Dark skies of the slightly eccentric WASP-18 b from its optical-to-infrared dayside emission

Context . Ultra-hot Jupiters (UHJs) are gas giant exoplanets that are strongly irradiated by their star, setting intense molecular dissociation that leads to atmospheric chemistry dominated by ions and atoms. These conditions inhibit day-to-night heat redistribution, which results in high temperature contrasts. Phase-curve observations over several passbands offer insights on the thermal structure and properties of these extreme atmospheres. Aims . We aim to perform a joint analysis of multiple observations of WASP-18 b from the visible to the mid-infrared, using data from CHEOPS, TESS, and Spitzer. Our purpose is to characterise the planetary atmosphere with a consistent view over the large wavelength range covered, including JWST data. Methods . We implemented a model for the planetary signal including transits, occultations, phase signal, ellipsoidal variations, Doppler boosting, and light travel time. We performed a joint fit of more than 250 eclipse events and derived the atmospheric properties using general circulation models (GCMs) and retrieval analyses. Results . We obtained new ephemerides with unprecedented precisions of 1 second and 1.4 millisecond on the time of inferior conjunction and orbital period, respectively. We computed a planetary radius of R p = 1.1926 ± 0.0077 R J with a precision of 0.65% (or 550 km). Based on a timing inconsistency with JWST, we discuss and confirm the orbital eccentricity ( e = 0.00852 ± 0.00091). We also constrain the argument of periastron to ω = 261.9 −1.4 +1.3 deg. We show that the large dayside emission implies the presence of magnetic drag and super-solar metallicity. We find a steep thermally inverted gradient in the planetary atmosphere, which is common for UHJs. We detected the presence of strong CO emission lines at 4.5 μm from an excess of dayside brightness in the Spitzer/IRAC/Channel 2 passband. Using these models to constrain the reflected contribution in the CHEOPS passband, we derived an extremely low geometric albedo of A g CHEOPS = 0.027 ± 0.011. Conclusions . The orbital eccentricity remains a potential challenge for planetary dynamics that might require further study given the short-period massive planet and despite the young age of the system. The characterisation of the atmosphere of WASP-18 b reveals the necessity to account for magnetic friction and super-solar metallicity to explain the full picture of the dayside emission. We find the planetary dayside to be extremely unreflective; however, when juxtaposing TESS and CHEOPS data, we get hints of increased scattering efficiency in the visible, likely due to Rayleigh scattering.

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Publication Details

Journal
Astronomy and Astrophysics
Published
2025-05-28
DOI
https://doi.org/10.1051/0004-6361/202450939
Citations
4
Primary Topic
Stellar, planetary, and galactic studies
Type
article
Field-Weighted Citation Impact
4.62

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article

Dark skies of the slightly eccentric WASP-18 b from its optical-to-infrared dayside emission

M. Deleuil, M. B. Davies, A. Collier Cameron, N. Billot et al.
4 citations
Astronomy and Astrophysics
Stellar, planetary, and galactic studies
4.62
article

Dark skies of the slightly eccentric WASP-18 b from its optical-to-infrared dayside emission

M. Deleuil, M. B. Davies, A. Collier Cameron, N. Billot, H. Rauer, D. Magrin, R. Ottensamer, Maximilian N. Günther, S. Sulis, Maria Bergomi, A. E. Simon, L. Borsato, D. Pollacco, V. Nascimbeni, M. Lendl, Ε. Πάλλη, D. Queloz, M. Fridlund, A. C. M. Correia, A. Deline, C. Mordasini, L. Carone, J. Korth, G. Piotto, S. Udry, Allison Smith, F. Ratti, T. G. Wilson, R. Alonso, G. Peter, A. Fortier, Roberto Ragazzoni, L. Fossati, D. Piazza, D. Ségransan, C. M. Persson, A. Derekas, L. L. Kiss, I. Ribas, T. Bárczy, L. Delrez, Y. Alibert, Ch. Helling, C. Broeg, M. Gillon, A. Heitzmann, G. Scandariato, N. A. Walton, G. Bruno, D. Barrado, A. Lecavelier des Étangs, W. Baumjohann, A. Erikson, W. Benz, N. C. Santos, Daniel Kitzmann, J. Hasiba, S. C. C. Barros, Nicolas Iro, P. E. Cubillos, D. Gandolfi, K. G. Isaak, K. Gazeas, J. Venturini, S. G. Sousa, Karsten Westerdorff, Brice-Olivier Demory, O. D. S. Demangeon, D. Ehrenreich, Billy Edwards, M. Stalport, A. Brandeker, G. Olofsson, B. Merín, M. Farnir, A. Bonfanti, L. Kreidberg, E. Villaver, V. Van Grootel, V. Singh, N. Rando, P. F. L. Maxted, J. Laskar, M. Güdel, I. Pagano, Gy. M. Szabó, M.-D. Busch, Sz. Csizmadia, K. W. F. Lam
article en
4 citations

