Nature, 493(7430).