Dark skies of the slightly eccentric WASP-18 b from its optical-to-infrared dayside emission
Journal article, 2025
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 omega = 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 mu m 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.
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 omega = 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 mu m 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.
techniques: photometric
planets and satellites: atmospheres
planets and satellites: individual: WASP-18 b
Author
A. Deline
University of Geneva
P. E. Cubillos
Austrian Academy of Sciences
Istituto nazionale di astrofisica (INAF)
L. Carone
Austrian Academy of Sciences
B. -o. Demory
University of Bern
M. Lendl
University of Geneva
W. Benz
University of Bern
A. Brandeker
Stockholm University
M. N. Guenther
European Space Agency (ESA)
A. Heitzmann
University of Geneva
S. C. C. Barros
University of Porto
L. Kreidberg
Max Planck Society
G. Bruno
Istituto nazionale di astrofisica (INAF)
D. Kitzmann
University of Bern
A. Bonfanti
Austrian Academy of Sciences
M. Farnir
University of Geneva
Carina Persson
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
S. G. Sousa
University of Porto
T. G. Wilson
The University of Warwick
D. Ehrenreich
University of Geneva
V. Singh
Istituto nazionale di astrofisica (INAF)
N. Iro
German Aerospace Center (DLR)
Y. Alibert
University of Bern
R. Alonso
Instituto de Astrofísica de Canarias
University of La Laguna
T. Barczy
Admatis
D. Barrado Navascues
Spanish National Research Council (CSIC)
W. Baumjohann
Austrian Academy of Sciences
M. Bergomi
Istituto nazionale di astrofisica (INAF)
N. Billot
University of Geneva
L. Borsato
Istituto nazionale di astrofisica (INAF)
C. Broeg
University of Bern
M. -d. Busch
University of Bern
A. Collier Cameron
University of St Andrews
A. C. M. Correia
University of Coimbra
Sz. Csizmadia
German Aerospace Center (DLR)
M. B. Davies
Lund University
M. Deleuil
Aix Marseille University
L. Delrez
KU Leuven
University of Liège
O. D. S. Demangeon
University of Porto
A. Derekas
Eötvös Loránd University (ELTE)
B. Edwards
Netherlands Institute for Space Research (SRON)
A. Erikson
German Aerospace Center (DLR)
A. Fortier
University of Bern
L. Fossati
Austrian Academy of Sciences
Malcolm Fridlund
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
D. Gandolfi
University of Turin
K. Gazeas
National and Kapodistrian University of Athens
M. Gillon
University of Liège
M. Guedel
University of Vienna
J. Hasiba
Austrian Academy of Sciences
Ch. Helling
Austrian Academy of Sciences
Technische Universität Graz
K. G. Isaak
European Space Agency (ESA)
L. L. Kiss
Hungarian Academy of Sciences
Eötvös Loránd University (ELTE)
J. Korth
Lund University
K. W. F. Lam
German Aerospace Center (DLR)
J. Laskar
Université Paris PSL
A. Lecavelier des etangs
Pierre and Marie Curie University (UPMC)
D. Magrin
Istituto nazionale di astrofisica (INAF)
P. F. L. Maxted
Keele University
B. Merin
European Space Agency (ESA)
C. Mordasini
University of Bern
V. Nascimbeni
Istituto nazionale di astrofisica (INAF)
G. Olofsson
Stockholm University
R. Ottensamer
University of Vienna
I. Pagano
Istituto nazionale di astrofisica (INAF)
E. Palle
University of La Laguna
Instituto de Astrofísica de Canarias
G. Peter
German Aerospace Center (DLR)
D. Piazza
University of Bern
G. Piotto
Istituto nazionale di astrofisica (INAF)
University of Padua
D. Pollacco
The University of Warwick
D. Queloz
Swiss Federal Institute of Technology in Zürich (ETH)
University of Cambridge
R. Ragazzoni
Istituto nazionale di astrofisica (INAF)
University of Padua
N. Rando
European Space Agency (ESA)
F. Ratti
European Space Agency (ESA)
H. Rauer
German Aerospace Center (DLR)
Freie Universität Berlin
I. Ribas
Spanish National Research Council (CSIC)
Inst Estudis Espacials Catalunya IEEC
N. C. Santos
University of Porto
G. Scandariato
Istituto nazionale di astrofisica (INAF)
D. Segransan
University of Geneva
A. E. Simon
University of Bern
A. M. S. Smith
German Aerospace Center (DLR)
M. Stalport
University of Liège
S. Sulis
Aix Marseille University
Gy. M. Szabo
Eötvös Loránd University (ELTE)
S. Udry
University of Geneva
V. Van Grootel
University of Liège
J. Venturini
University of Geneva
E. Villaver
Instituto de Astrofísica de Canarias
University of La Laguna
N. A. Walton
University of Cambridge
K. Westerdorff
German Aerospace Center (DLR)
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 699 A150Subject Categories (SSIF 2025)
Astronomy, Astrophysics, and Cosmology
DOI
10.1051/0004-6361/202450939