Abstract

Context . Ultra-hot Jupiters (UHJs) are gas giant exoplanets that are strongly irradiated by their star, setting intense molecular dissociation that leads to atmospheric chemistry dominated by ions and atoms. These conditions inhibit day-to-night heat redistribution, which results in high temperature contrasts. Phase-curve observations over several passbands offer insights on the thermal structure and properties of these extreme atmospheres. Aims . We aim to perform a joint analysis of multiple observations of WASP-18 b from the visible to the mid-infrared, using data from CHEOPS, TESS, and Spitzer. Our purpose is to characterise the planetary atmosphere with a consistent view over the large wavelength range covered, including JWST data. Methods . We implemented a model for the planetary signal including transits, occultations, phase signal, ellipsoidal variations, Doppler boosting, and light travel time. We performed a joint fit of more than 250 eclipse events and derived the atmospheric properties using general circulation models (GCMs) and retrieval analyses. Results . We obtained new ephemerides with unprecedented precisions of 1 second and 1.4 millisecond on the time of inferior conjunction and orbital period, respectively. We computed a planetary radius of R p = 1.1926 ± 0.0077 R J with a precision of 0.65% (or 550 km). Based on a timing inconsistency with JWST, we discuss and confirm the orbital eccentricity ( e = 0.00852 ± 0.00091). We also constrain the argument of periastron to ω = 261.9 −1.4 +1.3 deg. We show that the large dayside emission implies the presence of magnetic drag and super-solar metallicity. We find a steep thermally inverted gradient in the planetary atmosphere, which is common for UHJs. We detected the presence of strong CO emission lines at 4.5 μm from an excess of dayside brightness in the Spitzer/IRAC/Channel 2 passband. Using these models to constrain the reflected contribution in the CHEOPS passband, we derived an extremely low geometric albedo of A g CHEOPS = 0.027 ± 0.011. Conclusions . The orbital eccentricity remains a potential challenge for planetary dynamics that might require further study given the short-period massive planet and despite the young age of the system. The characterisation of the atmosphere of WASP-18 b reveals the necessity to account for magnetic friction and super-solar metallicity to explain the full picture of the dayside emission. We find the planetary dayside to be extremely unreflective; however, when juxtaposing TESS and CHEOPS data, we get hints of increased scattering efficiency in the visible, likely due to Rayleigh scattering.

Astronomy and AstrophysicsVol. 699
Eötvös Loránd University (HU), University of Geneva (CH), University of Bern (CH), Leiden University (NL), Centre National de la Recherche Scientifique (FR), University of Vienna (AT), Institute of Space Sciences (ES), Austrian Academy of Sciences (AT), University of Padua (IT), University of Liège (BE), Universidad de La Laguna (ES), Stockholm University (SE), University of St Andrews (GB), Universidade do Porto (PT), Lund University (SE), National and Kapodistrian University of Athens (GR), Aix-Marseille Université (FR), University of Cambridge (GB), Institut d'Astrophysique de Paris (FR), Konkoly Observatory (HU), Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR) (DE), ETH Zurich (CH), University of Warwick (GB), Sorbonne Université (FR), Graz University of Technology (AT), Institute of Astrophysics and Space Sciences (PT), Space Research Institute (AT), Institut d'Estudis Espacials de Catalunya (ES), European Space Astronomy Centre (ES), Osservatorio Astrofisico di Torino (IT), Cavendish Hospital (GB), Max Planck Institute for Astronomy (DE), Institute of Astronomy (RU), Osservatorio Astrofisico di Catania (IT), HUN-REN Research Centre for Astronomy and Earth Sciences (HU), Centro de Astrobiología (ES), Instituto de Astrofísica de Canarias (ES), AlbaNova (SE), Szent Imre Egyetemi Oktatókórház (HU), Osservatorio Astronomico di Padova (IT), Leiden Observatory, European Space Research and Technology Centre (NL), Space Research Organisation Netherlands (NL), Institut de Mécanique Céleste et de Calcul des Éphémérides (FR), University of Turin (IT), Keele University (GB), Chalmers University of Technology (SE), Freie Universität Berlin (DE), University of Coimbra (PT), KU Leuven (BE)
National Science Foundation, National Aeronautics and Space Administration, California Institute of Technology, Fondazione CRT, Canadian Celiac Association, UK Space Agency, Space Telescope Science Institute, Centres de Recerca de Catalunya, European Space Agency, University of Warwick, European Commission, Deutsche Forschungsgemeinschaft, Agence Nationale de la Recherche, Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung, Swedish National Space Agency, Austrian Science Fund, Fonds De La Recherche Scientifique - FNRS, Belgian Federal Science Policy Office, Generalitat de Catalunya, Centre National d’Etudes Spatiales, Ministerio de Ciencia e Innovación, Université de Liège, Nemzeti Kutatási Fejlesztési és Innovációs Hivatal, Instituto de Astrofísica de Canarias, Horizon 2020 Framework Programme, Science and Technology Facilities Council, Fundação para a Ciência e a Tecnologia, Staatssekretariat für Bildung, Forschung und Innovation, European Regional Development Fund, Agencia Estatal de Investigación, National Research, Development and Innovation Office
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